Tony Karp tkarp AT tlc DASH systems DOT com wrote:

We're going to be getting some estimates for geothermal for our house...

You might also consider a sunspace or solar siding.

Northern Virginia, just north of Manassas, about 30 miles west of DC.

NREL says 1040 Btu/ft^2 of sun falls a south wall on an average 30.6 F day
with a 40.1 daily max in Stirling VA, when a square foot of south glazing
might collect 0.9x1040 = 936 Btu and lose 6h(80-35)1ft^2/R1 = 270, for a net
gain of 666 [evil] Btu/day.

Last heating season, we used about 900 gallons of oil (it was colder than
normal).

So you need about 900x100KBtu/5006DD = 18K Btu/DD, eg (65-30.6)18K
= 618K Btu on an average January day, which might come from 618K/666
= 929 ft^2 of transparent polycarbonate siding.

More house insulation and airtightness could help as well.

Nick