5,500 sf home on one 5-ton AC unit?
Streamline
| Posted in Energy, Heating & Insulation on
I am working on a large custom home. At issue is the sizing of the HVAC unit. The house has radiant heating in the floors, so it only needs AC and and air handler (I’m putting in a heat recovery unit also). Here are a few facts about the house:
- 3 floors. Bottom floor is underground. It contains the stairs to lead the garage entrance to the main floor above. The other half of the living space (it its separate room) on this floor is a theatre room which will get infrequent use. It is heated with radiant heat, and I want AC in here also. 1500 sf.
- Second floor is main. Front entry, family room, kitchen, etc… 2000 sf.
- Third floor is bedrooms floors. 2000 sf
After submitting for 5 bids, I get responses from all over the place. Some want to use a single 5-ton AC unit with a single thermostat on the main floor. Others want to use 2 AC units (2nd and 3rd floors). The one I like best is one 5-ton AC unit with thermostat on 3 floors using a damper unit to zone all 3 floors.
How can one set of facts yield such a varied design so far apart? These are all seasoned contractors in the Seattle area. What would you do if this were your home?
Thx.
Replies
First and foremost... Did any of your contractors do a load calculation? Space below grade does not factor into a cooling load, however there is still a need to circulate air and remove humidity. Fancy schmanzy home theater rooms can throw off a lot of heat. Usually a good place for a small dedicated mini-split unit- depending on how seriously you use the space.
I find zoning to be expensive and complicated. Plenty of short duty cycles on the equipment, too.
Just how well do you think a 5 ton air conditioner will work cooling 2000 square feet? That's right, it will cool it off quickly, but the AC unit will never run long enough to pull out the humidity. Now you need to consider two stage condensers and variable speed drive air handlers to make it work right because you are cooling too small of a space with too big equipment.
You will find two smaller systems to be more expensive to install, but it will be much cheaper to operate and much more comfortable.
Five ton residential HVAC systems require attention to detail for proper air movement and performance. Bigger duct, harder to cram it into small mechanical rooms, make too much temptation to make duct that fits instead of duct that works. A few bad fittings at the air handling equipment can turn the whole system into a poor performing and noisy pile of junk. Few want to spend the money there, though because it cuts into the granite and SS appliance budget. Check out a copy of Manual D residential duct design.
Zoning is a something way too complicated that geeky homeowners are attracted to because they saw it on the internet. Or, homeowners that do not want to give up a few square feet in their McMansion for mechanicals want zoning for the few square feet they gain.
A properly designed duct system will work much better. Some contractors think they can shortchange some of the design work because of the zoning. In most instances, it is a bad idea.
I would strongly recommend that you install a furnace anyway as a back up. You save pennies on the equipment by going with an air handler only. You only have to add a gas pipe and a flue. You will appreciate it when the boiler goes down on a weekend and you fire up the furnace to postpone the service call to cheaper regular workweek hours.
As danski said, a load calc should tell all.
That said, a couple of points:
Foolish to put in AC only vs. heat pump in Seattle area. (I live in Renton, son in Kent).
Of course, if the HO of the custom home is happy shelling out $$ a month for gas and really really likes radiant**, OK; but HP during Sept-Nov and mid-Feb to June in Seattle is about 1/2 the heating costs vs. nat gas, plus in Seattle City light area, nearly all power is renewable energy sources! Over the last 3 years, warm enough that that hold true all year.
Examples: my house 5300 sq feet, full basement, 4 T HP, avg winter power heat bill about $120 (PSE, not city light)- could have done with a 3-1/2T unit only. This summer was record highs (99 a couple of days). My house has lots of trees, so the AC only needed to be on a total of about 2 hours on ONLY 2 days!, never needed it for the previous 4 years.
Son's house is 4800 sq ft, has no southern trees, lots of south facing windows, so does use AC whenever it goes over about 85F. Also has full basement . He has a 3 T HP, which is more than sufficient for AC. He has gas backup for winter, has not used the gas heat either for the last 3 years.
