We are in the process of building a new house on one acre lot in Connecticut.   The house we decided on will have approximately 3500 sq feet (900 on the second level and 2600 on main level).  The house is a two story design with four bedroom and 3 ½ baths.   The full- walkout basement when finished will add on other 1800 sq feet.  The exterior walls will be 2X6 and we will install energy efficient windows and insulate walls with spray foam.  The house will have radiant floor heat.  Since the home will be served by septic system, we have limited area available for loop installation.  In the front of the house we have access to area 130X60 and 110 X 40 on each side.  We would like to go with the horizontal loop since we own couple of excavators.     

I'm wondering if anyone can offer me some information on:

1.       Do we have adequate land available to install horizontal system? If yes,

Should we go with four pipe trenches or vertical slinkys?

2.       Is 7 tons unit will be efficient enough to provide cooling and heating for the whole house? One of the subs provided us with heat and cooling loss calculations.  129K Btuh for heating and 60K Btuh for cooling this does not include the basement area.  They seem a little high to us, but we are not experts. 

3.       Should we have 2 smaller units or one larger one?  We prefer to go with one however the local installer insists on two.  

I'd appreciated any info you all could give.  Thanks

129K for heating does sound high, about double what I would expect.

7 tons will give you about 90% of the heating load. If you do 4 pipe trench you will need 1300' of trench. Vertical slinky, 1100' of trench. Looks like you have enough space.

Most companies don't make a 7 ton unit, plus it doesn't sound like you need one that big.

I'm a bit skeptical of the heat/cool load as well. In my area 3/4" 6 pipe systems are about 150'/ton depending of course on soil....

I'm also skeptical of those loads. If you're using spray foam, and a window of .31 or lower U-value, those loads can't be right. Maybe I'm missing something.

I have worked with one installer before that insists on over-buffering his loads. He installed a 5 ton geothermal on a load that was 3.5 tons. It wouldn't cool, it wouldn't heat. The duct was only big enough for 3 tons, and 3.5 would have pushed it. Anyway, get someone else to do an ACCURATE load calculation. Unless your ceilings are maybe 20' high per level...

Loads seem high to me as well, based on square footage. You haven't specified how much glass you are including - huge walls of glass raise loads

It may be tough to fit a single large unit into a 3 story zoned setup since present 2 stage systems operate at 70% of capacity on low stage. If just one smaller zone calls on a 6 ton unit, 4 tons of air needs to move into the one zone.

An oversized, underducted system is a disaster - noisy, inefficient, uncomfortable, and potentially short-lived. Don't let that happen to you!

Thank you all for your input.  The windows are by Marvin; double pane w/Low E II coating.  The house will have 142 square foot of windows on the second level; 530 on main and 66 in basement.  The height of the each floor is 9 feet except the great room area (18’6”X29’2”) which will be 2 stories high.  The layout of the first floor is pretty much open except for the master bedroom.  The insulation values for walls: R-21 and ceiling: R-31.

Recently, we received a new quote.  The contractor estimated the heat load at 81k Btu's, he strongly recommends 2 units; 5 ton for the first level and 2 ton for the second. 

I still welcome any thoughts and suggestions. 

The load doesnt seem that far of to me 100k would be my guess . are you north of torrington or around springfeild ma. , cathediral ceiling's . have not yet touched water source but I do know slinky coils can cut the required land.

What type of insulation are you going to use? Fiberglass? (fiberglass = crappy insulation) Why only an R-31 in the ceiling? Go with at least R-44 ceilings.

Also, where is the duct to be located? In the attic? What outdoor temp. are they sizing to? Is it realistic? Unless you're using fiberglass, I just can't imagine your load is really that high. Maybe I'm missing something else. How much does humidity affect your heating loads? We don't have to deal with it in our neck of the woods...

In our area, I'd guess your load to be correct at the 81K if you're using fiberglass insulation, and there is attic duct work. If using cellulose, and no attic duct, you could see a load of around 65-70K. If using foam, you could be around 50-60K BTUh. I usually size to -10 deg. F. here as we typically hover in the -10 to -20 deg. range for weeks at a time during the night.

As a side note... I just discussed with a home owner today. He's insulating his entire structure with foam. He'll have radiant floor heat, and forced air cooling. It's a main, basement, and bonus room, totalling about 5400 s.f. There will be some attic duct, which he agrees to have spray foamed with at leas 4". The load I calculate for the heating of the home is about 52K BTUh, and cooling of about 33K BTUh. He understands that he must follow the recommendations to be able to keep the load to this sizing.

His duct installer is going to do 2 air zones. The main floor as a zone, including the bonus room, and the basement as its own zone. The breakdown of the cooling loads for these zones are:
main level: about 25K BTUh
basement: about 8K BTUh.

The duct guy is insisting that the main level needs 4.5 tons, and the basement needs 2 tons. He has finally done his own load calculation. I found out that his ACH, or air change rate, which is infiltration, is set at .50, and his SHR is set to .70. I can't convince this guy that foam uses an ACH for cooling of between .05 and .08. Also, in Utah, the lowest SHR (sensible heat ratio--has to do with humidity) should only be .85.

With an ACH of .50, the duct guy is not giving the benefit of the foam insulation. Why would someone want to ignore such a difference? I agree that if that were really the case--(btw it would take an aluminum frame, single pane window, unchinked log wall, or poor fiberglass installation to be that bad for cooling)--it really would take a 4.5 ton or more for just the main level's cooling.

My point in rambling on--it seems I tend to do this a lot--is that the load needs to be accurate to the actual situation. You should do everything you can to get the load down, and your geo installer should be able to make recommendations as to how to accomplish that. If they can't or won't, find someone else.

As far as cutting the required land, check out EarthLinked DX. Call Mel Hensch. He's the sales rep and should know some reputable installers for you. 508-328-4735 is his phone.Tell him I told you to call.

Any load calc worth a flip CALCULATES, rather than 'sets' SHR. Maybe the duct guy just came off a job with a Marijuana grow house in the basement or attic.

Clark, maybe your HO would be well advised to let his "duct installer" run ducts sized to provide the tonnage he thinks is needed. Then right size the equipment at the much lower tonnages actually needed taking into account foam and low ACH. This might actually result in a system with reasonably sized ducts for the lower load. Finally, set blower CFM per ton at rate needed for calculated SHR - could be 450+ CFM / ton.

I have my blower wandering along a dead slow 250 CFM per ton, but that's not right for the rest of the country.