Just had a new 4-ton AC installed in an old house with no previous central air. I notice that it cycles off and on quite a bit, even though we’ve had a very hot and humid summer thus far. How often should the compressor on a properly-sized AC unit run on a “peak” day?
I thought that they should run pretty much all the time if it’s 95+ degrees out, and only cycle on and off on days when the temperature is not as high.
Replies
How big is the house?
Did the contractor do a load calculation? Did you go with the low bid square foot per ton guy?
If it is sized right, it will run all the time on a peak day.
Get a digitial humidity guage so that you can see what the interior humidity is running. That the where oversized units will "fall down". They don't run long enough to remove enough humidity.
You can get home owners version (limited time use) of a load calculator.
http://www.hvaccomputer.com/
Or maybe they used this free sizing calcualtor.
http://www.hvaccomputer.com/hvac/sizer.asp
Bob- The house is 2,800 sq ft, 300 of which is the third floor and was previously un-conditioned. We had two window units (1.5 ton on 2nd floor, 1 ton on first) that cooled the house nicely last summer. Also, the house was not insulated until last week, and the HVAC contractor knew of our plans to add insulation - he even recommended the company we used.
Danski- He did a load calculation and came up with over 6 tons. I told him that based on the two window units cooling the house very nicely, there was no way that I'd install anything more than a 4 ton unit. And I thought that would be oversized.We only really got a bid from one contractor, who has an excellent reputation.
Bill- Excellent suggestion r.e. the humidity gauge. Are they common items (i.e. does HD or Keith's Hardward have them?). What range should the house be in?
I did a load calculation by hand, and it was pretty much the same as what the contractor came up with - 6 tons or more. Since there was such a large discrepancy between reality and the calculation, he agreed to install a 4-ton unit if I agreed that the house may heat up on peak days. Since the temperature has been well above 90 for several days and the AC has not had any problem keeping up, I think that I was right.
Then what am I worried about? My pregnant wife thinks it feels warm, although it feels OK to me most of the time. Could be that the humidity is not being removed as Bill suggests. In that case, we'll have to run the house colder in order to be comfortable. This will cause more heat to come into the house, making the oversized AC run more and costing more money.
A good chance that they would have one. If not WalMart, Target, or Radio Shack.Here is one from RS.http://tinyurl.com/zbsx"My PREGNANT wife thinks it feels warm"That might be the "problem". But yes, if the humidity level is high the body won't release as much moisture and feel hotter.I believe that 30-50% is the desired range.
Pregnant wife
That seems to be a common theme. May be something to that.
"My pregnant wife thinks it feels warm, although it feels OK to me most of the time"
Both times my wife was pregnant (once during winter & once during summer) it was next to impossible to keep her cool enough. When we were married, she always thought it was cold & I was always hot - it was always the reverse when she was pregnant.
It COULD be the humidity, but it is very likely your wifes metabolism during pregnancy. I'd crank the AC up a few notches (which is what I had to do with our first child) until the baby comes.
“My pregnant wife thinks it feels warm, although it feels OK to me…”<!----><!---->
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Lew, my wife was “ready to go” pregnant last December and she was sweating bullets every day complaining how hot the house was even thought the thermostat read 58°!!!! If the wife is your only second opinion on if the house is comfortable wait till her hormones rebalance about 2 months after the joyous arrival. By March my wife had the heat pumping at 76°, wearing 2 sweat shirts and asking me to chop wood for the fire every night. For now keep her drinks cold and good luck on waiting out the hot flashes (and not to mention the mood swings, Jezzzzz). <!---->
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Congrats and lots of luck on the new adventure.<!---->
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Ryan<!---->
As others have suggested, I would check the humidity and see what the ranges are. If the AC is oversized, it will have a lot of trouble to remove the latent heat from the structure. In particular, if the runtime suring peak days is less than 100%, you are definitively oversized, load calc or not.Unfortunately, I cannot think of an inexpensive way to deal with this other than an unloader that Jerry Scharf on the Wall pointed me to last year. It's basically a device that allows a one-speed condenser to modulate it's output. Let me search and see if I can find the original company.
if the runtime suring peak days is less than 100%,
Are you sure I'm reading that right?
I aim to average having my central a/c run only about 15 minutes in every hour--which is adjusted by immediate humidity & temperature conditions.
My electric bill would be about $800-900 with a near-100% run on my a/c. Now, getting the a/c to run that much would be easy enough, just set it for more than 20º of delta T (it's 102.4º for the heat index just now, in the shade it's 99.0º at a relatively dry 33%RH).Occupational hazard of my occupation not being around (sorry Bubba)
Allow me to disagree.Your AC system should run 100% of the day on a design day. If your AC system is only loaded to 25% on a design day, that implies that it's 4x too large for the home it was installed in. All things being equal, a 4x smaller system running 4x longer will result in the same energy consumption.However, a right-sized AC system has a much better chance of moisture removal than one that only runs 25% of the time. Superior moisture removal in turn allows you to set the thermostat higher yet remain comfortable because your skin has an easier time evaporating your sweat. Ergo, right-sized AC systems will always be more energy efficent than oversized ones.For what it's worth, I am currently in the market for two condenser systems (the AH are already installed). The heat gain I calculated and the gain calculated by my contractor are pretty close to 13BTU/(ft^2 x hr) or 923ft^2 per ton. I don't consider my home to be super-efficient, yet 2/3 of the heat gain is via the windows. Add some v-kool, etc. window-film to the mix, and I could be looking at consumption levels around 2000ft^2/ton.
Your AC system should run 100% of the day on a design day.
Hmm, we may be in a terminology muddle.
There were 23 or 25 CDD just yesterday, the 30 year normal for College Staion is 2776 CDD per year.