My take: Knowing WA state and Seattle energy codes, would be VERY surprised if a good load calculation showed more than 2-1/2 T for AC only were needed, esp with below ground areas!!! Ask to see the load calcs from the bidders, at least get the archys energy code spreadsheets (code requirement for permit) , which should give the same information.
** if a Seattle liberal, why is the hypocrite HO contributing to global warming with CO2 emmisions when HP is hydro in Seattle? <G>. If rare Seattle conservative, obviously one who doesn't care about others <G>
streamline,
You've got to do the load calculations. As others have said..
In addition as someone pointed out your mini climate makes a great deal of differance. I live on lakeshore and have a fair number of trees but my house is superefficent. (R30 walls and R50 ceilings done with SIP's so there is no energy lost due to the conduction of heat thru studs)
I had my A/C on 10 days this year compared to my neighbor who with a similar sized but stick built house kept his A/C running steady for the past three months.. Right next door but he has no overhead trees to provide shade, and greater southern exposure..
I was able to get by nicely with three tiny window units stuck thru the walls.. My house is about 5500 sq.ft. also.. I cooled it off to 67 and the total energy cost was$20.00 more than normal..
My parents house has 4 br on the second floor and we only had AC vents in the ceiling of the second floor -- house was comfortable -- colder air just naturally fell to the first floor. We needed the colder air to sleep -- it was really a nice set up. Now I have a house with ducts all over the place and the AC runs all the time. One thing I have learned is that humidly is really the culprit -- If you had the second floor cooled and a kicking butt dehumidifier in the lower level you might have the best of both worlds.
I'm with the earlier posterr on having a furnace installed -- you just never know when it will be needed -- cheap to run a generator to run the furnace (1/2 hp motor) if needed but a generator for a whole house radiant might be prohibitive.
A generator for whole house radiant would be easy- it is just a boiler and a couple of circulating pumps.
Generator for a heat pump is a different story.
You all have been very helpful. I will review the load calc to see how the 5-ton was derived. All the subs came back with a 5-ton system, and these are reliable HVAC companies around town.
I was swayed towards the 3-zone damper system, but your comments is making me rethink this. One thing I dislike in this is the complexity and mechanical movements in it.
I do have a variable speed AC unit, so it may work slow enough to draw out the moisture. But for a 3-story house, if I don't zone it, I may end up with different temperatures across all 3 floors. Shouldn't I at least 2-zone the house (top 2 floors)? I could regulate the basement at the register?
Thx.
Properly designed and installed ductwork will minimize temperature differences between floors. Zoning systems installed or sold to correct this "problem" are a crutch, not a solution.
Major temperature differences in residential applications come from poorly designed or installed ductwork 100% of the time. The catch is that the original HVAC company installed it within the budget that the builder or homeowner allows, so you get what you pay for.
I have seen lots of questionable, and even crappy, work installed by "reputable" HVAC contractors.
The equipment is normally sized to heat/cool the whole house, but the zoning system only uses a portion of the air you are paying to heat/cool and it dumps the rest somewhere else (the dump zone). Dump zones and barometric relief dampers can be avoided if the system is designed properly from the start.
If you had a commercial medium pressure VAV system or fancoils with chilled/hot water, then zoning will do exactly what you want it to- control each zone as intended. Despite the fancy marketing, residential forced air systems do not work that way.
One possibility is to set up the ductwork for floors 1,2 and 3 and install manual dampers in the main trunks. This will allow for the system to be balanced. The drawback is it will take up more space and cost more money.
Two smaller systems will control the temperature better than any zoning system and it may be competitive (price wise) to do it initially, and it will also be MUCH cheaper to operate two smaller systems than one big one that is zoned into two small systems. Two smaller systems WILL pay for itself and save money in time. Avoid sharing return duct between the two systems.
I know of two almost identical homes. One has two systems, the other has one big system with zoning. The electric bills (cooling) on the two system house are 1/2 of the other one. Same for gas.
