In a general sense, which uses less energy – turning off the AC when no one is home all day or raising the thermostat to say 78 deg? When I shut it off for the day the house goes up to 85 deg and it takes 4 or 5 hours to bring it down to 73. When I let the program run, it takes maybe one hour to lower it from 75 to 73.
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Replies
You just answered your self.
Not necessarily since you don't know how often it cycled during the day while he was gone.
Just because it only ran 1 hour when he got back doesn't mean that it didn't run half the time while he was out for the day.
Say it takes 5 hours to cool the house back down after being off all day vs 1 hour when left @ 78 degrees.
If the air conditioner ran 4 out of the eight or nine hours he was gone, then nothing was gained (not to mention the extra draw for all the times it had to restart during the day).
The only way to answer for sure would be to time the total amount it runs during the day when left on & see if it is more or less than the time it runs to cool it down after being off.
Or measure the meter readings in the two different scenarios.
True. Why didn't I think of that?
Either way, both days would have to be similar in terms of outdoor temperature, sunshine for it to be reasonably scientific.
Some programmable thermostats have a monitoring ability. You can get the run time from them under the two different options.Utility companies tend to say the set back version is a lower cost. Not sure that I would always believe them, but I've certainly heard that from a number of people.
Its a moot point if hes married .
Your are on it Mooney. It's going to cost either way and I'm betting the domestic situation will be more costly in the end!
Here in Kansas many homes have basements. The basements stay pretty cool during the summer - I don't know maybe 10 - 15 degrees cooler than the part of the house which is above ground.Many homes have the furnace and ductwork in the basement and the duct work stays pretty cool.Some HVAC technicians recommend leaving the FAN turned to the "ON" position 24 hours a day as the cool air runs through the ductwork and it works to cool the house even when the compressor is not running.^^^^^^
S N A F U (Situation Normal: All Fouled Up)
Ok, Ill work with ya .
Ive kept jobs heated in the winter with a salamander on thermostat.
This is THE extent of my knowledge on the subject. <G>
Ive tried it many times both ways. A GC brought my attention to it .
We burned much less fuel setting the thermostat at a set temp through the week than turning it off at night . Much less.
I dont remember trying turning it down 5 to 10 degrees because he was paying for it and willing to do it , so I always did it and charged for the fuel.
I just know its cheaper than turning it off and much more labor effective.
Tim
takes a lot of effort to achieve thermal mass... minimal to maintain it...Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!<!----><!---->
Forget the primal scream, just ROAR!!!
takes a lot of effort to achieve thermal mass... minimal to maintain it...
Yes, and it also depends on what you are doing I guess .
If I was hanging drywall then , no way as I was trying to heat the out side .
I would normally tape all the ceilings and then "set " the thermostat . After the job was fully taped then the fuel went way down.
Surfaces that are warmed dry better and I would crack the windows in the day time to let moisture out . A competitor would pull the heat and open all the windows and doors in freezing weather and did well with drying . He swore it would not freeze unless it was single digits. He claimed he finished a house in less time and he probably did in wet weather but he went through a lot of fuel he did admit . Lots more according to our shared notes .
Painting is a no brainer as it just sits on a cold wall and condences as you add heat.
The other trades wouldnt matter till you got to floor covering . It has to be prewarmed and constant as the floor covering it self has to reach room temp.
Tim
cool ..
that post used less words ... and bigger words ...
to say what I saw gonna say.
plus ... yers sounded more official than what I had planned.
Jeff Buck Construction
Artistry In Carpentry
Pittsburgh Pa
thanks...Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!<!----><!---->
Forget the primal scream, just ROAR!!!
I agree. Plus what an incredible work out it would be on the compressor to go from 85 to 73.
Not only that but your air conditioner takes the moistier out of the air.
Seems to me that constant running for several hours would be easier on the compressor than starting & stopping all the time.
Highway vs city driving?
Thats a fallacious analogy. It would be more appropriate to compare it to your pneumatic nailor. Would you unplug your compressor shoot a thousand nails then make the compressor play catch up? Or would you leave the compressor plugged in and let is gradually cycle on and off as needed?
Thats a fallacious analogy.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
I certainly would unplug my compressor (and drain it actually) If I wasn't going to be using it for 9-10 hours.