My air handler is running right under 2 air changes per hour at it's 15' per hour present run average. The digital thermometer is giving me about 19.5º (±0.35º) of Delta T from closest supply versus return.
Given that I have an under insulated house, with a medium-bad infiltration rate, I could set the stat to 72º, but that's not a temperature I'd see indoors until about 0130 or so. Then, it'd "hold" until about 1030 or so. So, that'd be 15 hours a day of running air handler & compressor.
I'll admit that my personal "design day" is also skewed a bit, too.Occupational hazard of my occupation not being around (sorry Bubba)
The way I understand it is the way Constantin explained it above. Around here, compressor/condenser units are sized for a 97° day. On that day, while it is 72° inside, the compressor should run all the time. Depending on the characteristics of your house (heat load calculations), you might need anywhere from a 1 ton to multiple 6-ton units to achieve the constant 72° temperature inside. Since this accounts for 1% of the time in a year, this does not amount to much electricity.
On days when it is hotter than 97°, which do happen occasionally, the compressor will run all the time, but the temperature will creep up inside of the house - maybe a degree or two an hour? Since that temperature extreme only lasts during the hottest part of the day, you can expect the inside temperature to stay below 80°.
"Cooler" days are where the properly sized unit pays off. In order to remove 2 tons/hr during a "cooler" day, a large unit (say 4 ton) may cycle on and off several times, running only 30 minutes/hour. A smaller unit (say 3 tons) will cycle 3/4 as much, and run for 45 minutes/hour. The electric cost will probably be about the same given the different sized motors (?), but the humidity in the house with the smaller unit will be lower. Dryer air means that sweating is more efficient, so it feels cooler, even though both houses are at 72°.
The savings would then be had by adjusting the "small-unit" house's thermostat to 73 or 74°F.
That sound right?
Well, when I refer to a design day, it's one of those outside the 99th percentile as calculated by ASHRAE/ACCA. Locally, that's 91 degrees. So, a design day here will result in 26CDD. Overall, my local climate is supposed to have 776 CDD per year. Meanwhile, I expect the cooling system to maintain 75 degrees indoors at an RH of 50%. My quandary revolves around the available condenser capacities being either 6.5kBTU too large or 3kBTU too small. At the moment, I am leaning towards undersizing and letting the third-floor inhabitants enjoy the summer weather a bit more than the rest of the building.
I'm a bit confused as to the "peak days" and the AC running 100% during this time.I would think that an AC that can keep a house at say, 72 (when it's 96 F outside with say, 50% humidity) and only runs say, 20 minutes every hour, (let's say during this 8 hour "window") would be "better" than one that can do the same, but have to run 8 straight hours.Or am I missing something?
One of the big purpose of an AC is removing humidity from the air.Now if you are in death valley this is not important.But if you are East of the Rockies the summer time the humidity can get very high with dewpoint of 65 - 75.If the AC only ran for a few minutes it would not remove much humidity.So you might end of with 72 degree air, but 90% RH.Not only is that a perfect labratory for every mold, mildew, and fungus know to man, it is very uncomfortable.Not to mention the cost of buy a system that is 5 times too big.
I go away for a week and miss all the fun. I would like to add a few points that were missed.
1- Design conditions vary from region to region but also from client to client. It is unwise to design an HVAC system to run 100% of the time on the warmest day of the year if the home will also be used to entertain during that time. Adding 30 to 100 people and the resultant exterior door use to a home such as this for, say, a July 4th party is going to result in discomfort for your guests. Any such unusual load, even baking, will overwhelm the system.
2- In hot humid climates, I recommend the use of an excellent dehumidifier, such as the DEC ultra-aire, to allow more flexible AC sizing and to control moisture levels in the swing seasons. I feel that this is a much better use of capital than a two speed compressor and thermidistat. Either can help overcome oversizing. In reality, oversizing occurs for months each year. (spring and fall)
3- One very valid reason for reducing the size of an AC system is cost. A smaller system will require a smaller duct system as well. The cost reduction in a smaller system can help offset more costly insulation and air sealing techniques as well as subsidize the addition of a dehumidifier and mechanical ventilation system.
4- We really haven't been given enough info to answer the original question and I was hoping to see humidity readings. Until we have more info(a lot more) all we can do is discuss HVAC sizing in general terms. In my climate and with typical modern construction this would definitely not be oversized.
As usual, you raise excellent points. The Manual-J calculators I have used make accomodations for human bodies at rest (i.e. 400BTUs of heat each). Like building conditions, the environment that you expect to use your home in are going to be vital in terms of determining just what the peak load will be.To give you an example, the lower air handler in our home supplies all the public spaces, i.e. the areas of the home where we can expect significant numbers of guests. Originally, the heat loss calculation there indicated the need for a 4 ton unit, though the needs dropped as the insulation got thicker and the infiltration was lowered. At this point, the total load is about 27kBTU with a 95%/5% sensible/latent mix when I do not account for occupancy.The 4-ton air handler is already installed, yet going by the tables in the Lennox manual I probably could have gotten away with a 2 ton condenser (and even gotten the latent/sensible ratio right). However, I chose to go with a 3 ton condenser for the very reason you raise, which is dealing with the variable loads of a kitchen and the dining/living room. Here, the extra 12kBTU are going to give me a lot of headroom to accomodate groups of people, opening front doors, and the like. With the 2-speed compressor, the operation should remain pretty efficient in non-peak periods as well.I agree that it's probably more cost-efficient to find a proper dehumidification system for super-humid areas than to rely on a general-purpose AC system to do all the work. I simply wanted to point out how you could tailor the output of an AC system to meet the needs of different local climates, home construction, etc. all of which can be pre-calculated using Manual-J or comparable methods.It is too bad that the truely-variable speed compressor system that carrier debuted some years ago turned into a flop. Whatever flaws thwere were in the actual implementation, the idea of being able to modulate the output of the condenser across a wide range, much like gas valves on most condensing, modulating boilers today, is very appealing.