A single system with a variable speed blower will allow you to operate the fan continuously at 50% capacity and even out the temperatures without costing an arm and a leg to run the fan when the electric bill comes. Very quiet, too. Possible tax credit.
Proper fitting design for good airflow is paramount when you start dealing with residential 5 ton sytems. Typical "cap and tap" residential work will cause nothing but problems, and it may never work right. A "cap and tap" (supply plenum off the furnace tapped directly into the supply duct above/below) connection at the air handler on the supply side is equivalent to 120 feet of duct. A fitting with a throat radius equal to the short side of the ductwork is equal to 20 feet of duct in the same application. Manual D is your friend.
Make sure you know what you are actually paying for with those bids. HVAC isn't quite rocket science, but it does take a fair amount of skill and knowledge to design and install it properly. HVAC is not a commodity. Too bad most people only care about the price... Then they complain when it doesn't work...
Danski,
Based on your comments, I am looking for ways to eliminate the automatic dampered zone design. This is my personal residence, so I really want it done right. It is not a matter of price. I just want to do it correctly, but the "experts" are all over the place on this. Can you offer me some instructions on what the duct layout for the job should be then?
I am spec'ing a Trane XLi 19.5 SEER 2-speed AC system, 5-ton air handler, Trane CleanEffects electronic air filter on the return side, and a Lifebreath 200Max, Honeywell Humidifier, and Honeywell VisionPro 8000 thermostats, plus all the associated duct work. Recall, the house has floor radiant heating everywhere.
First floor: The first floor (underground) has a theatre (not often used, but I do need air circulation and AC in it) and stairs/mud room lead garage to main 2nd floor above. For sound insulation, the theatre (500 sf) is sealed from the stairs entrance (500 sf), which would mean that they need separate return air. (1) Is this not a correct assumption? Otherwise it would be difficult for either to have return air access when all the doors are closed. For the theatre, I was thinking that with 2 air registers and a return air, I could regulate the temperature at the register. Same with mud room - I could regulate its 1 register (has its own return air) for cooling controls. These would be in the same system as the main floor above (2nd floor).
Second Floor: 2500 sf. Kitchen, family room, etc... This floor would be the same zone as the first floor below. Since I could regulate the temperature of the floor below based on infrequent use of the theatre, I would place the thermostat at the main floor for best comfort. It has about 11 registers, 1 return air.
Third Floor. 2000 sf of bedrooms. 8 registers and 1 return air. My challenge is if I have this top floor on the same zone as the main below, I would "waste" energy during the day when I don' t need all that cooling. (2) For a quality installation, do you believe that I must install a second AC unit for the top floor to separate the control of both floors (avoiding dampered zoning system)? (3) Suppose I don't mind heating the 3rd floor during the day, is it realistically doable (average HVAC company can do it without error) that a duct installation can be balanced across all 3 floors from a single 5-ton AC unit? If so, how would I test/verify this before drywall is put up?
In Seattle, there's about 1 week of 90+ degrees. You are authoritative on this topic and I would really appreciate your feedback on the 3 points above. How would you do this if it were your house? I have attached the floor plans for your review. Thx.
