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Dual-fuel high-efficiency natural gas / heat pump systems?

Jonathan Rich | Posted in Mechanicals on

Does anyone have any experience with or thoughts on the hybrid “dual fuel” natural gas furnace systems that are combined with an air-sourced heat pump (instead of an AC only unit)?

The intent is for the heat pump to provide heat when its most efficient, but then switch to gas when the outside temperature drops to the point where the heat pump is no longer efficient or effective.

I am planning a house in SE Michigan (zone 5), and this seems like a potential option. I am also considering a multi-split system, but I have not found a multi-split system that is effective below 5 deg F. If I go multi-split, I will need to have some sort of back up – probably electric resistance.

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jonathan,
    You are right that you can buy an air-source heat pump that doesn't operate well at cold temperatures, and then also buy a furnace to operate at cold temperatures. But that's a lot of equipment.

    Why not just buy an air-source heat pump that works well at cold temperatures? Mitsubishi makes ductless minisplit air-source heat pumps that work when the outdoor temperature drops to -13 degrees F.

    Here is a link to a Mitsubishi brochure:
    http://www.mitsubishicomfort.com/media/226460/h2i_brochure.pdf

  2. Jonathan Rich | | #2

    Martin,
    "Why not just buy an air-source heat pump that works well at cold temperatures?"
    What I'd like is a MULTI split that will work down to low temperatures. Mitsubishi does not (yet?) supply a residential multi-split capable unit like their MXZ 8B-48NA that works with their Hyperheat system. I'll talk to my local dealer about the MultiCity unit referenced in the link you provided, but seems like it might be overkill from a total capacity standpoint. If it works with the MSZ-FE indoor units - and doesn't break the bank - maybe this is an option.

  3. GBA Editor
    Martin Holladay | | #3

    Jonathan,
    The Lennox XP14 is an air-source heat pump with specs listed down to -15 degrees F. It has a heating capacity of 11,000 Btuh at 5 degrees F and 5,800 Btuh at -15 degrees F.

  4. Milan Jurich | | #4

    Jonathon,
    I'm having a home built in central OH, also zone 5. Very well insulated, good air sealing and mechanical ventilation. Original thoughts were to utilize a hybrid heat system from either Carrier or Bryant with their top of the line modulating natural gas furnace along with their modulating heat pump (Greenspeed or Evolution Extreme). At the end of the day, I opted for the Bryant modulating natural gas furnace combined with their Evolution 16-seer AC. Payback for the annual savings in operating costs just wasn't there to justify a higher seer AC or even the heat pump. 16-seer AC to their top heat pump amounted to only $40 annual savings at a $1700 higher cost. With loads being low to begin with when the home's envelope is well built, it just didn't make sense. Even though their best heat pumps can extract heat at temps in the teens, the economic break point was in the 40's (ie. significantly higher) when looking at heating with electric vs. gas. In other words, to run the heat pump below the 40's with natural gas heat available in a hybrid heat system would have been less economical. It really boils down to the #'s for your specific home and area of the country.

  5. Expert Member
    Dana Dorsett | | #5

    Since the house is still in the planning stage, you have control over the room-by-room heat loads, as well as the whole house load. Even without superinsulation it's possible to heat moderate sized houses with 2-3 well-placed heads. The notion that ANY normal house needs and 8 head multisplit is just silly, and the 1-head-per-room approach ends up with oversized heads everywhere and an oversized system, for both lower comfort and lower efficiency. In higher-R houses (even at well below PassiveHouse levels) it's often possible to use one head per floor. (A pair of -FE12NAs would come in well under $8K , installed price in most markets.)

    Dropping the heat loads of the doored-off rooms significantly with smaller and/or higher-performance windows (or even locally higher wall-R) and using the ventilation system to ease the temperature balances can work quite well. Even if it takes three FExx mini-splits it'll probably be cheaper up front and more efficient than a best-efficiency modulating ducted heat pump.

    A recent deep energy retrofit I was involved with is heated with one -FE18 per floor (it could have been -FE12s for at least two of the floors, but I digress...). Back-up consists of two $50 oil-filled electric radiators per floor. it could have been hard-wired baseboard, but why bother? (Maybe if the inspectors required it to be hard-wired you might.) When your heat loads are miniscule, so are the mechanical systems. Putting the real money into the building envelope performance buys you a lot more in comfort and security than any heating/cooling system can. The 99% outside design temp here is +5F, but it'll hit close to -10F once every five years or so. (This season's low was a relatively balmy -2F. Last year it was -8F.) But with the high performance envelope of the DER there is ZERO chance of a freeze up even if you killed power to the place during the cold snap of the century.

  6. D C | | #6

    Dual-fuel heat pumps are common in my part of the world (west Texas). They work well. You can choose the outdoor temperature at which it switches from heat pump to gas furnace. I'd bet these smart guys like Dana could calculate the optimum switchover temperature if the price of natural gas and electricity were known along with the specs on the unit.

    DC

  7. GBA Editor
    Martin Holladay | | #7

    D.C.,
    Yes, they work well. It's just a lot of unnecessary equipment.

  8. D C | | #8

    Martin,

    Does that mean that backup heat for traditional heat pumps is no longer needed? I prefer the warmer air provided by the gas furnace when it gets really cold but if heat pumps no longer need a backup, that does save some cash.

    DC

  9. GBA Editor
    Martin Holladay | | #9

    D.C.,
    The news is true: in Texas, air-source heat pumps no longer need a backup furnace -- as long as you specify the right air-source heat pump.

  10. John Williams | | #10

    looking at lennox xp25 dual-fuel (per builder's initial thoughts) for new build in franconia, nh, cant find technical specs on lennox website. anyone know where thye hide this info? looking to see what temps they are rated down to... of course ay firsthand knowledge is appreciated as well. thanks in advance guys

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