I know the car analogy isn't perfect, but I was merely making the point that driving 100 miles in one shot w/o stopping uses less fuel than if youwere to drive it in 10 mile intervals given that you have more fuel is burned starting & stopping the engine than if it were to run constant.
Back to the heating & cooling issue:
Regardless of how well sealed & insulated your home is, there will be a non-zero gain in heat over the course of the day.
If you are maintaining your house at 75 degrees while it is 95 outside, there will be MORE heat gain over the course of the day then if the temperature were to rise to 85 or more (since you are closer to equilibrium at that point).
It's also worth noting that in general, during the week, you will be home during the (relatively) cooler parts of the day, so the unit isn't competing with as great a temperature difference.
All that said, there are tradeoffs. If it is not comfortable when you come home and it takes to long for it to cool down, then it would seem more prudent (in my mind) to get a programable thermostat & have it setback to 80 during the day & change to 75 (or whatever) say 2 hours before you come home rather than turning it off completely during the day. Yes it costs a bit more, but as was stated by another poster, if you aren't comfortable, then why have air at all?
Edited 7/20/2006 1:22 pm ET by Soultrain
One important factor is that a significant amount of energy is required to start an electric motor. It's the start and stop, start, stop, start, stop, start, stop, start, stop, start, stop, start, stop, throughout the day that adds additional expense.^^^^^^
S N A F U (Situation Normal: All Fouled Up)
That may be true for heating where the only mechanism working against you is temp difference between inside air and outside air. With cooling you of course have the inside vs outside air temp difference to contend with, but on top of that is the more powerful effect of direct heating from the sun's rays (unless the ouse is completely shaded of course). BruceT
you can't ignore the thermal mass in heating or cooling..
that has a major position on recovery time and the amout of work that the unit has to do to get there..Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!<!----><!---->
Forget the primal scream, just ROAR!!!
I'd just point out that the thermal mass includes the building envelope and every dish, appliance, article of clothing, chair, table, etc in the house. The air is the smallest part of that, but cannot be cooled for longer cycles until the other items in the house have gotten close to the desired temp. Because of that, I like to just raise the thermostat a bit to find a happy medium between outside and inside temps during the day.
I like to start cooling in time for the air to be comfortable when I am home. If you start the cooling process while the sun is blasting the house, it will take longer but may still be comfortable because there will be dryer air blowing through the house as it cools. This will cool the skin. Once the desired temp is reached and the air shuts off it can be less comfortable than when it is warmer and the fan is blowing. Therefore, I like to have ceiling fans running, as well as several other fans to keep hot spots from developing. The AC fan can be placed on manual to help in this process.
Sometimes you just have to sit barefoot in your running shorts on a cool leather chair while sipping an ice cold brew.
Sounds to me like you've got the outline of a perfect science experiment. Read the meter on weekend and set it up one way and after
a week read the meter again. Then, set the system up the other way and read the meter again the following weekend. After two weeks then compare the two methods and then tell us your results.
My system is already GREEN! I don't have any air conditioning and my fan runs cheap. Even the dog likes to lay in front of it.
My guess is that it will take more power to maintain 75deg all day than to cool the house to 75 at the end of the day. The reason is insolation.
No, that's not a misspelling. Insolation is the heating of a house (or a car for that matter) by the sun's direct rays, especially through windows. It is a lot easier to cool a space at night, or at least after the sun drops low in the sky, because then you are only contending with the difference between outside and inside air temps. Conversely, it is easier to heat a house on a sunny cold day than on a cloudy cold day or at night because then insolation works FOR you. You feel the effect of insolation very directly when you step out of the shade into sunlight.
It sounds as if you may have little choice if it takes 5 hours to cool the place down after you get home from work. You deserve a little comfort after a long hot day, don't you? A timer/thermostat could help by starting the cooling process before you get home but after the peak temperature period of the day.
BruceT
No brainer. Turning it off uses less energy, of course.
Let's say it's 90F ouside. If the inside is at 85F, there is less heat loss (transfer) into the house than if it is at 78F.
DG/Builder
Turning it off uses less energy. A setback thermostat can be used to start the recovery early to get temps back to where you want them before the house is occupied.
For me, the additional cost of keeping the AC running all day isn't the only consideration. There's also the comfort issue.