A couple of thoughts,First of all, do a heat gain calculation and come to understand when you will need how much cooling. Stick to the 2% design-day conditions espoused by ASHRAE for your area and do NOT install more capacity than indicated by the heat gain calculation.Our contractor wanted to put in 7 tons of capacity, assuming a indoor condition 6 degrees colder than my wife and I like it, while assuming a 6 degree hotter condition outside than ASHRAE calls for. Even so, I suspected that 4 tons probably could have done the trick for the whole house but he insisted on a 3 ton unit downstairs in case we entertain in the summer. So now we have 5 tons of capacity for 5000 sq ft of house.I have since retrofit a energy monitoring system on the house... and our two-stage compressors have yet to reach stage 2 more than 1% of the time in the middle of summer. In other words, our system is likely to be oversized by at least 30% (stage 1 = 66% of compressor capacity, stage 2 = 100%). And if you want humidity control, that AC better be able to run for long lengths of time... My preference (if AC is to be used a lot) would be two units, perhaps a 3 ton and a two ton. That reduces your air ducting requirements significantly because you can reduce the length of the required runs to reach each room. In our case, we have on air handler in the basement, the second one is in the sealed attic. Don't bother with putting an AC in an unsealed attic though, because it's pretty much impossible to avoid duct losses, the holes in the ceilings lead to massive heat loss come winter, etc.Even more importanly, pay a lot of attention to proper return ducting. Many contractors love nothing more than a single big return in the hallway, ignoring the fact that doors get in the way of circulation. Should you undercut doors to allow circulation, you'll discover that this approach a) yields a very noisy house because you can hear everything, and b) does not work well for cooling either.
Which area of Seattle are you in? You can e-mail me.
Do you have trees? Were they taken into account for the AC calculations (probably not). **
Is the roof of the upper level a hip roof like the garage? What kind of ventilation is installed. A $50 vent fan for use during a few days in the summer is a much better investment than the 4th and 5th (and probably the 3rd) ton of AC.
The systems you have described as having been quoted really look like gross overkill for the Seattle area. Methinks everybody sees you have deep pockets and is willing to help you empty them.
I've had 2 quotes on and HVAC system in my life, one for own residence in 1973 and one for a chuch addition I did in 1980. HVAC contractors have not gotton any cheaper, and those 1970s ultra high quotes ($4K in 1973 dollars for instance) led me to learn how to do all loads and system design myself.
**note: if in a new subdivision, probably no trees as nearly every developer here clear cuts what they can.
Junkhound, I am building in Edmonds overlooking the water so I think the summer heat would be there. However, the sound winds tend to keep the area cooler than inland.
Did a quick search of the Sno. Co recent sales in Edmonds overlooking the water.
There were a few "teardown houses' built in the late 40's sold this and last year above the BNSF tracks overlooking the sound, so I'm guessing you are builidng on a $1.5 mil lot after a teardown of one of the older houses up on the cliff.
Thus assuming you have lots of bucks, go for the zoned system as dan described. I still think 5 T is way overkill for this area, and as previouslly mentioned, the ultra high seer rating is also overkill. If spending the extra $$ is trivial, no problem. Would not spend that extra $$ myself.
If you are up on the cliff, one additional thought overall. Make sure your drain lines are all clear all the way down to the tracks. I've often been along the tracks at the bottom, and see some of the drain lines that look like they clog and some erosion has started. BNSF probably has a monitoring system for erosion (numerous landslides), etc.
Junkhound, you are good! Should call you Columbo. Am not rich enough to spend money unnecessarily, thus I ask for advice from guys like you. I agree with your advice and will back off from the 19 SEERs AC. I also wonder if the I can't just use one 5-ton for the whole house if I get it balanced between the 2 floors. If not, I would probably go 2 AC systems. Are you an HVAC guy or builder?
Are you an HVAC guy or builder?
Actually, I'm a 'simple' DIY engineer. The only HVAC I do now is on my own and the kids houses (and some on airplanes at the big company in your area <G>).
Agree with nearly all Dan has said, with some slightly different personal biases. Dans advisories on air leaks etc. are absolutely to be followed.
Specifics if it were my choice:
If it was my house in a configuration as you describe, I'd put in a 1-1/2/ T, 13 or 14 Seer for the upstairs and a 2T for the downstairs. Probably put the air handler for the upstairs in the attic if accessible. I'd look at the cost differential, and may up the sizes to 2T and 2-1/2 ton if you deceide to heat in fall and spring with HP option discussed below.
