Is it OK to Run a Gas Furnace and Heat Pump While in Defrost Mode? – Ask the Expert Episode 110

Dual fuel systems consist of a heat pump and gas furnace, where the heat pump is the primary source of heat and the gas furnace acts as a source of backup heat.  In this setup only one source of heat runs at a time, but can you run both at the same time? Find out what Dan Danowski has to say in this episode of "Ask the Expert".

Video Script:

Our first question for the day is from Bill from the internet in America. My question is on a dual fuel system: is it OK to run a gas furnace and heat pump at the same time while in defrost mode?

Yeah, you can. If the heat pump is in defrost mode you can have the gas furnace going. If it's not in defrost mode, that's bad. You don't want to run the furnace and the heat pump at the same time. The hot discharge from the furnace will go onto the hot coil and cause the compressor to go out on high pressure.

It could explode and kill everyone within a 30 mile radius. I'm just kidding, it won't do that I don't think. I've never heard of that.

What I would recommend is if it gets to the time of year where you're going to have low temperatures, you're probably just going to want to run the gas furnace and maybe not the heat pump.

The point is, you'll want to run those separately unless the coil is in defrost and that's OK.

Have more questions? Just ask them in the comments below or on Facebook or on Twitter.

6 comments (view/add)
  • Dennis Goethe
    Dennis Goethe
    Posted on 8/7/2018

    I have gas forced air for my central heat. I just installed a Mr. Cool DIY 24,000 b.t.u. split system. Would there be any advantage to using the heat pump mode of the Mr. Cool when the outside air is 50 degrees instead of turning on the furnace? Electricity is a bit more expensive than natural gas in this area (most of the time).

  • Kyle
    Kyle from Ingrams
    Posted on 8/7/2018

    We would need more information to say for sure one way or the other. It is most likely any savings from the heat pump would be negligible.

  • Bill
    Posted on 9/20/2018

    What exactly is the difference between a communicating thermostat and a regular thermostat? Will a communicating thermostat operate the equipment better or more effectively?

  • Kyle
    Kyle from Ingrams
    Posted on 9/20/2018

    A communicating thermostat is usually defined as a programmable thermostat that can operate wirelessly. Properly programmed, they can definitely help save energy.

  • Cristian
    Posted on 10/6/2019

    Hi Dan, I have a RUUD high efficiency gas furnace model; U96VA0602317MSA W051902203 and is NOT a heat pump that was installed by a local company during the spring of this year. I started the furnace a few days ago for the first time and noticed that the control board displays a "df" during the entirety of the heating cycle. Is this a normal operation even though is not a heat pump? The outside temperature was low 50F's. System is central forced air electric and gas for heating.

  • Kyle
    Kyle from Ingrams
    Posted on 10/7/2019

    Hi Cristian. The information I have indicates that the “df” code is for defrost. I do not understand how the furnace would display this code unless either the unit was programmed for use with a heat pump or perhaps the board is bad? In either case the installer should be willing to come back and get it corrected for you.

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