I know that heat pumps are very efficient within a certain temperature range. Some say below 40F it becomes very inefficient. Others say it's below 32F. Do you know with more certainty?

I have a heat pump with gas backup so I can switch between the two. What would be the good temperature to switch to gas? Also, can I run on gas for long periods (like all winter) or would it hurt the heat pump for some reason?

Thanks,

 

I ran a curve of temperature at an indoor vent (bedroom register) versus the outdoor temperature. Here are some points along that curve.

OUTDOOR TEMP 64 50 40 30 25 23
BDRM VENT TEMP 104 101 95.5 88 82 80
These are in degrees F.
The location is SW Missouri. Humidity was not considered in these readings.
The system is an 18 year old Carrier 3 Ton split system heat pump.
As can be seen, there is still good heat to be had even at 23 deg.
I have the electric strips to cut in at about that point (23 deg)
If I could I would let the pump run along with strips to perhaps 20 degrees outdoors as it seems to me this lower cost heat could be added to the more expensive strip heat. The pump alone in 23 deg weather may not be sufficient to keep the house comfortable.
This data applies only to my system but you could run a curve for yours.

 

There in central Missouri we let them run all the way down.Most pumps have a COP of about 30.What that means is at 30o out side you get about 3 times more heat for your dollar as on a strip heater. The strips should turn on when the H/P cant do it .. That is why I dont like programmable tstat on a H/P when it tries to pick back up the temp in the home it will turn on the strip heaters. So like lose what you saved.

yes you can run all winter on the Gas if you want. For you there go to http://www.warmair.net and compare fuel cost . That will tell you what is best fuel to use there
ED