I'd make them both Heat pumps vs. straight AC, as in Edmonds the months of Oct and Nov. and late Feb thru May (and often into June) the comfort of forced air heat is just as good as radiant - there is also more dust in the air in those months than the winter and the air filter helps hold down household dust. Plus, the COP* (coefficient of performance) of a HP when the outside temp is in the lower 60s or 50s is well above 3, often over 3.5. This means less than 1/2 the heating bill in those months using the heat pumps vs. nat gas. My son has a gas furnance and 3T HP for his near 5000 sq ft house (in Kent), and has not used the NG for the last 3 years as the elec bill is lower than the gas bill for heating. I'm assuming you are on Sno. County PUD for electricity and PSE for NG.
Making them HP also takes care of the 'backup' considerations.
Would ONLY CONSIDER AC or HP with a Scroll compressor (prererably Copeland), any make (even Goodman) that uses a scroll will last a long time. I'd never buy any AC or HP with any other type compressor.
Many HP and AC have orifice devices vs. TXV. (thermostatic expansion valve). I'd always opt for the TXV, better efficiency, part cost is only $15 or $20 -- a good comparison of the contractors you have been talking to would be a discussion of this point and the cost differential.
Own choice would be using R22 vs. Puron (410). Lower pressures, etc. However, a lot of this decision is due to the fact I have an EPA license to purchase refrigerants such as R22, so can stockpile my own lifetime supply. (R22 still less than 1/2 the price of 410)
* at a 3.5 COP, you get 3.5 times the heat out of a HP as you would from electrical baseboard heaters for the exact same electrical input.
I would agree with the heat pump reasoning.
Maybe the heat pump will shine in a better light with the higher equipment efficiencies and the hotter discharge temp of the 410A refrigerant. The variable speed motors in the furnace/air handler is an added bonus for long run times.
I have considered installing a 16+ SEER heat pump in my own home mated to a variable speed 95+ efficient furnace (dual fuel). The two stage condensers finally address the need for a small cooling load and a high heating load in my area. Of couse, I could not afford to pay full retail for this setup, but I can do it myself.
For straight AC, the high SEER is not worth it here.
It really sucks that the homeowner tax credits max out at $500.00, yet a builder can qualify for $2000.00. Not much of an incentive for John Q. Public to do squat for energy efficiency, and a builder typically won't spend $.01 more than he has to...
The only major hangup is whether it will help or hurt at sale time.
R410-A systems automatically come with a TXV valve. They are a worthwhile addition on a R-22 system. A 13 SEER R-22 system+TXV+variable speed drive will almost give you 15 SEER.
Edited 9/11/2006 5:26 pm ET by danski0224
You are gonna be sorry for not making all that "crawlspace" into a "basement" :) Unless there are real reasons for not digging it out, I would cut the budget elsewhere to make it happen if it was my house.
I'm sure the XL19i is a nice unit, but I bet you will never see a payback on it in your area.
Oh, and if your air handler is not variable speed, you will not get the SEER rating you think you are.
I looked at the plans and bringing all of the ducts up from the base/crawl will be difficult- but not impossible. There is one chase located by the elevator. The main trunks (zoned or not) will probably be located in the crawlspace/basement area with the brances off of each zone.
Yes, you will need two returns in the basement if one area is sealed from the other.
The one major, glaring flaw in what you propose is the single return air on each floor. That design aspect alone will cause temperature differences. Most of the return air will come from areas closest to the grille on the wall. Central returns are used to save money by eliminating ductwork, not to increase comfort. You will not be able to test system performance with a central return until the walls are drywalled. Things like other open/closed doors will also have an impact.
Your register count on the second floor seems to be missing the closets with exterior wall exposure.
If your call for cooling can't be satisfied on the first stage, then you will need to move 2000 cfm of air through the one return on each floor. You will also need to move 2000 cfm through the supply registers. If the zoning system you choose does not have leakage built in to the dampers, then you will add a barometric relief damper.
If it was my home, I would do two systems- no doubt about it. If I had radiant heat, at least one of those systems would be a furnace for backup. probably the ground floor because heat rises. Another advantage of two systems is a structural one- now the number of walls that are cut up is reduced.
