Okay so I had a new house built with a 4 ton geothermal unit for 4500 sq feet in memphis zone 4. My manual J states that it should be okay until it hits 17 degrees then i need supplemental heat. In reality I get in trouble after it gets under 35. My HVac guys says the unit is working normally. They might be able to boost eficiency a little on it. The house is fairly tight with blower door test coming in at .5 2x6 construction with cellulose, and 1 inch blueboard sheathing. The contractor forgot to put in foundation insulation(which really ticks me off) and only placed r-30 in attic(manual J called for R-50). I have a radiant barrier in roof but so far have done okay in cooling. My question is...is it worth it to beef up atic insulation to r-50 and to bury the ducts in insulation to try to get my heating better? I understand the ducts may sweat if I bury them but I need some drastic things to help the efficiency of this unit. SHould I try to tighten up the house some more or will I hit a wall of diminishing returns. Thanks

ACCA Manual 'J' heating and cooling loads are determined by indoor, outdoor and construction conditions. If the house was not built to specification the Manual 'J' must be done over with as-built information. Once you have the new Manual "J" you will know what must be done and have a better idea on ROI.

Since your average mean temperature in January is 36°F foundation insulation would be in order. Without the revised Manual "J", not advised, I would look to the attic and and the first couple feet down at the perimeter and look to my builder to pay for it.

It is refreshing to see a heating appliance that is not grossly over-sized and a condition that can be fixed while improving comfort and lowering the fuel bill.

I did make sure to have them extend the 1inch blueboard over the sides of the foundation as far as possible. Should i dig it up to add more or will additional not be worth it at that point. My contractor and I did not part on good terms so I would have to sue to get him to do the work and that would be pricey in and of itself. Thanks

Driebel, Was that 0.5 ACH 50 or 0.5 ACHN? If the house was tested at 0.5ACHN then a lot of air sealing is in order. If it is 0.5 ach50 then you are in great shape. (0.5 ACHN is about 10 ACH50) Did you have the ducts tested for leakage? If allot of the ducts are in the attic, air leakage can be a huge contributor. One option may be to have the ducts air sealed, then covered with 2” of CC foam, and then bury them in cellulose. If you have the room and the ceiling can handle the weight, run up to about R-55 and bury the ducts. Cheers, Eric

Is there a reason you did not put this on the geothermal forums?
There is a trouble shooters guide there that asks questions. The answers to those could help us help you. Not knowing where you are, what insulation is on your ducts if any, what your current heat load is etc. makes it impossible to do anything but speculate.

My first SWAG would be the thermostat is overengaging auxiliary (not allowing geo time to work).........

Im looking into getting a more accurate ACH number and an updated Manual J. Sorry Joe, I thought that this was more of an insulation problem than geothermal issue. I like the foam over the ducts option and will get that priced out.

If you input the blower door test with a proper Manual 'J' (we use Wrightsoft) you will have a very accurate load calc and can then consider options for meeting the loads.

Your self diagnosis is limiting the information you provide and completely undermining any chance of helping you. I pointed out some of the things that you could share that might help us help you and the only question you cleared up is why you didn't post on the geo forum.

You titled this "Undersized unit" and mention that you have a manual J, but haven't shared your loss and gain btu's. Not knowing that or the nearest major city I have no way to know if you are oversized or undersized. This means more insulation may or may not help (it certainly can't hurt of course).

Good Luck

Joe....go back and reread the OP. He tells where he is.

robin, there is a 5F design difference between Lansing and Detroit. Though Howell is on the fringe of the Detroit zone, I use Lansing design temps here.

Anyway, what's your point, I still don't know gain/loss. OP determined the unit is undersized and that there's an insulation problem. Maybe that's correct. If I had answers I could confirm or debunk the theory to assisst in rectifyingthe problem.

Posted By drlebel on 02 Jul 2014 10:27 AM
Okay so I had a new house built with a 4 ton geothermal unit for 4500 sq feet in memphis zone 4. My manual J states that it should be okay until it hits 17 degrees then i need supplemental heat. In reality I get in trouble after it gets under 35. My HVac guys says the unit is working normally. They might be able to boost eficiency a little on it. The house is fairly tight with blower door test coming in at .5 2x6 construction with cellulose, and 1 inch blueboard sheathing. The contractor forgot to put in foundation insulation(which really ticks me off) and only placed r-30 in attic(manual J called for R-50). I have a radiant barrier in roof but so far have done okay in cooling. My question is...is it worth it to beef up atic insulation to r-50 and to bury the ducts in insulation to try to get my heating better? I understand the ducts may sweat if I bury them but I need some drastic things to help the efficiency of this unit. SHould I try to tighten up the house some more or will I hit a wall of diminishing returns. Thanks

Placing ducts & air handlers above the insulation increases the heating & cooling loads the system needs to serve, and is a bad (if legal & common) practice.

R30 is below the R49 specified minimum for climate zone 4 per IRC 2012 , so yes, it's worth adding to the attic insulation, independently of issues with the mechanical system's performance or sizing.  That upgrade alone is unlikely to make a fully (35F-17F=)  18F difference in the crossover point to where auxilliary heating is necessary, but it's still going to make some difference, and will be worth it on a long term energy cost savings basis.

If the ducts are poorly insulated you may indeed run into condensation issues when you bury them- pictures of the installation & specs for the existing ducts might be useful- it's pretty easy to screw up the duct flow volumes with a sub-par installation even if they get the duct sizes right. You say the house is "fairly tight", but has the duct tightness been verified with a duct-blaster? (If yes, what was the cfm/25 leakage?)  When ducts cross through the pressure boundary of the house like that duct leakage adds very substantially to the infiltration rates, and thus the heating/cooling loads whenever the air handler is running (a primary reason why installing ducts & air handlers in vented attics above the insulation is bad practice.)

Insulating at the roof deck to bring the whole-shebang inside of conditioned space would probably have a more significant effect on the heat load & crossover point, but that solution comes with a hefty price tag compared to adding another R20-R30 as cellulose on top of the existing attic insulation.