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Concluding you need bigger because it's not doing the job is a trap that might get you into trouble.

Audit the house. Check duct design & leakage.

Originally Posted by tedkidd

I'm not a good one to address that situation - 1152 sf I would call 2 ton grossly over sized. I put 1.5 ton on 1700 square foot mobile homes.

You would think, but 2 ton is not doing the job, and it runs fine. What temp difference are you calculating upstate?

I'm not a good one to address that situation - 1152 sf I would call 2 ton grossly over sized. I put 1.5 ton on 1700 square foot mobile homes.

Originally Posted by tedkidd

Be funny if it weren't such a common practice.

Actually we got a call from a guy in a 1970s house. 24'x48', he had the AC put in 10 years ago, it never worked right. The installer just put in a 2 ton with no measuring. My load calc with a large amount of glass on the west side came slightly above 3 ton. Yup, that is what we have to compete with, I also have a feeling he thinks my cost is too high based on what he paid for an entire system 10 years ago when he got what he paid for.

Originally Posted by ChaseAir

Do you size them from the sidewalk btw your thumb and forefinger?

Be funny if it weren't such a common practice.

Originally Posted by meh70087

When we first started doing foam houses we didn't trust the load calc but stuck with it and have not had the first problem. Approx 3200sqft with a cool 2.5 tons. At start up I was kinda feeling like we may have some issues. Told HO we would be back in two days, when I returned she had the biggest smile ! Give it some time. No one has mentioned AIR FLOW so that may be something to check? This is also in south LA .

ALL FOAM HOMES GET HEAT PUMPS unless HO refuses.

Originally Posted by arc8

I once ran into something like this but it was the other way; a furnace.

I sized up this new small house. The heat loss was 32,000 @ -24 degree, so I installed a 92% 40,000 furnace.

The installation happened in the middle of winter, we got the furnace going, the drywallers were mudding. It was insulated everywhere except the basement which were poured concrete walls.

Needless to say, the furnace could not keep up, it would not warm the house above 65 degrees on average cold days for a few weeks. I hoped that I calculated the Heat Load correctly. I was a little nervous. But after a month, it corrected itself. The house was completed and the folks moved in and to this day it is able to heat the house above 75 degrees on the coldest days.

I guess the Heat Loads were meant for finished and stable (constant) temps on the inside.

Good Luck to your project.

Originally Posted by frumper15

This is something that I have found as well - our first Geothermal install also happened to be a foam insulation job (house fire rebuild). Of course, the homeowner didn't tell me that during the design phase, and the plans didn't specify so my load calcs and equipment sizing were based on conventional insulation, etc. Well, they sprayed the underside of the roof (cathedralized?) and made the attic a conditioned space so the interior volume went up by another 25% or so. I was a little nervous at startup because 1) it was my first Geothermal and I was trusting my training and calculations and 2) I had sized a 4 ton geothermal where we probably would have installed an 80k BTU furnace. When the system first started up it was middle December and below freezing outside. Thankfully, the HO had been keeping things warmish with a kerosene torpedo heater so I wasn't trying to come up from too low, but without resorting to the auxiliary heat it took a while to chip away at those degrees.

The lesson I learned is that an appropriate sized system will do a great job of keeping the space at temperature, but extreme recovery is another story. New startup in the middle of record high temperatures? A few days at the very least before getting worried.

We had a similar issue with our service side last month - record highs for a few weeks and lots of broken down systems. The house would be 90 degrees inside when we got the system running again and the customers would be calling 2 hours later complaining that it wasn't 70 degrees in the house yet. I told all of them to call back 24 hours later if it wasn't the temperature they wanted - I didn't get any calls.

Originally Posted by air2spare

always remember, with foam homes its not an issue of "R" value but rather infiltration and exfiltration....that's what changes the load

Originally Posted by ChaseAir

Timber, I have done couple of homes like this in North Florida. The loads many times come out to over 1000 sq ft per ton. We have three ton 2 stage unit that cools a 4400 sq ft. Home like a champ. This home has no direct east or West facing glass. What is your subcooling, superheat, td, pressures,etc. The foam must be thick to work. 10" is what I mean.

Originally Posted by cuchulain

I would not recommend going with a system that is larger then the calculation calls for. It may seem extremely small, but I've seen systems calculated out using traditional insulation methods put into foamed houses and then get calls that the inside of the house is literally raining, because the system can't run long enough to pull the humidity out before satisfying the stat.

They are correct in saying that don't expect the system to run optimally if it's designed for 97 and it's 107 outside. The house has to come down to a good relative ambient before you'll see a difference. Check every thing and make sure your running where it should be and then when the outside temp reaches design then go back and check it and that's the point you'll know if it's working properly.