I would not be too concerned about "saving money" by not cooling the second floor. It is cheaper to maintain a reasonable temperature than to turn it off and on when you want to.
The ground floor/basement system would be located in an easily serviced/accessible location. I would not cram it into the furthest corner of the house. Ductwork in the basement. I would consider insulating the duct in the basement.
The second floor, based on your drawing, if it was my house, I would eliminate the shower/toilet area between bedroom #2 and #3 and make that a mechanical closet. Bedroom #2 WIC would be on the exterior wall so the mech room could be accessed through the hallway. The 2nd vanity in the bath would be eliminated, and the mech room a little smaller for a toilet space. Mech room walls would be double 2x4 with an air gap to eliminate noise transmission- one of those stud walls would be filled with rock wool sound batts- or the whole thing filled with blown in cellulose. Ductwork would be in the attic thru the mech room ceiling and insulated with R6 minimum, and the equipment would be on the second floor. I would tile the floor and make it like a shower pan with a drain in case of a leak. The equipment would be placed in a metal sealed primary drain pan with a safety float switch. Round plastic supply diffusers on the 2nd floor ceiling are easy to adjust for airflow.
Both systems would have sealed ductwork and branch runs. No ductboard in my house. Flex duct would be permitted (1) if it is accessible and (2) no more than 5' lengths.
Both of my systems would have a variable speed drive and 410A refrigerant with properly brazed and installed (no extra oil traps) copper. I doubt I would do high SEER in that area (or mine). I would skip the fancy air cleaner and stick with a basic media filter- maybe an Aprilaire 5000 for a premium. I don't have allergies, so the UV/hepa stuff is a waste. About the only place I would use UV is for the air conditioner coil. I would make sure that everything within sight of the UV light is UV safe (sounds simple but it isn't).
I would consider an ERV/HRV if I was building a real tight home.
My house will be insulated with blown in cellulose, and the exterior walls will be caulked to the floor.
That's how I would do it.
You can go here: http://www.hvaccomputer.com and get a limited use license for like $50 and do your own load calc. The program sizes duct, too. It is easy to use. Pay attention to the required cooling BTU's- not the size of the equipment it tells you. A "5 ton" system will not provide 60,000 btu's of cooling.
I bet you will be surprised at the size of equipment you need.
Edited 9/9/2006 9:34 am ET by danski0224
Dan, thanks for the advice. I need to seek some clarifications from your comments.
Dan: If your call for cooling can't be satisfied on the first stage, then you will need to move 2000 cfm of air through the one return on each floor. You will also need to move 2000 cfm through the supply registers. If the zoning system you choose does not have leakage built in to the dampers, then you will add a barometric relief damper. Streamline: Why would I need a relief damper if I have 2000 cfm supply, and 2000 cfm RA? Based on your advice, I want to simplify the design to one zone for the whole house. Thus, barometric relief should not be needed as everything is wide open, correct?
Dan: If it was my home, I would do two systems- no doubt about it. Streamline: If properly balanced at the start, and Seattle only has a week of over 90 degrees, would it be wiser to use 1 AC system? Why would a 2-stage system not as effective and simpler than a 2-AC system?
Dan: If I had radiant heat, at least one of those systems would be a furnace for backup. probably the ground floor because heat rises. Streamline: The variable air handler is probably going to be placed in the attic. Wise?
Dan: The ground floor/basement system would be located in an easily serviced/accessible location. Streamline: The duct runs will be in the 4' crawl space in the basement, then climbing up near the elevator chase. Would this be reasonable?
Why would I need a relief damper if I have 2000 cfm supply, and 2000 cfm RA? Based on your advice, I want to simplify the design to one zone for the whole house. Thus, barometric relief should not be needed as everything is wide open, correct?