Good post Constantin-
Yes, I agree that variable speed compressors are appealing. The ones that are available right now are about a grand more expensive than a single speed scroll type compressor. When it needs to be replaced it will be very expensive again. The setup for them is also harder to get right. I have used several of them in the past but now I use a dehu to absolutely control the humidity levels at times when the cooling load is not sufficient. Neither approach is vastly superior though.
On a side note, my car has a fully variable electric scroll type AC compressor that uses a thermidistat to control the moisture removal. It automatically controls the fan speed, recirc/vent, compressor speed, and output location to maintain a user defined temperature setpoint and manufacturer set humidity level. There are of course overrides for all these settings. The efficiency of this unit is around twice that of a modern belt driven auto AC compressor. The car? Toyota Prius.
Now, why can't our residential AC system be this well designed? It's because we don't build our homes in a factory to the same size and design criteria. Those control systems become unwieldy in a residential application. I keep the controls separate for temperature, humidity, and fresh air ventilation on all the houses I build.
I know that they make variable speed DC blowers. I wasn't aware of anybody making the variable speed compressors yet. Who sells those?
The 1000$ upcharge is actually for the two speed compressor models.
There are several companies that make variable speed AC compressors but they are not yet made by the big major brands in the US and are used in small commercial projects here as far as I can tell. I think they have some design problems that are hampering bringing them to market in the mainstream residential arena.
Companies offering this technology include Temtek, Mitsubishi, Fujitsu, and ####. The problem with these high efficiency units is that the US only requires a SEER of 10 so 80% of all units sold in this country are SEER 10. The remaining 20% of the market is shared by all the remaining units, most of those being 12 SEER. There simply isn't enough profit motive in this environment to provide drastically more efficient systems in this country.
If we regulated and marketed our energy consuming products for the energy market realities of the next 20 years instead of the last 20 years, we would be in much better shape 10 years from now. The 10 SEER units being installed today will be costing people in high energy costs for years to come. Many people replace their old AC equipment to save energy instead of because they are worn out because of the low requirements of the past. The minimum standard should be raised to 14 SEER and to 16 in five years. As a country, we are not managing an efficient use of our resources.
Oops, I guess I ran on about my opinions so I'll stop it here.
No expert here, but maybe it's the wrong product as much as wrong size. I'm a little surprised by talk here of presumed limited options - so maybe that depends on market.
We bought a Heil/Keepright central AC unit for last house - at a real entry-level price point - which came with high and low compressor settings. About five acquainted first-time owners ended up buying practially identical units; and the only upgrades paid for were noise control. Cheap like borscht, really.
Last year it was a cluster of Mitsubishi split systems. These have a good variety of settings which can do straight cooling; straight de-humidifying, and fully automatic combination based on relative humidity by temp and perceived comfort. No these aren't the cheapest crap on the market, but neither are they the platinum option.
Actually starting in Jan no more central AC's less than 13 SEER can be made.Invert technology has gotten so that a lot of people are now using 3 phase motors and running them off of inexpensive inverters from single phase power for home workshop equipment. Very common for lathes where the VS is needed.I think that VS compressors systems will be common soon.
Well, I was involved in setting the coming 2003 AC/HP standards. Back in 1998-2000, the assumption was that energy in real terms would get less expensive over time due to deregulation. In the final week of its presidency, the Clinton white house signed off on the 13/13 SEER standard, that GWB later tried to lower to 12/13. Were we to look at the same questions again today, I am not sure that the SEER standard could be justifiably raised a great deal. The reason is that the cost of higher-SEER HXs, compressors, etc. have risen in line with the raw material price shocks in recent years. Furthermore, while the price of electricity has shot up in some markets, other markets that are served by coal and nukes have not seen major price increases. Thus, a lot of the variables that determine the minimum efficiency standards have changed dramatically.Remember, the minimum efficiency standards have to be shown to be economically justifiable, with a payback after a couple of years, etc. ... and only on the basis of kWh, not taking other system-wide benefits (such as fewer powerplants, lower infrastructure costs, etc.) into account. Another thing to consider is that the upcharge of going with a higher-SEER air conditioner can be tempered by the energystar and utility rebates. Up here in Boston, I get $500 a condenser for an 16+ SEER installation, which takes some of the sting out of the cost.However, the main reason I went with a super-high efficiency AC is the expectation that energy prices in the northeast will continue to rise quicker than the CPI, making a high-SEER installation justifiable even though the climate is usually mild around here. The downside of higher maintenance costs can always be covered with the 10-year warranty that the OEMs offer.
Ray-
1. I guess I can throw a good party next year. You comin'?
2. I've got one running in the basement that ran wuite a bit last year. I could ring it upstairs and see if it makes a difference.
3. We also got the place insulated (finished today), they did seal most of the ducts - although I was not impressed with their technique, and the mechanical ventilation (I assume you mean automated dampers).
4. Did some humidity experiments tonight. Dew began forming at about 53F with a 71F dry bulb, putting the RH in the low 50%'s. Sound OK? It was very hot and humid today and the AC has een running consistenyly tonight (first time I can recall it doing this).
Thanks
Oh yeah, glad to see Timbo and Constantin (and POTR) make up. Now I will be able to sleep tonight!
