We have a 1970s - built house in Southern Oregon, which is currently heated by a heat pump. I'm considering replacing this system with a hydronic system, but am curious about what the operating costs will be. For the sake of arguement, let's say that the house is ~1600 SF, and has 2x4 insulated walls. I've done some preliminary calcs, and sized hydronic radiators to deliver 30,000 btu/h. If I'm using a gas-fired micro-boiler at between 84 and 90% efficiency, how do i go about determining how many therms of gas it would take to operate?

Since there are 100k BTU / Therm, if I needed 30,000 BTU/H continuously, that would equate to
(30,000 BTU/H x 24H/Day x 30.5D/Mo)/ 100KBTU/Therm = 219.6 Therms / Month? I'm sure that's way too much.

What percentage of the time does a boiler need to be boiling?