Over the last 10 years we've had to replace our furnace twice, and happen to have both gas and a heat pump (a hybrid system). Each time, we had a different contractor do the installation...but both guys told us that they like to run gas below like 30 degrees. The second guy said he runs gas exclusively - no heat pump installed in his home. I asked what the primary differences were in terms of efficiency, and both claimed that temperature at the register is significantly hotter, producing more heat with less effort and therefore more efficiency.
I haven't done any scientific testing; the only data I have to measure the difference is my energy bill (which of course is subject to rate changes). But I have indeed observed that with gas, our bill is lower during the very cold months, and I'm able to see this because I have years of data on what we spend each month on energy.
I have a aux heater that kicks in when the heat pump can't keep up, and I have noticed this. When the heat pump is running, the air coming out of the vents is barely warmer than ambient. Definitely less than 70F. When the aux heater kicks in, the air is actually hot.
A side effect is that the heat pump needs to run a lot, because heating a house to 62F takes a long time with air that it 65F.
I haven't done any scientific testing; the only data I have to measure the difference is my energy bill (which of course is subject to rate changes). But I have indeed observed that with gas, our bill is lower during the very cold months, and I'm able to see this because I have years of data on what we spend each month on energy.
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