Lew
Hurnik, Bill hits the issues right on the head. An AC that is allowed to run pretty much continuously will, all things being equal, remove far more humidity from the interior air than one that runs only 1/5th the time. Whenever there is no flow across the evaporator coil, there cannot be any moisture removal.The total heat gain that an AC deals with is comprised of latent heat and sensible heat. Latent heat basically translates into humidity, while sensible heat you can feel outright. As the latent heat makes it more difficult for your body to perspire effectively, a room with a lower overall temperature but a higher humidity may feel warmer than a room that is hotter but drier. The usual way that oversized ACs manifest themselves, as Bill points out, is that rooms are both cold and clammy because very little humidity is being removed. In order to feel cool, homeowners then lower the thermostat ever further, though the high humidity will then make them feel clammy. So, to maximize comfort, you have to control temperature and humidity.By increasing the runtime of the air conditioner, you can maxmize the removal of latent heat. By reducing the interior humidity you can then set the thermostat at a higher setting and still feel comfortable. It is one of the main reasons that two-stage or variable-speed compressors are becoming all the more popular, as they allow to better match the heat gain of a house and maximize the runtime, humidity removal, etc. of the equipment inside.One thing to consider whenever you are doing a heat gain and loss calculation is the exact ratio of latent/sensible heat. All Manual-J compliant heat gain/loss calculators will give you a breakdown. Furthermore, most AC equipment is specified assuming a 30%/70% latent/sensible mix, which may or may not be representative of your site (it depends on the local climate, interior loads, home construction, etc.). Ideally, your equipment choices will match the sensible/latent ratio for your site. The only way to find out is to go through the detailed ARI equipment specifications that the manufacturers publish.In my case, the latent heat made up only 10% of the total load, so it made sense to oversize the air handler compared to the condenser, as that increases the sensible-heat capacity of the system. Thus, a 2ton/3ton combination of condensers is feeding a 3ton/4ton combination of air handlers. I have heard of other installations where an installer did the reverse to increase the latent heat removal.Perhaps Timbo will also chime in and enlighten us further.
There are whole house dehumidifiers which might be a solution to your problem. Expect to pay something like 1,200 plus installation - which isn't going to be cheap either.
Lew, does you house have efficient windows and a goodly amount of insulation? And is your calculation taking this into account?
Almost sounds like your HVAC guy sized it based on sq ft without taking into account a well insulated envelope.
jt8
In an underdeveloped country don't drink the water. In a developed country don't breathe the air --Jonathan Raban
sized it based on sq ft without taking into account a well insulated envelope
Having used both long and short form calculations for heat gain/loss, it's surprising how close the 500sf/ton figure really is. That ciphers to 6 tons of a/c for 2800 sf.
Going from R-19 to R-30 in the walls will usually not knock more than a ton off the total (it's the square footage are of windows & doors that remain constant at very low R values that limit the change in end result).
Now, that's ciphering on a pad and/or using software--"real" people meddling with the thermostat change the results after any unit is actually installed.Occupational hazard of my occupation not being around (sorry Bubba)
If one used a "500 sf per ton" rule of thumb around here, the system would be way oversized.
Somebody must have done something wrong to come up with a 6 ton load calc for 2800 square feet, unless you live in a glass house. ACCA Manual J specs for the Louisville area are less than where I am, so someone goofed.
You can get a one time use program at http://www.hvaccomputer.com for a small fee.
"Rules of thumb" are close, but usually end up with oversized equipment.
Remember, there are sensible and latent loads for cooling. Sensible is what you feel as temperature and latent is humidity.
You could try putting the blower motor on a slower speed for cooling, but you have to make sure there is still enough airflow so the coil does not freeze. Slower air speed will remove more moisture.
I live in a house that is 95 years old, had no insulation (although they did know that I was planning to insulate with blown-n cellulose), and a lot of equally old windows. No double-pane, low-e stuff, very few storm windows. 10' ceilings on 1st floor, 9' on second = lots of sq ft for heat loss.
I did the calculations by hand myself straight out of a Trane text book. That's why it's nice to have real world experience (the window units), although it didn't do me much good since I went with the 4-ton unit instead of a 3 or 3.5-ton one.
The owner is coming by next week to touch up some loose ends. I'm going to make a pitch to trade the 10 SEER unit for a 3 or 3.5 12 or 14 SEER unit, but I doubt that he will do it.
I happen to have a 2800 or so square foot home on my Manual J program. Changing ALL the windows from single pane, no storm, no special glass to double pane plus storm, no special glass resulted in a net change of about 4k btu's for the whole house.
Try hvac computer, spend the $40 bucks....
Did the installers check the charge? Could be overcharged.
Edited 7/7/2005 10:57 pm ET by danski0224
Could it be there is something not installed right or is faulty. Causing the unit to cycle on and off. It could be it is just to large make sure there is no mechanical problems before you chuck it up as being to big.
That's possible. I also failed to mention that the house has three zones on automatic dampers, so whenever any zone needs cooling it will cause the AC to kick on if it is not already going.
Would this suggest the A/C unit should be sized for the zone and not the whole house at once, 'cause ineffect a 4ton unit is way to big unless whole house cooling. Maybe fix open the zone dampers and seee how it works.
If you need cooling in all three zones then it would be too small if it were sized for any single zone. I suggested that we install two units - one in the attic for the 2nd/ 3rd floors, the other in the basement for the 1st floor. Obviously these would each have been much smaller units. But, more rotating equipment = more maintenance, even if the initial outlay was about the same (according to my contractor it would have been more expensive to install two units, even though the ductwork would have been much simpler. Of course he didn't have to go back and frame & drywall around the new trunk).
I do owe some more information to this discussion. I have yet to get a humidistat to collect any humidity data (been busy building kitchen cabinets). I did however have a conversation with the HVAC company owner and installation manager:
They returned a Honeywell programmable thermostat that they removed during installation (they replaced it with a White-Rogers digital/ non-programmable unit). I laid the original stat on a table next to the new one while we walked around talking. When I came back I noticed that the old one reads two degrees higher than the new one. So, I pulled out my Fluke meter, and it reads closer to the new one. I decided to lower the new stat's setting to 71 (which equals the 73 reading on the old one).
