DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   HVAC (
-   -   Improper A/C sizing - Responsibility to Fix? (

Dragon 06-28-2007 01:39 PM

Improper A/C sizing - Responsibility to Fix?
I have finally been able to move into my newly constructed ICF (insulated Concrete Form) house located in southern Wisconsin.

One of my concerns from the beginning of construction on my house appears to have occurred. The A/C unit appears to be improperly sized. The A/C unit can quickly cool the house to any desired temperature but does not appear to be removing the humidity. This is referred to as short cycling. The unit is so big that it quickly cools the house before the unit has the chance to pull the moisture out of the air. The humidity level in my house currently hovers in the mid 60% range and will drop to just about 60% when the A/C is running but goes no lower than that. When you are in the house it feels cool but clammy and you do not get that refreshing evaporative effect on your skin.

The contractor sized the A/C unit based on calculations done using software called ResCheck. Those calculations apparently called for the installation of a 2 ˝ ton unit.

Since than I have purchased a software package specifically designed to perform heat calculations for ICF homes, it is called “HVAC sizing for Concrete Homes” and is distributed by the Portland Cement Association. I wanted to double check the other calculations. This program appears to indicate that the A/C we have twice the size actually needed. At the beginning of the building process I told the contractor about this program but he assured us that their calculations would be accurate and that program wasn’t necessary.

So I have just begun discussion with the contractor regarding this humidity issue. He is in the process on reviewing our file and calculations with their engineer.

All other sources of humidity are properly controlled with exhaust fans, etc. (bathrooms & kitchen)

I would like to hear what you opinions are regarding this. Do you think the contractor is on the hook to replace the unit if the A/C installed is improperly sized? If it does turn out to be correct that they installed a unit twice as big as necessary should I expect $$$ back for the difference in price on the different sized units?

I would love to hear your advice, suggestions, stories and questions! Thanks.

Buckeyetech 06-28-2007 06:38 PM

It does sound like the a/c is too big by the symptoms you describe. I'm surprised the other program show it being twice as large as needed. I would really believe that the HVAC contractor is responsible to replace it. $$$ back. Not much difference in price between a 2 1/2 and 1-1/2.

Go Buckeyes

bgd73 06-29-2007 09:42 PM

Is there another setting aside from thermostat, like how cold it works?
Coincidentally on a much smaller scale I have a large window unit in aprox 180-200 sq ft (one small room) with a 450 sq ft capable a/c. Same problems- blasting cold enough for goose bumps but no humidity vacated. creepy feeling that is, like pneumonia on the way...
anyway, as it turns out, I don't have a thermostat on the a/c it is a function for cooler or warmer. I left it low to run longer and it worked.
Most a/c is thermostat , there ought to be a setting for such scenarios.
try slower speed, if no other settings?

handy man88 06-30-2007 10:21 PM

If you close or adjust some of the vents, it'll slow down the cooling of your house to a degree which will allow more moisture to be removed.

bigMikeB 07-01-2007 07:19 AM

If he closes vents and slows air flow without changing the charge (freon) he can also freeze up the coil and slug the compressor with liquid. There is no quick fix for this.
If this is a new house there should be permits, ie the contractor should have had to submit a heat loss/ heat gain to the inspecting authority for size verification. Regardless of what your own calculations came out to be, I have never seen a house of any construction that really works with an a/c system that is half of 2 1/2 tons. Tonnage should be @600 sq. ft. per ton of a/c.

pjpjpjpj 07-02-2007 11:29 AM

I agree with bigMikeB, but another thought I had was location of the thermostat. Is there a supply grille blowing air on (or very near) it? This can give a false reading and make the unit cycle off too quickly. Of course, you did say the unit "quickly cooled the house", so it sounds like this is not an issue. Just thinkin' out loud....

I am not familiar with ICF construction - does your house have normal amounts of windows and such? What sort of wall insulation does it have?

Jeekinz 07-02-2007 11:46 AM

What about the return ducts?

Dragon 07-03-2007 09:46 AM

Thanks for the responses. Here is some feedback to your comments and questions.

I’ll start off by giving a better explanation of how an ICF house is constructed and what of. The foundation and 1st floor walls of my house are constructed of poured concrete that is sandwiched between 2 1/8 inches of foam on the interior and 2 1/8 inches of foam on the exterior. The ICF foam arrive in blocks at the work site and are assembled like Legos. Once up they are braced and then the concrete is poured into the wall forms. (yes, the foundation and upper walls are poured on different days, generally the maximum pour in a single day is about 8-10 feet) The thickness of the foundation walls is 8 inches of concrete and the 1st floor walls are 5 inches of concrete.

So, with the walls constructed out of concrete sandwiched between the foam I have a super insulated home. Looking at pure R-values the wall of concrete and foam is at least an R-25 if not more. There are two other factors that need to be taken into account. One is air infiltration. My house has very little air infiltration. There are no gaps in the walls at all, it is one continuous slab of poured concrete covered with foam. The second factor to be taken into account is Thermal Mass. With the mass of the walls and the floors (in addition: the floor between the basement and first floor is 10 inches of Spancrete topped with a 4 inch concrete slab encasing the PEX tubing for my Radiant floor heat system) I do not get wild temperature swings throughout the day because the large Thermal Mass prevents these large swings of temperature.