Leaving it running all day (even if at a slightly higher temp) keeps the humidity under control much better than trying to lower the temp all at once. In fact, without the humdity under control, the temp needs to be lower to be comfortable, so it might be more expensive tu use the setback feature.
And besides, I don't want to come home and hear and feel the AC running for hours.
-Don
And besides, I don't want to come home and hear and feel the AC running for hours.
Theres really somthing to that . Its like hearing a daughter empty a 50 gallon hot water heater in the shower . If you didnt hear it you would be fine at least till you paid the bill. BUT nearly all that time the water is running you could chew nails .
Tim
There are a few issues here.
The greater the Delta T (temperature difference) the more heat gain you will get. Turning the AC off will save energy on this issue alone.
The temperature of the outside air affects the efficiency of the condensisng unit. Try not to run it during the hottest part of the day. Also try not to run it when the condensing unit is in the sun. This issue is dependant on your schedule.
I have mine set to set up the temperature at 11:30 AM which is just before the heat of the day and just before the sun gets around to that side of the house. It comes back on at around 9 PM as things are cooling down and the sun is gone. This works for us because our house is so efficient that on a 100 degree day the temlp only rises about 2.5 degrees. You would just need to figure out what works best for you as far as avoiding running the unit in the hottest part of the day. It also helps with peak demand at you electric utility, so you'll be doing your part to avoid power plant construction and their associated costs.
On the issue of moisture, it is much better to have longer run times. Once the coil is loaded with water, it is best to let it run. Each time it shuts off, it will evaporate some of the moisture back into the living space. For this reason you should never leave the fan set to on during the cooling season. This practice will evaporate all the moisture left on the coil at the end of a call for cooling, back into the space. That is very inefficient and will lead to higher interior moisture levels.
I hope this helps to clear up the issue.
I would take issue with your statement about leaving the fan running. The amount of water left on a properly drained evaporator is fairly inconsequental, and the re-evaporation of it will produce cooling, offsetting to a degree the cost of recondensing it.I'd say run the fan or not, based on personal preference, though it doesn't make sense to run the fan when the house is unoccupied.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
A loaded coil will contain around a pint of water depending on the size of the coil. This represents a substantial latent load if re-evaporated. Allowing the fan to be off for at least 20 minutes will allow it to drain off as much as possible and reduce the amount of moisture that is reintroduced to the interior.
But if you've just had a reasonably long AC run time you've drained off gallons. A pint isn't going to make much difference.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
"..around a pint of water depending on the size of the coil. This represents a substantial latent load if re-evaporated.."
In real numbers, a pint of water at 60 degF takes 1100 btu to evaporate. This is the equivalent of a 100w bulb burning for 3 hours, or the amount of latent and sensible load that three people at rest would generate over the course of 45 minutes.
Let's run some numbers on this.
When vapor is condensed into liquid, the reorganization of the molecules into a liquid requires additional energy that we call latent heat. The amount is 1000 btu per pound of water. One pint of water weighs 1.042 pounds so will require the removal of 1042 btus in order to condense that much liquid. I will make a few assumptions to try to model a typical home.
1042 btus condensed by a 13 SEER air conditioner will use 80 watt hours. If you pay 10 cents per KWh, then the cost will be .8 cents for every pint of water removed. If your AC unit cycles on twice per hour, that equates to $11.52 per month. If your AC fan uses an average of 300 watts to operate(typical) and the unit runs 50% of the time for cooling demand then the additional run time for the fan if set to "on" will be 360 hours at 300 watts at 10 cents per KWh, or a total of $10.80 per month.
There will be a cooling effect caused by evaporating the water off the coil and a heating effect generated by running the motor. Continually recondensing that moisture will cost money but will also have other unintended consequences depending on your building and climate.
If your fan coil unit and duct work are in a vented attic, then they are outside the building's air pressure boundary. The net supply side leakage(substantial) will create a negative pressure in the building that will draw outside air into the building. This will add heat, moisture, dust and pollen to the interior space depending on the outdoor environment. This will be a load penalty that can easily add 10% to your cooling bill. This is an unfiltered air exchange that is generated by the fan running.