They opened up the AH housing and lowered the fan speed. Their theory is that the third floor was the only zone calling for heat, and since there are only two vents the fan was having trouble pushing everything through. As a result, the outlet air temperature was too cold and the controller was shutting it down to prevent freeze-up. I'm not sure that I agree with the logic (seems like slower air movement will make the air colder?), but I don't do this for a living. If this was THE problem, we should be running longer now. I guess since the air is moving slower, more water should condense as well? Maybe that was more of a reason to slow it down.To test it out, they lowered the setpoint on the first floor by two degrees and it ran for 15 minutes or more. Again, I'm not sure about this test since the setpoint is a larger change than normally occurs during the day.
My wife just told me that she still does not feel comfortable (again), so I guess that I'm going to have to do some humidity testing. I'd really rather get the cabinets done so we can have a kitchen sink again, but whatever.
Thanks again-
HVAC engineer?
Yes. For the last 17 years.
Allow me to quibble...
In my estimation, the HDD and CDD data can be used to help size HVAC equipment, as it's another route to estimating heat gain and heat loss in existing systems.
For example, if I take an oil boiler of a known AFUE, sum the gallons used in a season, and spread that across the HDD in a given year, then I can estimate the BTU/DD losses and hence also estimate the boiler size needed for a given structure. Granted, if the boiler is grossly oversized, it will burn a lot more oil in short-cycle than if it was properly-sized, but this approach can give you a first-order approximation to the actual heat loss on a design day.
Similarly, you could sit in the basement on a design day and see how much the boiler cycles. If it's not on at least 80% of the time, your boiler is oversized as far as the house heating needs are concerned (DWH heating is a whole other animal). Naturally, this is less of a concern with modulating, condensing boilers because they simply modulate down, but residential oil systems and most AC/HP's out there are simple on-off appliances.
On the AC side, such estimates become a bit trickier because determining the fuel usage (i.e. kWh) is much more difficult. On the other hand, the other sizing approach, i.e. determining how much the unit is cycling on a design day is still valid.
Timbo,
I doubt that the acrimony in your posting is either necessary or deliberate, however regrettable.
For the record, I am not an HVAC engineer, nor pretend to be one. However, I did run a Manual-J calculation on my house which came to within 5% with HVAC-Calc what my contractor managed with Wright-soft, so either we're both off or I did something right. Plus, while my engineering curriculum may not have prepared me fully for the HVAC-trade, I do have some of the basics.
I have enclosed a work in progress that addresses what you call a "worthless math excercise". The excel spreadsheet certainly does not address all the variables, nor is the AFUE a good measure of seasonal combustion efficiency. But like I said, this is a work in progress. Several folk in the trade tried it out and found it to be a useful means of checking their regular Manual-J heat losses in pre-existing structures.
I would also like you to consider that using prior performance data on the AC side can work just as well. Lew was happy with a 2.5 ton system, an allegedly-accurate Manual-J calculation shows a heat gain of 6 tons, and the 4 ton system they settled on apparently is still oversized. To me that indicates either that the heat gain calc was way off or that Lews house is better insulated/shaded than the calc allowed for.
Either way, Lew didn't need an HVAC engineering degree, 17 years of experience, etc. to know that the calculation being put forward by his contractor was way off. I welcome any constructive criticisms you may have. Cheers!
Edited 7/12/2005 2:06 pm ET by Constantin
Timbo,
I agree that using a Manual-J approach is the best approach in new construction to determine heat gain and heat loss. Under such circumstances, you have a fighting chance to know what'll go into the walls, what the R-values of windows, walls, etc. will be. In an existing structure, you can use the current energy consumption as a valuable yardstick to determine the heating/cooling needs of the structure.
If you know how many BTUs are being fired up inside the home and have a reasonable guess as to how many of those BTUs are going up the flue vs. out via the structure, then you can start approximating the heating need of a home. Using the local HDD data, you can average out the consumption over a longer period of time, such as a year, to account for multiple fill-ups and averaging demand.
Once you know how many BTUs were used for heating (i.e. not DWH, etc.), how many HDD there were, etc. you can estimate the number of BTUs needed per HDD to keep the home comfortable. Ergo you can extrapolate how many BTUs are needed on a design day to keep the structure comfortable as well (you just need to know what the number of HDD are on a design day).
At the end of the day, either approach may be more accurate, predicting performance by looking at the atributes of the structure and the outdoors vs. measuring the actual heating demand of the structure to keep it warm.
The main downside to the consumption-based approach is that grossly-oversized equipment will short-cycle during the shoulder months and hence consume a lot more fuel (particularly if it isn't tempered with a buffer tank) than a system that is close to normal. That in turn will predict a higher-than-actual heatloss for the structure.
Nevermind bad piping arrangements, clogged boiler/furnace primary HX's, or other BOP issues that can really cut down on efficiency. Plus, the AFUE measure doesn't even come close to being accurate as far as seasonal efficiency is concerned unless you're using a modulating, condensing heating appliance.
Clear as mud?
Timbo,
Your points about data accuracy are well taken. That is one of the reasons why observing fuel usage during normal patterns of occupancy can be more accurate than a Manual-J calculation, building survey, etc. After all, a Manual-J or whatever other method you're using every day depends on hundreds of data points whose cumulative range of results is huge, while the method I'm looking into uses a total of 6 data points.
As my FEA professor would gleefully exclaim "GIGO!", whenever he showed us another trick as to how FEA programs can show totally wrong results due to limitations in the code.
For example, an extant building survey may not tell you whether a pane of glass has heat-mirror / v-kool / etc. inside it, etc, something that could reduce the heat gain in my home by 46% according to Manual J, 7th Ed.. The only way to know is if the specs or documentation show the exact U-factor, etc. for the glass used, something that may or may not be possible, particularly in residential construction.
