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Community and Q&A

Air-Source Heat Pump with Forced-Air Natural Gas Backup

MattFulkerson | Posted in General Questions on

I’m wondering if it is possible to run a forced air air-sourced heat pump, while keeping an existing (new) natural gas furnace for backup, that shares the same duct work.

I know this is easy if you have a heat pump with forced air and say baseboard on separate systems.

What would be needed besides a thermostat that could switch between the two systems?  Speaking as a Minneapolis/St. Paul metro area resident, where if everyone went with air-sourced heat pumps, we’d probably take out the grid when the polar vortex hits.

Thanks,
Matt

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Replies

  1. BirchwoodBill | | #1

    Are you looking at an air to water heat pump? What about a hydronic hot water coil through your air handler. Make that stage 1 and then turn on the air burner for stage 2? Use a heat pump for the mild weather and the natural gas for cold weather. I am the ne side of the cities and looking at a heat pump with boiler for backup.

    You could use the heat pump for DHW.

  2. charlie_sullivan | | #2

    This is a very standard combination for central ducted heat pumps, and the thermostats sold for them have the ability to set various criteria for when it switches over. The only limitation is that you can't run both simultaneously. You use the furnace blower to move the air through the heat pump coils. Good heat pumps want control of the fan speed, so you want a furnace with a variable speed blower, and the right interfacing, which can get tricky with mixing brands and ages of equipment.

  3. Jon_R | | #3

    As long as air doesn't flow over the furnace heat exchanger before the HP heat exchanger, what is the problem with running both simultaneously?

    > if everyone went with air-sourced heat pumps, we’d probably take out the grid

    No doubt that it will cause a large increase in peak grid load - far worse that the summer AC peaks we see now. Maybe someday this expensive peak load will be reflected in electricity pricing.

    1. Expert Member
      BILL WICHERS | | #4

      >"No doubt that it will cause a large increase in peak grid load - far worse that the summer AC peaks we see now. Maybe someday this expensive peak load will be reflected in electricity pricing."

      The peak pricing IS already reflected if you're on one of the "time of day" rates that have varying electric rates based on the time of day, and day of the week. I'm on such a rate (by choice), and my rate is around 18-20 cents/kwh for on-peak (11am through 7pm weekdays), and around 8 cents/kwh for the rest of the time (8pm through 11am weekdays and all day on weekends). I save around $30/month at minimum on this rate, and I have my car setup to charge starting at 7pm on weekdays so that it charges on the cheaper rate.

      The time of day rates are a good way to save money if you have the ability to shift your loads around somewhat, and they help the utility to balance loads throughout the day. Note that ALL of the larger service commerical rates are time of day type of rates, and the demand component heavily influences the rate larger commerical customers pay. This is why there are things like ice chillers :-)

      If everyone were to switch to all electric I think you are correct the grid couldn't handle it. I work in the utility industry too so I see more detailed info about this than most. To offset the current natural gas use (and to a lesser extent, propane and heating oil use), a significant amount of additional electrical generation would be needed, and associated transmission assets too. I don't see a transition like this happening anytime soon.

      Bill

    2. charlie_sullivan | | #5

      I think you could run both simultaneously with the HP coil first. I don't think that's the standard setup, but I don't know any reason it couldn't work. You'd probably lose the condensing feature of the furnace, and maybe that's an issue for getting a rating on the system.

  4. AlanB4 | | #6

    I don't see why not. In a sense this is how an AC unit works, its added to the plenum and has the ability to activate the blower motor when the coil is cooled. So in theory a similar type system for your air sourced heat pump will work fine when integrated the same way.

    What situations are you envisioning needing the heat pump vs natural gas furnace?

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