Hvac peoples

I've got a heat pump in my place. 3 floors, over 4k Sq ft. It's a bargain unit sized for the upper two floors (too small) and there are minimal registers in the basement. It keeps the place no lower than 65 when we hit 0, but really wants to burn propane (no NG in my neighborhood) once it's under 20F. I have way too much glass for as far north as we are, but I didn't design the place and bought it for the land more than the house. Anyway, high ceilings, drafts (too many doors too), and lots of glass means heat loss so the pump always runs. If you oversize to get more heat, then typically it will over cool and that can actually cause damage.. So it is what it is. A good heat pump install relies on above average insulation if you're dealing with low outdoor temps. Heat pumps are primarily good for cooling. They heat as a benefit, not necessarily by virtue of their design.

The coefficient of performance or COP is a measure of efficiency. A COP of 1 is 100% efficient. A good unit will run 1.2-1.8 in its design range which they tend to target unrealistic envelopes like 40-165F. The reason they can go over 100% efficiency has to do with what happens when you compress a gas to a liquid or evaporate it back to a gas and which side of the conditioned envelope the heat exchanger is on.

Fiesr thing I'd be doing is looking at your last couple years of gas bills and try to determine how many btu you're using in a season. Trying to figure out "number of heating days" can help you determine a average btu per day based on weather. From that you can do some math to compare btu to cubic feet of gas to kWh and COP to determine if a pump would benefit you or not. Being 120% efficient is awesome, but not if gas is 40% cheaper per unit energy than electricity.

For example, I did all the math on my place once and found that a kWh is cheaper than a comparable measure of propane, so it makes sense for me to use as much electricity as possible. If I had NG, it would be the opposite. NG came out something like 20% cheaper per energy unit, but since I'm miles away and across the river from a hookup it's moot. Then again, a furnace wouldn't cool my place and in the summer we run the ac pretty consistent for a month or so, and it makes a world of difference.

There are worksheets out there on hvac forums to help with the math on all this stuff. A good hvac company would do the work for you and explain it all. If they can't or won't, then they're not worth working with. Their job is to get the customer what will work best. If you go with the cheapest install, you're bound to regret it.

Also keep in mind that if you're in an older home that was never air conditioned before it may be unwise to install one without significant remodeling. If you get interior surfaces of your home cool enough to go below the dew point in summer, you're bound to get condensation in walls and cavities and you'll have mold and rot rather quickly. Again, a quality installer can help you through these kinds of considerations.