In summary, there are upsides, downsides, and unintended consequences to running the air handler fan continuously. The upsides are a movement of air that may add to a feeling of coolness by evaporating the sweat on your skin, and a continual mixing of air that will tend to minimize warm areas from developing. The downsides are the cost and elevated moisture levels. The unintended consequences are the addition of moisture, dust and pollen to elevated levels in the home due to the creation of negative pressures caused by placing equipment in a vented attic.
Your own circumstances may vary in many ways.
> Your own circumstances may vary in many ways.Right. In our case all the ductwork is inside the conditioned space, and we have a DC fan motor that uses maybe 50 watts on low speed (don't know exactly as it's too low to measure with my clamp-on ammeter).In addition, the absorption of heat through evaporation reduces the cycling rate of the AC, effectively reducing your $11.52/month estimate to maybe half that.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
Based on your numbers, gaining over 10 degrees during the course of 10 - 12 hours, I have to believe that your house is not very well shaded, sealed or insulated, probably all of the above. I leave my house at 6:30 in the morning, 73 deg. When I get home at about 6:00 pm, it my be 76 or 77 unless the outside air gets over 95. If it gets that hot, my set back at 78 keeps the house from getting any hotter. At that point, I don't care about the very small amount of extra electricity I buy, I want the comfort.
Speculating, based on the comparison, you will save electricity by turning it off while your gone. I can't recover 12 degrees in 4 hours, at least not while the sun is up., but then my house is very well insulated and sealed, and well shaded. I also have what most would consider a severly undersized system at 1250 sf/ton. The hottest time of the day is usually at 5 pm, probably about the time you get home, which is the least effective time for your condenser to run. Long, as in several hours continuously, run times are great for dehumidifying.
IF you could compare the two operating schemes from one identical month to anther, the difference would be minor. Probably less than $20 on the electric bill. If that makes a difference to you, go for it.
Edited 7/19/2006 8:58 am by Tim
It's unclear to me whether you have an old mercury thermostat or a digital one.
I recommend a digital one and if no one is home during the day, set it at 85 or 90, and have it kick in a half hour before you generally arrive. This has been efficient in my experience.
Why have AC if you come home to an 85 degree house every day, and have to wait 4 hours for it to become comfortable?
AC is for comfort....set the thermostat up a bit during the day, but keeping it on will keep the humidity constant, and you will come home to a more comfortable home.
Personally, we only use ac for about 3 1/2 months here, and I could care less about saving even $100 per month if it means I have to be hot every day.
I have about 2,500 sf of space in an 1850's Greek Rev farmhouse. I use four 6400btu units in bdrms, and two 12,400 btu units on first floor. Bdrm units always on...74 all day, 71 at night. First floor units at 71 from 9am to 11 pm, off at night. My DW likes it this way (remember the "comfort factor".....if she gets too "hot"....my life is not any more comfortable)
AC adds about $200/month to my elec bill, thats about $7/day.....less than lunch or a good six-pack.....all are cool and happy, life is good.
Thanks to everyone for repsonding, the only clear answer I could glean was, it probably doesn't make that much of a difference, other than imediate comfort level... House is 1962 rancher, ~R 35-40 in the attic, double pane, but not low-E windows on a southern exposure- windows shaded during the day- located in central MD. Thermostat is programable, but someone returns home at different times every day, so I haven't been using it to it's full potential. Usually doesn't go up as high as 85, but in this heat wave.... My question arose from a debate with the DW, who knows next to nothing about anything hosue related, but occaisionally pulls a truthful fact out of never never land. She was certain thta it was better to leave it running all day at a higher setting. I thought I had read that it was better to shut it off and cool it down in one cycle instead of cycleing throughout the day.
Don't know if anyone else has brought this up, but i'll throw it out. Another factor, not to mention pets or plants, are any sensitive electronics in the house (i.e., your computer, backup batteries, etc.). A computer generates a lot of heat just by itself and if you turn off the ac during the day it gets even hotter. 90 degrees inside the house could muck up some expensive hardware. And people tend to put their computers in out-of-the-way areas where ventilation is bad anyway.We've lost one computer due to a bad fan. And i've heard the saying that computers like the same temperatures people do. If you decide to leave the ac off as your option, you might want to make sure it's not going to interfere with anything else during the day.
