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Sizing of gas furnace in dual fuel system

ireneheim | Posted in General Questions on

I am replacing an aging gas furnace with a new one that includes a heat pump coil.  The existing furnace used to heat the house all by itself and is 66,000 BTU.  In the new set-up, I will have 24,000 BTUs worth of minisplits, plus a 18,000 BTU coil in the furnace.  Still, the contractor wants the new gas furnace to be almost as powerful as the old one, namely 60,000 BTU.  I asked whether that wasn’t oversized, given the help from the heat pumps (which are all Mitsubishi Hyper-Heat models).  He replied that it wasn’t, because “we have to make sure the furnace can handle the whole heating load in emergency temps”.  Should I accept this answer?
(In case it’s relevant:  The house is in Massachusetts and has about 1,650 sq ft of living space.  The old furnace never left us too cold, even before some recent upgrades in insulation.)

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Replies

  1. Expert Member
    DCcontrarian | | #1

    What is the purpose of the gas furnace? Is it because the heat pumps are undersized for the weather that you get? Is it because you want to have a backup in case the heat pumps conk out? Or is it because below a certain temperature gas will be cheaper to operate?

    Those are three different reasons, and the answer to your question depends on which you're sizing for.

    1. | | #

      The first reason you list is the main one: The existing heat pumps are undersized for heating the house by themselves in the coldest months. And I am not considering getting bigger or additional heat pumps at this time, because (a) I already bought the two I have and they are relatively new and (b) I have no outdoor space for a third one.
      A second reason is that I prefer not to heat my house through the night, and it seems like heat pumps are not good at raising the temperature quickly in the morning (say, from 58 to 68 degrees in no more than an hour).
      I am less concerned about cost and would be okay with paying a little more for cleaner energy, but within limits, i.e. not if my heating bills were to double.

  2. ireneheim | | #2

    And forgot to mention: I have only 100amp service in the house, which would probably not be enough for the kind of electric backup heat I would need if there was no gas.

  3. gusfhb | | #3

    60k is already a small furnace. I see them down to 40k. I kind of doubt you are paying much more for the larger, or that the larger is significantly less efficient.
    As much as it would personally gall me to accept the larger, I don't think it is a big deal, considering it is essentially back up .
    When I did the boiler in my house, I had to basically bring the contractors in and say' Quote the install of this boiler model #' They were counting feet of baseboard and other silly things.

    In Massachusetts it will probably be cheaper to run a 90 percent efficient gas furnace than heat pumps. Not that money is everything, but our electricity is high.

  4. krackadile | | #4

    Why would you want to buy a smaller furnace? What is the cost savings or is there any at all? Would a smaller unit fit in the same location or would the ductwork require reworking and cost more than just replacing the unit with the same sized unit? It is probably not worth the cost to reduce the size but regardless you'd probably be just fine either way. Looking at prices online between different sizes of furnaces you're probably looking at a $250 savings to go from a 60k btu $1700 furnace to a 30k btu $1450 furnace.

  5. gusfhb | | #5

    Just make sure they are not installing some old school furnace, whatever the size. Because you anticipate low hours and use at max output, as long as it is a sealed combustion furnace I would think the 10 year cost to operate between any of the options[86-98 percent] would be in the noise. I don't even know what they allow but contractors can be amazingly resourceful at finding janky stuff when they want to.

    1. Expert Member
      BILL WICHERS | | #6

      You're unlikely to find anything less efficient than an 80% unit. I would normally consider an 80% furnace if it's only a backup and won't run much, because their IS a big difference in cost between the 80% units and the "condensing" units in the 90+ percent range. The big downsides to the 80% furnaces, besides the lower efficiency, is that they are not sealed combustion (i.e. they draw their combustion air from the room where the furnace is installed), and they need metal vent pipe.

      Sealed combustion 90+ percent units have seperate in and out lines for combustion air to be drawn and exhausted to the outdoors, and you can use PVC pipe for those lines. The big downside is higher cost.

      If you are building a very well air sealed house, the sealed combustion part of the higher efficiency furnaces is a big plus. That would be my preference unless you're tight on money, even though the backup units are probably not going to run that many operating hours compared to what they would be doing if they were the primary heat source for the structure.

      Bill

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