Sizing the ground loop (Ground loop oversized??!)

[therik]

Hey guys, I've visiting this site for a while now learning all about ground source heat pumps and would really like to use one on our new home we are building. I've had a manual J and D completed, and now the ground loop design in coming back and I just don't understand how these folks are coming up with their numbers. Here are the details:

House size: ~5170 sqft. (I know this really doesn't matter, but thought you would like to know)
Location: Mountain area just East of Albuquerque NM (Very similar to Santa Fe, NM)
Elevation: 6890'
Ground temp: 66-68° (according to all the charts that I have seen)
Ground material: Limestone (we had a soils report done)
Heat loss: 90,510 Btu/h

Loop design will be vertical, we have 2 acres, but there is limestone just under the surface and trenching would be difficult.

Some folks (I don't know who came up with these numbers exactly) believe we need the following:

Bore holes: 9
Depth: 300'
Pipe size: not listed

What do you guys think? I know some of you have access to the computer software that can size these loops, would you care to run my numbers? I think that the size should be more like 8 bore holes at 250' each, and that may be over doing it still.

Please help, over sizing this loop will put this system out of reach financially!!

[54regcab]

90,000 BTU heat loss is quite high for a brand new home. Perhaps put more money into sealing the house and downsize the geo system. Weigh the cost of using backup heat on those super cold days vs. the cost of the larger loop. You may find payback time for the larger loop isn't worth the cost of of installing it. If your heat loss is 60,000BTU or less on all but 1 month of the year, is it worth installing 50% more loop to save from using backup heat to make up the 30,000BTU differnce for that month?

[SkyHeating]

What is your cooling load? Cooling load can be very important in determining loop size even in heating dominant climates. I did some VERY rough calculations and came up with 2,500 feet or boring assuming a 80K cooling load and two 4 ton systems through average rock with .75" piping(keep in mind i have no knowledge of your area and do not know limestone since we have none in my area)
I think 9 bore holes at 300' is probably correct and that 8 at 250' will be to short but I would need the cooling load to get a better calculation.

[Munkywrench]

What about going with an open loop well? As Skyheating said I dont know your area, I just know that here in Ga I have installed a few of these and they have worked great, it also eliminates the need for multiple holes. Again I know NM is a lot drier than normal, but is it not possible to tap into the aquifer at all?

[geodean]

the OP has moved on with his install. This thread is dead.

[VTP99]

the OP has moved on with his install. This thread is dead.

So what was his final choice ?

[geodean]

You can read all about it here http://www.greenbuildingtalk.com/For...lt.aspx#108748