To really save energy the answer is both.Given that the A/C system was not designed (sized) for your current heat wave you need to take an active role in maintaing the comfort level in your home. Your system was probably designed around an outside temp of 85 degrees and 50% humidity and an indoor temp of 75 degrees and 25% humidity (just a guess) - to size the system to cool the house during your current heat wave would have installed a grossly oversized system that would have short cycled and had all sorts of problems when the temps were lower.That being said - you need to pay attention to the weather reports. When it's going to be super hot outside, set the system lower during the day. When the temp are more moderate, raise the setback temps up a few degrees. As someone mentioned, thermal mass is a big issue. If you let the house "heat soak" during the day, AND it's outside of the A/C's design parameters, AND the sun is beating on the house, you will experience just what you are going thru - long cooling cycle run times.Take a more active role in the A/C operation and I think you will be far more comfortable at a lower cost.
A related question.
I have heard and read more than once that a slightly undersized compressor is actually more efficient for cooling. Is there any truth to this?
I rarely ever use A/C so it is not a big issue for me. I recently expanded my house though, but kept the old compressor for now.
I have heard and read more than once that a slightly undersized compressor is actually more efficient for cooling.
TRUE.
A slightly smaller unit will "run" for longer periods of time giving you better humidity control, and longer periods of air movement.
Conversely, (and this happens all too often......lots of people think that if 2.5 tons is good, then 3.5 must be better) an oversized unit will cool the "air temp" down really quickly and then cycle off, stopping the air movement, and not running long enough to extract the humidity from the inside air.
Thanks, that is exactly how it was explained to me, though the HVAC contractor did want to sell me a larger unit, I think my present one is 2.5 tons BTW. As I mentioned I rarely use it, and if I do I would use it only to take the edge off the scorcher days, not make the house as cool as a fridge like too many do.
I have a friend who's house would not cool down below 80 despite running the system non-stop.He called the a/c technician and the tech cleaned the outside a/c unit with a garden hose and the house then cooled down to 75 degrees and was comfortable.The outside a/c unit collects dust, hair, lint, etc and needs to be cleaned more often than most people realize. I think mine needs cleaned every 30 days.^^^^^^
S N A F U (Situation Normal: All Fouled Up)
"that a slightly undersized compressor is actually more efficient for cooling. Is there any truth to this?"
I'm going to nitpick on Pickings answer a little bit here. The answer, in the strictist sense is NO. The pysical characteristics of the system (size of the coils in relation to air flows) more than anything else determine the efficiency of the system. That is how much work it does, for a given anount of energy input. So now I'll take off my ME hat and answer your question
The answer to the question, in a practical sense, is YES. The reasons being are that a smaller sized system will meet the load more often than not, will run longer (and therefore dehumidify the air more), have less starts and stops, therefore last longer, and so will be much more effective because all of these characteristics. Additionally, the smaller system will cost less up front to buy and cost less to operate on a daily basis. If you were to compare the feeling of comfort of 78 degrees F at 30% relative humidity versus 72 degress F at 55% rh, you would find them very comparable. That is the approximate difference an extra 1/2 ton will have on a typical house.
The answer to the question, in a practical sense, is YES. The reasons being are that a smaller sized system will meet the load more often than not, will run longer (and therefore dehumidify the air more), have less starts and stops, therefore last longer, and so will be much more effective because all of these characteristics.
Hey.......that's what I said.
How are you going to get an AC coil to dehumidify to 30% rh? Without reheat, how are you even going to get it much below 50%?
Depending on the installation, the conditions, and the construction, use and occupancy, its possible. I seldom achieve less than 40% without over cooling. 320 to 360 cfm/ton helps lower the s/t ratio where below 50% is regularly acheivable, but it depends on the system is loading. 50% and below is good, 40% and below is very good and 30% is excellent dh performance out of a normal split system. In my case, a well sealed, well insulated home, 1250 sf/ton with an evaporator flow rate of 360 cfm/ton and a deep setback to get the run times up, gets me below 50% at 75 degF. On dryer, hotter days, it gets below 40% with 12 to 16 hours run time. This is when db temps exceed design day numbers and wb temps are well below.
What is the deltaT across your coil?
It changes as the air dries. At the start of coming out of setback with a high latent load, I see an initial 20 degrees across the evaporator. As the air dries, I get a greater drop. Starting at 80/67 it'll take a while (several hours) to get the discharge down to below 55. Its been a few years since I measured and monitored everything closely, some I'm trusting an aging memory, but when I have 75/60 entering the coil, I'll have 52 or leaving, and shortly thereafter shut down, having reach the temperature I'm comfortable with.