You will have one main duct off the equipment with zoned branches (2 or 3). Zoning systems prioritize calls for heat/cooling- only one zone will be "open" at once, and the system will modulate between the calls for heat/cooling if two calls are made at the same time until one/all are satisfied. When one zone is open, that zone ductwork must be designed to move the full output of the air handler. The ductwork size between zones must be within the manufacturers percentage to avoid the use of a barometric relief damper- newer systems have "leakage" designed into the damper system for this reason.
I am not familiar with all zoning systems, but what I am familiar with will not open two zones at the same time, but some thermostats can be programmed to give a particular zone priority.
Therefore, you are cooling roughly 2000+/- sf at any one time with 5 tons of equipment if the call for cooling is not satisfied/times out on the first stage. It will short cycle and never run long enough to pull out the humidity. Unless the ductwork is designed and sized properly, it will be noisy. If you do not move 5 tons of airflow through your central return, the coil will freeze. Roughly 2000 cfm through a residential grille will be very noisy.
I realize you are planning to use a two stage AC unit, but it must all be sized to work on the full output.
Streamline: If properly balanced at the start, and Seattle only has a week of over 90 degrees, would it be wiser to use 1 AC system? Why would a 2-stage system not as effective and simpler than a 2-AC system?
There is usually only one week of honest 90* plus weather here at a time (of course, weather patterns are changing:). Given your drawings, if it was my home, I would install two systems. The HVAC system will not be balanceable until the house is finished inside. Some of the dampers- especially in the attic- may not be accessible later. Dampers on residential registers are not effective for balancing. Two stage AC is like a two stage furnace- it is designed to lower the output on low cooling/heating demand days so the equipment runs longer and makes the space more comfortable. For the price of that high end condenser, I bet you could buy two smaller "plain" ones. The cost of the zone dampers, controls and installation will offset most of the cost of the second air handler. The cost and installation of the ductwork remains about the same.
In the end, you have replaced a complicated condensing unit with two less complicated ones. You have eliminated the zone dampers and their control panel and the problems caused if/when they fail. You are cooling each zone with properly sized equipment. The ductwork on the smaller equipment will be smaller. Operation of a simple air handler, thermostat and condenser is a known quantity.
In your case, I would seriously consider sizing the equipment to the larger of the zones you want to create, not sizing one piece of equipment to cool the whole house and then splitting it up and using a two stage AC as a crutch.
Streamline: The variable air handler is probably going to be placed in the attic. Wise?
If it was my home, and I was designing it (or had input on the matter), I would *NEVER* place the equipment in the attic. For this house, if it was mine, I have detailed the changes I would make. Attic based equipment is usually difficult to service and homeowners do not want to go up there to maintain the filters.
I would give mechanical systems priority (within reason) for ease of operation and serviceability. I realize my viewpoint is different from the majority that wants to bury it all to the furthest corners of the home.
Streamline: The duct runs will be in the 4' crawl space in the basement, then climbing up near the elevator chase. Would this be reasonable?
Nothing wrong with that if it all fits.
I still say that your "crawlspace" square footage will be the cheapest space you will create. Half of the concrete and excavation is already paid for.
Danski, I should call you "Master" instead. He's what I learned from you. Please let me know if this is a good solution based on what I have:
I would have 2 AC systems. One running top bedroom floor. Probably 3-ton system. Lower the SEER to something like 14 SEER because for the weeks it is operational, the savings is not worth the extra equipment cost. Should be a 2-stage for comfort.
The second AC system operates the main floor and basement, one zone. This should be a larger system, as it is operating 3500 sf. Probably 4-ton 2 stage (per manual J). Also lower SEER. 2 vents in theatre room and regulate temperature at the vent opening as thermostat is upstairs in main floor.
All equipment moved to the basement in designated mechnical room for servicability. Variable speed air handler should be converted to furnance for backup.
This design allows ducts to be smaller, quieter air flow, no complicated zoning systems.
Does this sound like a quality design for the house?
Well, I certainly don't know about the "master" part- I just believe in the KISS principle (Keep It Simple, Stupid).