With these additional factors (thermal mass & air infiltration) taken into account I have a house that performs at about a level equivalent to an R-50. If you are interested in learning more just Google ICF concrete homes and check out the various websites.

Other items specific to my house: There are what I would consider a normal amount of windows. That is to say there are not huge expanses of windows all over the place. The eves on the house are a full 3 feet which provide excellent shade to prevent heat gain from the sun. Due to the very tight construction of the house I also have a Guardian Air Exchanger to bring in fresh air and exhaust stale air. This system works in conjunction with the air handling system. The ceiling/attic insulation is blown in and is a minimum of at least an R-50 if not more.

Location of the thermostat: The thermostat is located in a central hallway and there are no supply grilles in that area of the house that could blow on the thermostat. Nor is the thermostat exposed to direct sunlight.

Calculating tonnage for A/C system: Every single source dealing with ICF construction states that tonnage should never be calculated using a “rule of thumb” estimate calculated by basing tonnage on the square footage of the house. This will always result in over sizing of the A/C system. Using the previously mentioned 1 ton to 600sqft. that would require a 4 ton unit just for my first floor and not even take into consideration my finished basement. (I have 2476sqft on the main floor and the same amount in the basement) The computer program I previously mentioned for ICF homes takes into account the special properties of this type of construction and calculates HVAC loads accordingly. At this point I wish I had insisted that they utilize this program from the beginning. That way I would not be in the position of pointing out to them that they have improper calculations even though they insisted everything would be fine. You know what they say about hindsight right….

Return ducts: Not sure what was meant by that. There are return ducts throughout the house.

Closing vents: As mentioned in another post, this is not a long term viable solution for the reasons given.

I did not state before that the company I am dealing with is a large, reputable, professional firm. I am very fortunate in that I am not worried about them being a shady outfit I will have to hunt down to get an answer from.

It was good to hear at least one response indicating that the contractor is on the hook for the system design and any problems with it. Does anyone have other experiences where a contractor had to replace a system in a newly constructed house due to improper size (either too big or too small)? What was your experience? What can I expect as a response from the contractor?

Thanks again, I look forward to your responses and further questions.

pjpjpjpj 07-03-2007 12:49 PM

Daaaannngggg. That sounds pretty darn GREEN. Sorry, I'm in the commercial design biz and haven't heard of that sort of construction. Is ICF common up there where you live?

My gut feeling when reading your description of the house construction is "yup, that contractor is on the hook if he didn't take into account this unusually-good method of construction". I'm betting that the contractor still used a "sq ft per ton" method, even if it was less (well, more) than 600.

The good news is that a smaller AC unit should still work with the current ducts. Better to have oversized ducts than undersized!

With that sort of construction in Wisconsin, I'm betting you hardly need summer AC at all - just a little winter heating, eh? :thumbsup:

MechanicalDVR 07-03-2007 10:23 PM

"The good news is that a smaller AC unit should still work with the current ducts. Better to have oversized ducts than undersized!"

Yo Dude get a clue, duct work is sized by the cfm (cubic feet per minute) that it carries, so no way in hell can you downsize a unit and expect it to work properly with ducts sized for more cfm. The three main factors in duct design are air velocity, air volume, and friction loss. All three of these factors will change significantly going from say duct work sized for three tons of air (1200 cfm) down to two tons (800 cfm) the velocity would be gone and so would moving air. This isn't water your trying to move here, which is 1000x more forgiving.

dmaceld 07-03-2007 11:55 PM

Sorry to hear of your problems. You are dealing with exactly the problem I am desperately trying to avoid ahead of time, especially for the heat side of the calculation. I used the PCA program for my 2200 sq ft one story ICF house to be with conditioned crawl and attic, and came up with about a 19,000 Btuh heat and cooling load. My nephew HVAC contractor comes up with about 38,000 heating and 19,000 cooling. He uses WrightSoft, a Cadillac of HVAC programs and has ICF wall structure parameters available in it. I ordered the HVAC Calc program and it comes up with numbers close to my nephew's.

So which one is right? I would put more stock in the PCA program for the simple reason it calculates heat and cooling on an hour-by-hour basis using 10 year average hourly temperatures for the city selected and takes into account the latency of heat traveling through the concrete. The PCA program uses the Dept of Energy DOE2.2 energy load calculation program for large commercial buildings, which almost requires a PhD to run!! The PCA program is a simplified Excel front end tailored for residential use. Regular HVAC programs calculate heat and cooling only on the max and min temperature and assume heat travels almost instantaneously through the walls. They also assume a greater air infiltration rate than the PCA program does.

In emails with the PCA program author, he says he often hears about the discrepancies between the Manual J programs and the PCA program. He did admit the PCA program runs the DOE program at the low end of what it's designed for, which can lead to errors. Differences in air filtration numbers often lead to the differences in the program outputs. PCA uses a lot lower infiltration rate.

