rustico

the question is how efficient the house is at insulation.

the other is your design limit.

The Average cold weather is 49F

9 days have broken 32F in 140 years.

The coldest day recorded was 25F.

i would suggest designing to 33F, and if you get an extreme cold weather event get some space heaters.

and set 4241 degree days for heating.

http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

here’s a good calculator, set your Insulation and details, and calculate your losses to match the BTU’s in your tank