You are on a much better track now. I do not have anything against two stage AC condensers, but if the system is sized properly, you will not need it. I doubt you will ever recover the cost of the unit. Definitely a 410A system, as R-22 is being phased out. R410A systems must be installed properly for long life.
If you plan on doing frequent entertaining, 2 stage condensers can help by making the unit a little bigger and run on the proper size most of the time. Another option is to pre-cool the space before everyone gets there.
I suspect that the equipment you *think* you need is still way too big for the area. Your basement space will not factor much into the cooling load. Window orientation, insulation and low-e coatings make a difference. However, a high zoot home theater space will. You will be better off with a small mini split unit dedicated to the theater room if you plan on loading it with people/equipment.
A manual J using the local temperature values will tell what size stuff you need. Do not fudge the numbers trying to get upsize equipment. Residential HVAC (and all SEER ratings) are based on an 80* indoor temperature. If you want it colder, that must be allowed for, and the output BTU's of the equipment derated accordingly. Properly sized equipment will run all the time on a design temperature day, and as the temperature indoors will rise as the temperature outdoors climbs above design temp.
Upsizing equipment is a dangerous game that leads people to beleive they need two stage stuff to be "comfortable". Usually, the equipment will short cycle and lead to discomfort at best if the equipment is too big.
I would definitely make one of the units a furnace for backup.
Both units in the basement would be a plus- but only if it all fits/works in there. Sometimes, it isn't possible to bring all the ducts from the basement to the second floor (framing issues like steel, mixed 12" and 16" on center framing or LVL's or other trades like the plumber). Both you and the builder might need to allow a chase or a fattened wall or two to make it all work. More time with the plans would tell.
Make sure the duct is designed for proper airflow and not faster/cheaper installation. Sealing/taping the ducts and branches makes a difference. Return air duct in each room served by a supply will be much, much better than central returns.
Edited 9/11/2006 6:55 am ET by danski0224
What are your thoughts for coolant?
I was reading in one trade magazine or another a while back that R22 is going to be phased on in the near future and it would be wise to purchase using R410A.
Yes, R-22 is being phased out. If you search for "R22 phaseout" you will be directed to EPA information.
Like R-12, R-22 will continue to be available for a while. Prices will go up and "safe" replacements will be developed- just like R-12. The EPA site will give all the phaseout timetables, so you will know the real deal without the BS.
Currently, R-410A is the accepted standard for residential HVAC equipment. Some applications it is better than 22, others not so, but it is what it is. Commercial refrigeration is pointing to using CO2- Europe is much further along in this regard.
If I was doing an all new installation or retrofit, I would get the R-410A.
It is not "new technology". Carrier has been marketing their Puron brand for many years.
R-22 is more forgiving to improper installation than R-410A. Lots of shortcuts are taken on slam bam residential work with R-22. Those same shortcuts with R-410A will lead to major problems. Make sure your HVAC contractor knows what they are doing.
"(2) For a quality installation, do you believe that I must install a second AC unit for the top floor to separate the control of both floors (avoiding dampered zoning system)? "I built my house in 79 and at that time options where more limited.The house is on a hill side facing a lake. The back and about 1/3 of thhe sides are underground. The front side has a large amount of glass. It is slab on grade.The 2nd floor is more conventional framing and the number/position of windows.I went through about a dozen different HVAC contractors until I found one that would do other than "stick a couple of vents in the ceiling to heat the 'basement'". That 'basement' was the living, dining and kitchen area. And I know that a couple of vents in the ceiling it would be COLD in thewinter.So I ended up with two complete systems. But I only have AC on the 2nd floor, by design. Now there is only 800 sq ft/floor and thus the stair well is a fair percentage of the space. But the 2nd floor AC keeps the first floor reasonably comforatable. Not perfect, but fine for me.BTW, the returns in the 2nd floor are high and I think that helps.I also have a very small 2.5 story landing (about 50 sq ft) that is where the entrance from the garage and street side door is (remember this is a hillside and the house is "upside down"). So vents are return in that area. And during the summer that spot gets very hot.