HELP...Two system - heat pump vs. gas water baseboards.

cobrasneat

I have a two system heater. One is gas hot water baseboard and the other is electric hot air pump. What is the best outside temperature to switch from electric to gas to give us the best cost savings and most efficient heating system? Thanks.

Jamie Hall

Depends on the characteristics of the heat pump. There is no single good answer. Certainly a higher temperature than when the backup electric resistance grids switch on, though! And probably somewhat higher than that. What you need to look at is the COP of your specific heat pump vs. temperature.

cobrasneat

boiler is afue of 81.8 and heat pump is afue of 100. what's a cop ?

Jamie Hall

The COP is a measure of how much energy (electric) it takes to deliver a certain amount of heat energy. Can vary from 3 or 4 on warm days, or with a warm source (meaning you get about 3 times as much heat as you would get with resistance heating) to considerably less than 1 on cold days (meaning you'd be better off with resistance heat). The AFUE of a heat pump is meaningless in the real world. IMHO

JUGHNE

Assuming the heat pump is ducted with outside unit?
I would drop the heat pump use when the outside temp frosts up the outside coil and requires auto defrost.
Defrost requires the inside electric resistance elements to come on (assuming you have them).
The efficiency of air/air heat pump drops off with the outside ambient temp.
COP might be 3 above 40 and drops from there.
I would go for the most comfort of the house.
Avoid any defrost time. IIWM

mikeg2015

The answer depends on the price of your electricity (factor in stepped winter rates if you have them) and price of your gas and the inefficiency of each. Economic Balance point could be anywhere from 15F-40F. Above 40F, most all heat pumps, even older ones are make BTU's cheaper than a boiler, unless you have very high electricity. For comfort, above 45F, heat pumps are much more even, but they can be drafty if oversized (most AC systems are)

On my system with 8.5HSPF, I added a 3% loss for piping and boiler effectiveness since it's in the basement and my economic point is around 15F which was a COP of I think about 2.5. But my thermal balance point was around ( where AC starts falling behind) 40F for my downstairs HP and 30F for my upstairs HP. But my I can disable strips in defrost.

My Lennox S30 thermostat has been doing an excellent job intelligently choosing which to use based on the temp differentials and time delays I've set. Temperature swings less than 1F each hour.

Al that being said, under 40F, the comfort of the steam radiators is much better. But above 40F, the steam system overshoots quite a bit.

However, I also run a whole house bypass humidifier, so the heat from the heat pump makes that more effective.