1st foam house - seem to be undersized ?

[MM#7]

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.

nice post!

[EugeneTheJeep]

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.

That is how my foam jobs are, bid the print based on fiberglass and during the install get the surprise that it will be insulated with foam during rough in. Then worry it might be too big or too small.

[frumper15]

That is how my foam jobs are, bid the print based on fiberglass and during the install get the surprise that it will be insulated with foam during rough in. Then worry it might be too big or too small.

Yeah - the HO didn't seem to think it was a big deal - the Foam contractor was also supposed to be installing a fresh air damper as part of his work (which he didn't) so I need to get back there to check IAQ. I'm sure their CO2 is high unless they keep the windows open a lot.
At least the Geo was 2 stage, but even 1st stage cooling capacity made me a little worried about latent capacity and dehumid capability during the summer months with the reduced cooling load. That reminds me, I probably need to give the HO a ring and see how things have been going. Pretty much seen all 4 seasons now with the system.

[air2spare]

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

[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.

[ChaseAir]

[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?

[ChaseAir]

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.

[air2spare]

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

[jpnatlanta]

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.

[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.

[teddy bear]

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

[jimj]

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*.

[EugeneTheJeep]

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