Using HDD data, fuel usage, etc. allows me to back out the maximum fuel usage over a given time period. Presumably, the fuel company and the meterological stations keep the errors in this regard to a minimum. The AFUE is another matter and should be replaced, but it's the best we've got until the standards get changed on the heating side to be more aligned with reality as they are on the AC side (i.e. SEER).
At the very least, the errors using the underlying fuel as a starting point are limited, much like options are... the heat loss in BTUs cannot exceed the thermal efficiency of the heating plant in BTUs, while the calculations you deal with are like futures with virtually unlimited error bands in both directions. That your results are accurate speaks to the diligence, experience, and training that the successful practitioners in your trade bring to the table.
I certainly do not advocate this calculation as a replacement to manual-J calculation, it's simply another way to sanity-check the Manual-J results, just as putting a cycle-meter on a boiler would . Arguing that the method doesn't stand up to scrutiny is amusing at best in the light of Lews real-world experience where an allegedly skilled practioner predicted the need for an AC system that was 200% oversized.
Given that 2.5 tons cooled Lews home adequately, any heat gain prediction of 6 tons should have sent up red flags. 'Compromising' on 4 tons is, at best, an imperfect solution. 3 tons probably would have done it, and with superior latent heat removal to boot. Thus, perhaps a simpler approach would have served him/her better in the real world where homeowners cannot afford a licensed HVAC PE to determine just what they need.
Since you're open to trying new things, please do me a favor. Put some real-life data into the spreadsheet and compare the accuracy of the plant-sizing prediction to the manual-J or whatever you calculated, observed, etc. Cheers!
Timbo, it appears that you read whatever you want to read in my posts and ignore the rest. Thus, please re-read my posts without the obvious anger and biases that your responses contain to date. As best as I can tell, you are not approaching this topic with the open mind you claim to possess.
If you read my posts above, I stated quite clearly (and multiple times) that the Manual-J approach is the way to go. I also stated that the approach I did a little spreadsheet around is, merely another way to back-check the Manual-J results in extant structures. Obviously, doing a consumption-based calculation only works in buildings that have a season or two of data to look back onto.
You were the one who brought up the possibilities of cumulative errors piling on top of each other. IIRC, the proper Manual-J heat loss calculation is to do a room-by-room analysis so that follow-on efforts (like duct-sizing via Manual-D, baseboard sizing, etc.) are accurate as well. In most residential projects, that means lots of data points to capture the varying sizes of windows, rooms, orientation, shading, etc. of every component that interacts with the outdoors.
Your commercial projects may be a lot simpler because the variability of rooms, windows, and the like may be smaller. However, I doubt that any of your projects only needs 6 data input points (including design conditions) to calculate a heat loss.
Why you throw out arrogant statements like "I have never discussed this with someone so resistent to the simplicity that practical engineering experience affords some of the rest of us" when I agree with your position that established practices are the best way to determine heat loss and heat gain is beyond me. For whatever reason, I expected better of you. Cheers.
Edited 7/13/2005 11:53 pm ET by Constantin
Lew,
I apologize for hijacking your thread. Hope that all works out.
Be cautious of the "information" presented here by some. Pretending to know something and offering advice is very dangerous, though, those who do so (and there are unfortunately more than a few that post here regularly) cannot be convinced otherwise.
Ahem, it's not OFFERING advice that's dangerous, it's BELIEVING it. Remember, you can't believe half of 90% of what's said here.
Agreed... though it is nice to see Timbo setting a good example by removing some of his posts to improve the signal-to-noise ratio. :-P (just kidding Timbo, I don't claim to know why you deleted all your posts and that's your business)
Anyway, does anyone have a good link to ASHRAE design-day conditions? My Manual-J states 88 degrees on a summer design day, while my contractor is designing to 96 degrees. This is in the Boston area. I know that global warming is an issue, but I doubt that the average local design-day temperature has jumped that much since manual-J 7th ed. was released.
Edited 7/14/2005 4:20 pm ET by Constantin
You can't blame the believers, although a healthy dose of skepticism is advised. I am guilty of getting drawn into a discussion about which I had familiar but incomplete knowledge and proceeded to spew forth garbarge I had no right to. Wethead Warrier, as I recall, curtly and aptly put me in my place. Since, I have tried to offer only what I truely know enough about to do so with confidence that I am not going beyond my field of real actual experience and knowledge (something, BTW, you may wish to work on yourself ;>). I would hope that more of the actual experts on the forum take it upon themselves to challenge the bullshooters a little more often. There was a thread recently, in which some moron stated that units with TXV's removed more moisture than those that didn't have TXV's. I played dumb and asked some whys's and how's. I was amazed at the BS that was put forth by several people, who "knew" so much. Some were so clueless as to describe the complete opposite of what actually occurs which such a device.
Two knowledgeable craftsmen/technicians/experts will often disagree on the details and approaches, philosphies, etc., however, that is not the case in recent discussions here. I deleted my many posts on this thread because, except for first one, they did not pertain to the subject at hand and therefore, did not belong.
Often under cooler outdoor conditions, especially if warm & humid inside, a fixed orifice system can have very high superheat and be properly charged. Superheats upwards of 30 degrees can be normal these days. If you have 45 degree entering refrigerant, that means that part to 1/2 of your coil could well be above the dewpoint and not dehumidifying. But the same system with a TXV will run around 15 degrees superheat and have a much colder coil. That's a more efficient use of refrigerant.It is possible that I am a moron as you claim. However, I do not think that my statements concerning TXVs is proof of that.You are entitled to your opinion, but please keep the name calling out of it.