I have a variable speed furnace, and with the flip of a dip switch or two, can set the air flow up or down a little. When I care to mess with it, I'll slow it down in the spring and fall when dry bulbs drop and the wet bulbs rise. I have slowed it down to the point of frosting the coil, once.
A 23 degree delta is really high. Anything over a 20 degree delta is really going to do a good job of wringing the moisture out of the air. The problem I see with your numbers is that a 23 degree delta only results in 45% relative humidity in the conditioned space. 40% rh requires a 26 degree delta and 35% rh requires a delta of 30 degrees across the coil. Can you explain how my calculations are off? I'm not an ME but I do have a psychrometric chart handy at all times.
Perhaps you are reading the rh from a faulty hygrometer. I'd love to learn a new trick for removeing moisture efficiently for the interior space.
I'm also very impressed with 1250 sq ft per ton of AC but I haven't been able to do it here in TX. My home is around 800 ft per ton and I've designed at 1100 feet per ton but if I can pull it off at 1250 in a home that average Americans can be comfortable in here in TX I'm going to retire on the royalties. I'm working hard to do just that.
Respectfully,
Ray
I didn't say that my system acheives 30% rh, BTW, I was using numbers to demonstrate to another person the relative comfort that people experience at higher dry bulb temperatures with lower coincident humidity. Years ago (2002) when I set up and balanced this system, I could have quoted you chapter and verse on the performance. 23 degress is high, and during some "experimentation" I froze the coil at least once. After some initial tweaking, I moved on to other items of interest.
Now, as far as your calculations are concerned on the psych chart, manually it is an iterative process. As the conditions in the space dry, and no moisture is being added, the the entering air conditions continually have to be revised. This is done assuming a given load on the coil. To run the numbers on my system, you have to make a lot of guesses. Theoretically, though the real life charateristics of the refrigerant and the coils will be the limiting factor, in a closed environment, you could remove moisture completely. Dehumidiying, like any other inside air conditioning endeavor, is greatly improved with superior envelope sealing and insulation. During the reconstruction of my home, we went to great lengths to seal it up very well.
My "meter" is a cheapo, and I acheive only high 40's in the summer. and only then at the higher db's. I realize the the accuracy is not quite "scientific". Today, we are expecting 100 degF db, with a 71 degF dewpoint. I doubt if I'll be below 50% rh this afternoon, though I bypassed the setback.
I do, however, with heavy shade, double exterior walls & R-50 in the ceilings, have a 4 ton system cooling 5000 sf. Designed for my ASHRAE 0.4% cooling db of 91.1 degF. On 100 degree days, it runs 20 plus hours. I would not expect that my northern Illinois design to be compatable with any portion of Texas, certainly not in the Hill country.
In theory, with no moisture added, you'd eventually get down to where the dewpoint was only a hair higher than the surface temp of the coils.Seems to me the achievable RH in a "real" situation depends roughly on the ratio of heat load vs moisture load. You can hold the temp at 65 in a locker room and still have 80% humidity, while in a tightly-sealed but poorly insulated, unoccupied (no breathing/sweating) house you might achieve 30% on a hot day.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
In terms of calculating the resultant relative humidity from a cooling/dehumidifying system, requires just that: the ratio of the sensible cooling load to the total of the sensible and latent cooling loads.
This is referred to as the S/T ratio. When you're plotting the perfomance of a system in a specified space, in which have calculated the S/T, and you determine the LAT from the evaporator (or cooling coil in a chilled watre system), the S/T is the slope of the line that when drawn across the chart will intersect the design indoor dry bulb temperature line at the humidity level that will occur, if your calculation is correct.
In a typical comfort system, with a direct expansion coil (i.e. the standard residential split system), the suction temperature of the refrigerant entering the evaporator coil is around 45 degF. So that, with a coil "effectiveness" factor of about 0.7 to account for the fact that the passing through a coil does not all contact the coil fins, means the best you can see coming off of a properly sized cooling coil is air at 53 to 55 degF dewpoint (55/53). IF the S/T =1.0, that would result in an rh in the space of about 47%. This is why 30% with a residential system, without dry air from another source is not reasonalby acheivable.
