You're right - they said -20F. I didn't leave out the minus sign, I didn't even see it!, and it's interesting why. Thirty years ago my parents had a house havin

You're right - they said -20F. I didn't leave out the minus sign, I didn't even see it!, and it's interesting why. Thirty years ago my parents had a house having both a gas furnace and a heat pump that worked in tandem. They had a problem with their furnace that they didn't immediately fix because they were using the heat pump. Then they got the bill. It was nearly 3x more expensive than usual. My dad immediately got the furnace fixed. What I recall my dad saying is when the furnace guy came out to fix the furnace my dad was talking to the furnace guy about the bill. The furnace guy said heat pumps get very inefficient below 20F. So my dad's system was designed to run the heat pump until the outside temperature got to 20F and then it would switch over to the furnace, if available. If it detected the furnace not coming on then it would continue to run the heat pump. So I've had it in my head ever since that you don't want to run a heat pump below 20F. Are the new heat pumps different? Is it now -20F instead of 20F before your costs soar?