Heat pump, do I need to turn on aux heat when it's 10 degrees outside?

So, our upstairs is heated by a heat pump furnace. Someone told us if it's below like 30 degrees or something, we're supposed to turn on the aux heat. Right now it's like 10 degrees outside but the furnace seems to be able to maintain the 68 degrees. Is there any reason such as it's somehow bad for my furnace to run it in regular mode as long as my furnace can keep up? I'd rather not turn the aux heat on. I was told it was similar to heating my upstairs by a huge hair dryer. Costs a fortune to run. Figure it's cheaper with my furnace running almost non stop as it is in normal mode.

 

So you have a regular heat pump with electric backup?
Or a hybrid heat pump with fossil fuel backup?
If you have a conventional, your auxiliary heat is already running on its own. Cycling as needed for supplemental heat. And comes on when the heat pump goes into defrost.
Setting your thermostat to emergency heat shuts off the heat pump and only uses auxiliary heat.

 

I run natural gas heat in the winter along with my heat pump, but I live in Maine. If I only had electric heat as an alternative, I would not run that with the hp, but then the hp does lose efficiency at abt 20 so the cost of electric for that would be increased anyway. Our utility co has a reduced rate when you go over a certain value and own a heat pump, so you may ask them if they have a discount also.

 

The problem is that you cannot run a gas furnace and a heat pump at the same time. The heat from the furnace can cause the heat pump to run too hot.

If you have electric reheats.... they should be running automatically if your room temperature drops more than three degrees under the set temperature. If you have a gas furnace as your backup.... the heat pump should shut down and then just use the furnace. The furnace can run indefinitely during the cold period.

 

Your heat pump is basically identical to my GEO thermal, the elec heat is there for supplemental backup when the system cant maintain the selected heat setting, it's automatic so you don't "switch" it on. The advantage of GEO is that the water temp is relatively constant but the demand still goes up when cold!

It's a higher elec draw but not that much compared to the unit running non stop and is limited to the coldest times of the night.

Close all the curtains, shut down unused rooms, turn the temp down a few degrees, they all help reduce the overall heat demand!

 

All good heat pump with aux heat installations have a adjustable thermostat that automatically turns off the heat pump and powers the aux heat at the set temperature on a falling temperature. The set temperature should be derived by calculating the cost to operate the heat pump versus operating the aux heat for the same BTU output. The adjustable thermostat operates in reverse with a temperature rise at the set temperature with some hysteresis. Of course, the cost of electricity is a variable but once the set temperature is calculated, most people don't change the thermostat based on changes of electricity cost.

 

Every modern heat pump automatically switches to "AUX heat" when it gets too cold. You shouldn't need to manually switch the heat pump to AUX.
What is "too cold" depends on the specific system, "basic" heat pumps you find in areas that get a mild winter usually switch to AUX heat once it gets down into the 20s. For colder areas, newer 2 stage "high efficiency" heat pumps don't switch to AUX heat until it gets down to single digit temperatures.

 

Our furnace is only a few years old. I believe its high efficiency. So, I guess it's probably doing what its supposed to be doing.

 

A heat pump thermostat is not supposed to turn off the heat pump and turn on just the AUX unless an outside temp sensor is involved. Emergency heat turns the HP off and runs just the aux.

The correct thermostat for a heat pump with a gas furnace as backup needs to have "dual fuel" as a system choice. A thermostat set to dual fuel will not run the HP and furnace at the same time unless defrosting.

The bulk of all heat pump thermostats work in the same mode..... the heat pump provides the heating. IF the room temperature drops 3 or more degrees below the set point will start the AUX heat. The AUX will run with the heat pump until the system recovers. A better thermostat will have an outdoor sensor that locks the heat pump off if it's below a certain temperature. Many smart thermostats can also do this. They use the temperature as posted online.... not the actual outside temperature. Not as accurate.

 

PJMax, I don't disagree with your explanation of how some heat pump controls work, however my heat pump works as I described. The transfer thermostat is outside. It is an older installation and I am sure there have been advances in control designs as programmable devices become cost effective. I would also venture that no 2 manufacturers have the same program so there is a myriad of operational differences.

 

Yes..... you have a manual lockout determined by outside temperature.
That usually works pretty well.

 

If the heat pump condenser was physically before the gas furnace (as aux. heat) heat exchanger in the air heating path (main duct) then why would operating the furnace and heat pump simultaneously (say to raise the house temperature by a large increment quickly) cause the heat pump to run too hot?

Meanwhile there should be very few occasions when the main heat source (heat pump) and aux. heat source (e.g. fueled furnace) should need to run simultaneously.