Even if not all the air going through the air handler is being cooled to the dewpoint, some is. So long as moisture is dripping off the coil water is being removed. Lacking any input of moisture the RH will continue to drop.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
What Tim said is accurate. You will not in this condition remove moisture below 47%. It's physics at work. When I have more time I'll explain it on a molecular level, or Tim can.
Leaving the molecules for a different day, at best a cold surface (ie. the extended suface of a copper tube with aluminum fins) can do to air is reduce it to a saturated condition at the temperature of the surface. A 45 degF coil cannot remove moisture beyond a 45 degF dewpoint. Once the air reaches the effective dewpoint of the coil, water will cease condensing on it.
When 55 degree air (db) with a 45 degF dewpoint is warmed to room conditions, say 75 degF dry bulb, the rh is 35%. Reality of the physical limitations of economically produced coils, the best you can ever expect to see out of a 2 or 3 row coil in most evaporators is 55/53 (thats dry bulb/wet bulb) which correspons to a dewpoint of 52 degF. Heat that air to 75 degrees and its at abut 47% rh. No more moisture can be removed. OTH, 47% rh at 75 degF is more comfortable than most commercial or residential systems will provide, 75/50% being typical design conditions, which in actual practice are not commonly acheived, 75/60% being more commonplace, at least in the non-desert areas.
Edited 8/1/2006 9:22 am by Tim
What I was saying is that it's not the average exit temp of the air but rather the temperature at the coil surface that determines the lowest achievable dewpoint. I'm guessing that the lowest practical surface temp is around 35F, to safely prevent frost. The air may still exit at 55F because not every molecule of air is equally cooled.Not having the charts available, I don't know what a 35F dewpoint works out to in terms of 75F RH, however.Note I'm not saying that the above-described conditions provide for RAPID moisture removal. Clearly much, much more moisture/hour is removed when the dewpoint is closer to the average exit temp.I'm also not saying that the average AC is set up to produce a 35F surface temp, but since a clogged filter can cause frost-up in some cases, I'm guessing that a lot of systems can produce that temp, at least under conditions of low air flow.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
"and I've designed at 1100 feet per ton but if I can pull it off at 1250 in a home that average Americans can be comfortable in here in TX I'm going to retire on the royalties."
Ray,
It's more a matter of educating the consumer, than the design. If a furnace here, runs 45 minutes/hour on a design heating day (which usually occurs at night) of -15, they think its broke, or not big enough. OTH, I think its as close to perfect as you can get, though no setbacks below zero. Same for cooling, for given design conditions, if you are operating at those design conditions, which are short term, the system short not be able to get ahead. IF you could convince the customer, that when Austin is at or above your ASHRAE 0.4% design of 98.8, which does happen regularly I know (I am originally from Dallas, BTW), the AC should run constantly, then more than half your battle is won.
There is less energy used by turning off your AC when you leave. It is a simple and irrefutable law of physics. The amount of heat transfer into your house is dependent upon the temperature difference between the inside of you house and the outside. When it is 90 outside and 70 inside, much more heat is transfered all day long into your house than if it is 80 inside and of course when the temperatures equilibrate at 90 then there is no heat transfer. The amount of heat transferred is dependent upon the Temperature differential X Time. This is what your AC unit has to work against. So to burn more energy set your AC temp low and leave it there for a long time. Do the check with the electric meter as others have suggested, it will show you the truth in the science. This is true in the cold weather also. So in the summer let your house warm up when your not home and in the winter let it cool off, you will save energy. In spite what seems like eternal running of the AC or heat when you come home.
PS-I agree that if you are married, it is all moot.
That is true in the winter, especially for anything but a heat pump.
In the summer time A/C season, it becomes more complex. Because the efficiency of the A/C system goes down a lot in the hottest part of the day, some houses will be more efficient with the system left on all day, and others will be more efficient with the thermostat turned up. There is no one answer here that works for all houses / climates.
hmj,
It really depends!
(wow isn't that news) <G> My house has a trememndous amount of thermal mass. The energy it takes to heat up or cool down that thermal mass and the high level of insulation I have means it's cheaper to remain constant..
Your results may vary, objects in mrror are clsoer than they appear, close cover before striking,............