[MN Renovator]

This is very true about thermal mass, as a test I've had my house at 40 degrees, raised the temperature to 70 and after three hours it will be at 65 degrees but getting it to 70 from there battling all of the thermal inertia behind the thermal mass of everything in the house really slows things down. Did this last January heating the house up and allowing it to be at the set temperature for 12 hours (after the thermostat shut off the first time) and my heat load on the house went from 285BTUhr/f measuring it when it was at a solid temp before the start of a very cold period to about 400BTU/hr. Even after 12 hours the thermal mass still had a significantly large part of the load. I learned not to measure heat load that way. This morning it was 260BTUhr/F and it was -3.5f average outside from midnight to 7:30am. -13 is the design temp for my area 260BTUhr/f * 83 = 21580. 10.28BTUhr/sq ft, I'll take it. Actually I won't, still have work to do. 40k is the smallest condensing furnace I can buy but I'm not sure I'm going to go with a 2-stage since I keep reading that the lower stage is still less efficient than running high stage. As long as I can get a good thermostat that will keep the cycles efficiently long I think I should be fine though. Either that or using moderately deep setbacks like I do when I'm not heat load testing isn't such a bad idea. With this colder temperature the gas runtime is now at 17 mins instead of the 10-12 mins I get with warmer outdoor temperatures. I'd probably have 30 min cycles with a 40k condensing furnace, maybe a little more if I can set the temperature swing a little higher.