Based on your symptoms, your a/c unit is way oversized. You would be much better off to have an undersized unit! Also consider using an evaporator rated smaller than the condenser. That helps remove humidity. I did that in a Louisiana house I used to own. The combo of 3 ton condenser with 2.5 ton evaporator worked good, although I should have used a smaller unit for both.

Talk seriously with your a/c man. Most likely you will have to come to some compromise on the cost of changing out the system. You probably shouldn't be too hard on him if he has had no experience with ICF in the past, especially if he's embarrassed and contrite, but don't let him off the hook if it looks like he gave short shrift to the fact your house is ICF construction. Double check what ResCheck does. I'm thinking it's more an energy check list or audit program rather than a heavy duty heating/cooling load calculation program. In the Green Building forums there are strong differences of opinion, and experience, in how to correctly size HVAC for ICF. One contractor is very adamant that the best size is somewhere between Manual J and PCA.

Keep us posted. I especially would like to know more specifics about your situation, and what you come up with because, as I said at the beginning, I'm wrestling with this issue in the HVAC design phase for the house I'm getting ready to build. Email me through the profile link if you wish.

dmaceld 07-04-2007 12:04 AM


Originally Posted by MechanicalDVR (Post 51433)
All three of these factors will change significantly going from say duct work sized for three tons of air (1200 cfm) down to two tons (800 cfm) the velocity would be gone and so would moving air.

Surely an option would be to keep the same air handler, thus the same air volume, and reduce the evaporator size. The blower could be set to run at the lowest design speed for the system as built. The net result would be the air wouldn't be as cool as the normal design parameters call for. But as long as the air is cool enough to be comfortable and the the desired Btuh heat extraction is occurring, it would work, wouldn't it?

pjpjpjpj 07-04-2007 01:35 PM


Originally Posted by MechanicalDVR (Post 51433)
"Yo Dude get a clue, duct work is sized by the cfm (cubic feet per minute) that it carries, so no way in hell can you downsize a unit and expect it to work properly with ducts sized for more cfm."

Yo, dude, you get a clue. As in, be nice.

It all depends on the situation, the duct size, etc. Unless you've been to this guy's house, measured his ducts and grilles, and run the calcs, you're talking trash with no proof. No need to be rude.

If he drops from a 2.5T unit (1000 cfm nominal) to a 1.5T unit (600 cfm nominal), it's probably not going to make much difference. At that low flow, the ducts are all small anyway. As long as you have enough velocity at a floor grille to get cold air off the floor, or enough velocity at a ceiling grille to get heat off the ceiling, you are fine. Besides, it sounds like this ICF building will hardly need AC at all, especially in Wisconsin.

But most of all, most residential ductwork is so poorly constructed, leaky, without proper transitions to reduce friction losses, etc., that it is actually good to slightly oversize it. You might actually get the air to go where you want it. Ever seen 45-degree rectangular taps, conical or scoop spin-ins, or elbow turning vanes in a residential application? Yeah, me neither. :laughing:

Obviously you lose velocity when you oversize - but I'd rather push 800 cfm through a 24x12 than try to push 1600 cfm through a 6" round, ya know?

harleyrider 07-04-2007 03:03 PM

You all are way over thinking this whole issue, simple slow down the blower and lower the piston size by 1X. In addition to that I would run a de-humidifier. In Wisconson the cooling season is VERY short, these two things should solve the problem.

Dragon 07-05-2007 01:35 PM

Update 2
The latest news is as follows:

I spoke to the company Rep. and we further discussed the issues. They were in the process of sending out a service tech on an unrelated issue. (rattling zone damper) So he had the tech adjust the blower settings while he was out replacing the damper.

The service tech called me after working on the unit and told me the following: The blower had not been adjusted from the factory set maximum of 1200cfm. He said that this was the blower setting for a 3 ton unit and that mine should have been set lower when installed. He said that he adjusted the unit down to 1000cfm and then further adjusted the “dip switches” for a further 15% reduction which by my calculation should put me at about 850cfm. This should require the unit to run longer to achieve the proper temp and in the process will be able to better reduce the humidity levels. He also said that he ran the system and hooked up his instruments to ensure that the unit would not freeze up at this new setting.

We will see how this works for the time being. I also still requested a sit down meeting with the engineer to discuss the HVAC calculations and the differences between the calculations from the various programs.

Even if the adjustments that have just been made work just fine I want to ensure that I am not going to pay a substantial difference in energy costs to run this system in the long term. If the difference turns out to be not that bad I’ll probably live with it. But if the long term costs appear substantial then a discussion about replacing the system with one that is properly sized will need to take place.

What are your thoughts about what the service tech said about the adjustments that were made today? Based on the other replies I am going to have to ask the company Rep. about the duct sizing to determine if it was sized for the larger or smaller rating. What issues can I expect with the reduced CFM? How will I know it is an issue? (besides my current problems?)

Harlyrider: What are you referring to when you talk about reducing the “piston size”?

I looked online at Whole House Dehumidifiers (Aprilaire) that can be tied into the central system. Wow, they want around $1700 for those things.

All times are GMT -5. The time now is 02:45 PM.

Copyright © 2003-2014 Escalate Media LP. All Rights Reserved