Edited 7/16/2005 8:56 pm ET by photography on the run
"It is possible that I am a moron as you claim."
I'm not going to speculate on this one, however, I specifically did not mention any one person, because there were many posters of incorrect information. If you took this to apply specifically to you, then I apologize and will agree that addition of the word "moron" was not appropriate, whether applicable or not.
A simple, blanket statement such as: "A system with a txv will remove more moisture than one without a txv." is incorrect and inaccurate. Sure, you can pose some unlikely or uncommon scenario (as you have; cool outside & warm/humid inside) where it might be true. In the event the above conditions occured in a residence, most bright souls would turn off the AC and open the windows. I can describe many more scenarios in which the statement is not true (like usual design condtions in most of the country). Describing the system operation with a TXV in terms of superheat, in lieu of subcooling, further leads me to believe that you may not possess the depth knowledge necessary to accurately dispense advice on this subject.
Edited 7/18/2005 10:19 am ET by Timbo
Allergies is my reason to run the A/C and leave the windows closed when it is cool outside and warm/humid inside. External noise or precipitation might also be good reasons.
The information I found on TXV vs humidity came from an engineer in the HVAC industry. I am not an expert in airconditioning systems, but I do understand the terms superheat and subcool enough to know what they mean.
In my discussions with the HVAC engineer and other HVAC experts, nobody came up with a scenario where the TXV system would remove less humidity than a fixed orifice system.
I am on this forum to learn. Could you please describe a situation where the fixed orifice system would be better for humidity control?
photography on the run, I think the main issue here is that Timbo is quick to criticize anyone who dares enter his domain of HVAC without the requisite 18 years of information, an HVAC engineering degree, etc. I don't claim to know nearly as much as he does about the subject, though I find it revealing that he focuses in his posts less on explaining his positions/theories/knowledge/whatever and more on attacking anyone who even minutely dissents with him as being unqualified to dispense any kind of advice on the subject. Whatever.Furthermore, there has be some reason that all high-SEER systems I have seen specs for use TXVs as a means of metering the evaporator coils. It is possible that the TXV is there simply because of some regulatory reason, but I doubt it. Rather, I suspect that the variable metering that a TXV allows in an evaporator will allow it to better respond to a wide range of loads, environmental conditions, etc. and hence boost the SEER rating of the system in an economical fashion. If you want more professional responses to your questions, I suggest you check out Dr. Silbersteins forum on the Wall at Heatinghelp.com. He's genuinely helpful, does not carry a big chip on his shoulder, and may give you some very helpful insights.
"I think the main issue here is that Timbo is quick to criticize anyone who dares enter his domain of HVAC without the requisite 18 years of information.." And this is where you miss the point. A beginner learning the HVAC business, though, has the ability to see the flaw in your argument.
I criticize anyone who pretends to be an expert on a subject in which they are not. I would not say that I am quick to do so because I have read your BS for several months before waving the BS flag. You seem offended that you fall in this category. Your choice, my observation. You chose to "quibble" and offered nothing but speculation to dispute with. You have yet to make a viable argument on sizing and selection of equipment based on historical climate data avarages, which you claim is much easier and more accurate than engineering calcuations (notice I did not mention the Manual J, which is not what I meant).
All subjects are learned by asking and exploring and experiencing and making mistakes. I've made plenty. Anyone who can challenge knowledge (mine or anyone else's) based on actual experience and real knowledge, facts, test reslts, studies, or some concrete or at leats presentable form will not be criticized by me. Unless we are simpley having a communication problem, you have not done so. If the data you present is skimpy, incomplete or unverifyable, that will be challenged as well.
Edited 7/18/2005 6:30 pm ET by Timbo
Like I said, Timbo, you read whatever you want to read, and ignore the rest. I don't know how many times I have re-iterated it, but the current accepted design approach of using Manual-J or whatever (naturally superior) engineering studies you prefer to size equipment loads, is unquestioningly better than using sizer.xls to determine a heat loss.
Furthermore, I've previously put down for the record that I am not an 'expert' in HVAC systems as you claim I have. Go through my posts... I've never claimed to be anything more than an interested homeowner in any forum that I participate on. Your accusations are simply ludicrous. So get over it and don't make me into something I'm not.
With the sizer.xls spreadsheet, I simply explored an alternative approach to calculating the heating load of a house using fuel consumption, design-day conditions, and a few other measures. That there may be a flaw in these calculations is almost a certainty given the many factors that go into actual fuel consumption and the few variables that I am using. These issues I wanted to explore, not defend myself from character-assassination attempts.
You've previously claimed (now deleted) that you have an open mind, yet by your own admission never explored sizer.xls other than to download it, open it once, and declare it invalid because it ran counter to what you expect a proper approach to be. Brilliant.
So let me re-iterate: The sizer.xls file is NOT intended to replace a proper heat loss calculation. It's simply an idea that I am exploring for the sheer fun of it. There are several issues with the spreadsheet that I can think of from the top of my head, yet you haven't raised one of them.
This situation is all the more unfortunate given that you have a deep set of knowledge, vast experience, etc. that I respect a lot. You probably forget more about HVAC in a day than I'll ever learn. However, your personality seems to get in the way of sharing your knowledge and experience in a friendly and professional manner, which is a shame.
If you decide to make some constructive criticisms in a professional manner I'll be persuaded to reply. All the best in the meantime.
I apologize for my shortcomings and bad manners. I have given the sizer more than a cursury review and will find the time to give a rigorous trial. When I do so, I look forward to discussing it more completely with you. I know that my approach is at times "abrupt". A point of personal improvement.
I look forward to your thoughts. Perhaps a new thread would be the appropriate place to revisit the issue. Cheers.
