It seems intuitive that heat exchanger eff increases with decreased flow rate. Also air handling is always best done as slowly as possible, because you're tryi

It seems intuitive that heat exchanger eff increases with decreased flow rate. Also air handling is always best done as slowly as possible, because you're trying to move air, not make noise and turbulence. There doesn't seem to be any intuitive reason to think eff would be higher at low rates unless its doing something insane like varying fuel/air ratios to get a desired heat flow or some kind of slow PWM with repeated ignition losses. Real furnaces don't work like that, so... My otherwise boring furnace is variable speed and the longer its on, the faster it runs. It's really roaring after about 30 minutes. Its more than a decade old, this is pretty old COTS tech. My first zillion BTU cost me somewhat less than my last zillion BTU in any heating cycle. I live in a cold northern climate with great insulation and low heating bills and a variable speed furnace and the house is basically never empty (spouse and I work at home often enough, etc). The whole "nest" idea doesn't work financially for me although it is interesting to watch.