Nice posts guys! This comes down to educating the homeowner, helping them understand reasonable expectations and the importance of not oversizing. They simply need to understand the home is more like a large yacht than a water ski boat.

Maybe show them this thread...

Originally Posted by EugeneTheJeep

Zone was off?

Tstat never called

Originally Posted by ChaseAir

I was at one of our foam houses early on summer. It was 85 outside and we were replacing a txv on one of the units for 3 hrs in late afternoon. The other unit did not come on once.

Zone was off?

Originally Posted by Timber

Was there this afternoon, house was at setpoint , 76. 101 outside.

Customer is happy?

I was at one of our foam houses early on summer. It was 85 outside and we were replacing a txv on one of the units for 3 hrs in late afternoon. The other unit did not come on once.

I was at one of our foam houses early on summer. It was 85 outside and we were replacing a txv on one of the units for 3 hrs in late afternoon. The other unit did not come on once.

Originally Posted by EugeneTheJeep

So Timber, it has been a week, did the house cool off yet?

Was there this afternoon, house was at setpoint , 76. 101 outside.

So Timber, it has been a week, did the house cool off yet?

Originally Posted by air2spare

always remember, with foam homes its not an issue of "R" value but rather infiltration and exfiltration....that's what changes the load

AND the ducting has been moved into the air conditioned space!

In my world that means maybe 80* to 85* attic verus 135* to 145*.

Originally Posted by cuchulain

I would not recommend going with a system that is larger then the calculation calls for. It may seem extremely small, but I've seen systems calculated out using traditional insulation methods put into foamed houses and then get calls that the inside of the house is literally raining, because the system can't run long enough to pull the humidity out before satisfying the stat.

They are correct in saying that don't expect the system to run optimally if it's designed for 97 and it's 107 outside. The house has to come down to a good relative ambient before you'll see a difference. Check every thing and make sure your running where it should be and then when the outside temp reaches design then go back and check it and that's the point you'll know if it's working properly.

Every home get downs to where the moisture content of the air (dew point)in the home is the same as outside air dew point plus the moisture added from the occupants. This happens when there is low/no cooling load. And you are right that it can be near raining inside the home. The smallest a/c in the world will not be able to keep the space <50%RH without significant steady cooling load.
Humidity control by the a/c requires the perfect setup of the a/c cooling temperature and enough cooling load per hour to remove the 2-4 lbs. of moisture per hour from the fresh air ventilation and occupants moisture. This is the equivalent to about 1 ton of continuous cooling load to maintain <50%RH with 3 occupants and 75 cfrm of infiltration/ventilation air @ 65-70^F dew point.
Any moderate evening followed by rainy 65-70^F days presents the type of condition the a/c is unable to deal with. If you several days of this type of weather, you need supplemental dehumidification to maintain <50%RH.
The moral of this story is to get an adequeate sized a/c to maintain your desired temperature during peak cooling loads with occupants and fresh air ventilation. Add 10% a/c capacity to factor in a/c capacity decline over the next 10 years and add 2-3^F to the peak temperature for climate change. Have the a/c setup by a capable tech to maintain <50%RH (-25^F coil temp). Provide an adequately sized whole house dehumidifier to maintain <50%RH with occupants and fresh air ventilation.
Anything else is hoping that you will not encounter a couple weeks of wet cool weather that will make you uncomfortable in you new home. Or worse case, start mold growing in your green grass climate home.
Regards TB

I would not recommend going with a system that is larger then the calculation calls for. It may seem extremely small, but I've seen systems calculated out using traditional insulation methods put into foamed houses and then get calls that the inside of the house is literally raining, because the system can't run long enough to pull the humidity out before satisfying the stat.

They are correct in saying that don't expect the system to run optimally if it's designed for 97 and it's 107 outside. The house has to come down to a good relative ambient before you'll see a difference. Check every thing and make sure your running where it should be and then when the outside temp reaches design then go back and check it and that's the point you'll know if it's working properly.

When was the foam installed? I install foam as well as HVAC systems. The foam's exotherm reaction takes about a week to dissapate heat.

Originally Posted by ChaseAir

My version of wrightsoft software seems to reflect a difference in load with greater r-value in addition to encapsulation. We have calculated several homes sometimes installing 70% less equipment than our worthy competition.

well, the thickness of the foam is relevant

Originally Posted by air2spare

always remember, with foam homes its not an issue of "R" value but rather infiltration and exfiltration....that's what changes the load

My version of wrightsoft software seems to reflect a difference in load with greater r-value in addition to encapsulation. We have calculated several homes sometimes installing 70% less equipment than our worthy competition.

[QUOTE=EugeneTheJeep;11007532]Done a few foam houses, I always size the same as non foam as well.

Do you size them from the sidewalk btw your thumb and forefinger?