"Could you please describe a situation where the fixed orifice system would be better for humidity control?" First, you need to be clear on the words. I said more effective at removing moisture, not humidity control. The two go hand in hand, but are separate properties of a system. One is the ability of a system to cool air beyond its dew point and condese moisture out of the air stream. The other is a more complicated control issue, beyond the scope of this discussion.
Any time there is less than full 100% design load, an orifice-based system is operating at a colder evaporator coil temperature and, therefore, removing more moisture than a txv system. I am making the following assumptions: that both systems pass the same mass of refrigerant at full load conditions, were discussion R-22 based systems, that the systems are sized such that they can meet the design condtions load and that all other aspects of the systems are the same.
The use of TXV's is to improve the SEER and not the moisture removal capacity. Many of the changes to refrigerant systems over the years have been geared toward meeting higher SEER ratings, and some actually have a detrimental effect on the performance of the system. Performance in an environment in which dehumidification is more imprtant than sensible cooling can be measured (or at least a good indicator of the ability to perform and careful system selection) is evaporator coil discharge temperatures. 55 degF is nominal, 54 is good and 53 is better. One change that increases the appearant efficiency (and SEER is an appearant efficiency) is increasing the head pressure, which, by virture of thermodyanamics and the properties of the refrigerant, increases the heat of rejection and make the system more thermodynamically efficiency. How do increase the head pressure/temperature? An adjustable restriction - Viola! - a txv.
The reason I used superheat and subcooling as point of issue is because of the way the two different types of systems are set up and adjusted. Under given load conditions, the amount of refrigerant is adjusted such that the evaporator coil "boils" all of the semi-liquid refrigerant passing through it, to a degree that that testing has shown gives the most satifactory results and that the system is design for. In an orifice based system, this done based on the back pressure/suction temperature. In a txv based system, since the txv manipulates the back pressure based on appearant load, the system charge has to be adjusted based on the liquid line (high side) pressure/temperature, which is (or should) in the subcooled region. Telling me the superheat values in a txv system indicates a lack in understanding of the operation of the txv.
When the back pressure (and the temperature of the refrigerant vapor as it leaves the evaporator coil) drops under a decreasing load, the txv begins to close, restricting the flow of refrigerant. Less refrigerant to evaporate, less energy removed per pound of air (the flow of air in the system is for all intents and purposes, a constant), warmer coil, less moisture removed. This is based on a mass and energy balance.
The only benefit, that I can see as far as moisture removal is concerned, is that as the refrigerant flow is restricted and capacity of the system decreased, an oversized system will tend to cycle less and even with a warmer coil, an operating unit removes more moisture than one that is not operating. These, by the way, are all points made during the last time we discussed this.
The situation in in a txv restricted system will operate at lower evap coil temperature is a mystery to me. I don't believe that one exists, even the one you described earlier. I would like to hear some of the cycle details on what the refrigerant is doing in that situation.
Edited 7/18/2005 6:29 pm ET by Timbo
Thank you - An intelligent answer - THAT is what I was looking for.Information I gleaned from people that are supposed to know HVAC systems well.
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All other considerations being the same, a TXV metered system will remove humidity better by keeping the evaporator coil fully flooded. Being fully flooded the coil maintains being colder over a fuller range of the coil surface. The main point is that more of the coil surface is below dew point temperature for a longer period of operational time.
This is where we disagree. I believe that a txv does not keep the coil flooded, but rather does the opposite.
It increasingly restricts the flow of freon into the coil as the air passing across the coil becomes increasingly lower in temperature to prevents the coil from flooding and becoming too low in temperature. The restrcition lowers the mass upon which the compressor does work and decreases the energy consumed. Less energy consumed, and at the higher back pressure and higher temperatures, overall, increases the thermodynamic efficiency. To my understanding, this is the sole purpose of the TXV. Ask the folks you know to comment on my understanding, if they are willing. I would like to hear the reasoning behind their/your understanding.
I will have to dredge up some compelling info on this to hopefully confirm what I believe to be correct or learn a new lesson.
Edited 7/18/2005 10:41 pm ET by Timbo
I am probably into an area where my ignorance will really show now.If the outside temperature is fairly low - such as during a rain storm, and the load on the A/C system is low, then the pressure coming out of the condenser coil is going to be much lower than it is during full load conditions. It seems to me, that with a fixed orifice system, the amount of freon going through the evaporator coil is very dependent on the pressure differential across the fixed orifice. So, it appears to me that it is quite possible for the amount of freon going through the fixed orifice to drop to perhaps only 1/3 the amount under full load conditions. The TXV is not affected as much by the pressure drop and lets more freon through under these conditions than a fixed orifice system would, thus maintaining a colder coil and removing more moisture.I am outside my area of expertise here but it sounds good to me anyway.
I was hoping that you might be able to get some real details about the performance of a system as described. You said that this was described to you by an "HVAC Engineer", as I recall. They (or you) should be able to provide some supporting performance conditions and/or performance data for this phenomena of TXV's, if it is true. What you have provided, speculation and exaggeration, does not make your position too convincing, even though it does sound good. I'd love to discuss the point further, but it must be on technical details. I can't debate what seems or appears to be (well, I can but I won't). Quiz you freind(s) and find some details.
So toss out 45% of what is said? :)
Or maybe 55% -- I didn't say anything about the other 10%.
Getting back to the OP's problem: If indeed you're stuck with an oversized unit you might as well make a little lemonade.
Check with your power co to see if they have a "Summer Saver Switch" or some such. This is a gizmo that goes on the outside AC unit and allows the power co to switch the AC off about 15 minutes at a time, in order to handle peak loads.
With a "properly sized" unit, when they're switching it off 30-50% of the time on a hot day then the AC can't keep up and your house gets warmer. But if the AC is oversized you'll be able to keep up -- and still qualify for typically a 15% discount on your summer electric bills.