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Community and Q&A

Heat Pump +/- gas furnace

Stockwell | Posted in Mechanicals on

I am building in the mountains of western NC, Zone 4. The bid calls for a Trane 17 SEER, 9.6 HSPF two-stage, variable speed heat pump with a gas furnace backup.The house will be about 4000sf on 2 floors over a crawl, very well insulated, tight, triple pain windows with 10kW of solar. Do you think this is the correct choice for HVAC or is there something better(always considering cost, of course)? Is the gas furnace backup needed?

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  1. Expert Member
    Dana Dorsett | | #1

    To answer the question we would need to know the design heat load of the house, and the outdoor design temperature, the model number & tonnage of the heat pump, and the BTU ratings of the furnace.

    There are some 4000' houses that have a heat load of 20,000 BTU/hr @ +15F (or whatever your 99% outside design temperature is), others that have a heat load of 60,000 BTU/hr. That's the difference between a 2 ton heat pump and a 5 tonner.

    Have an engineer (not an HVAC contractor) run the heating and cooling load numbers using Manual-J methods, and AGGRESSIVE rather than conservative assumptions, and make sure they understand the U-factors and SHGC numbers of your triple-panes, the R-values and stackup of the walls foundation, attic, etc. and make sure they assume air tightness that's tighter than a code-max 3ACH/50, and heat recovery ventilation rather than ventilation a ASHRAE 62.2 rates without heat recovery. Even if it costs you a grand up front for that service (it would be less than that in my neighborhood), it will save you more than that in up-front equipment costs.

    It's cheaper to use resistance heater heat strips in a right-sized heat pump as the "backup" than installing a gas furnace. With the heat strips engaged the heat pump is still delivering the lion's share of the heat, and only when needed, in the amount needed. Dual-fuel setups are pretty kludgey, and can only be operated either the gas furnace, or the heat pump, and not both at the same time. With the heat strips sized to cover the design heat load you're covered even if the heat pump fails, and when it's just a matter of running out of capacity at outdoor temperatures colder than your 99% outside design temp, the amount of resistance heating use is small.

    If you can avoid the cost of hooking up to the gas-grid altogether and plumbing your house with gas, applying that money toward more PV solar (to cover, say charging up your electric vehicle, or higher than expected use of the heat strips) would probably be a better investment.

  2. GBA Editor
    Martin Holladay | | #2

    This is a big topic.

    Briefly, the first step to designing a heating system is to perform a heat load calculation. If you're not familiar with heat load calculations, see these articles:

    Saving Energy With Manual J and Manual D

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    Who Can Perform My Load Calculations?

    For more information on designing a forced-air heating system, see this article:

    All About Furnaces and Duct Systems

    A few more points:

    1. Most green builders spend a little bit more on their thermal envelope (better windows, thicker insulation, and more extensive air sealing measures) so they can have a simpler HVAC system.

    2. If you have a well designed system with an air source heat pump (or minisplits), you don't need a gas furnace for backup.

  3. walta100 | | #3

    Our houses and weather seem very similar.

    If you have low cost city gas available on site that is most likely the most comfortable and lowest cost heat source.

    If your goal is be a net zero home or your gas is propane then a heat pump with electric back up makes more sense.

    The duel fuel sounds like a good idea at first thought but then you realize you have run up the equipment and install costs for little savings.

    I think the XR17 in your bid is one of their lower end units and lacks features I feel are necessary for high performance system.

    1 Variable speed compressor.
    2 Two electronic expansion valves one indoors and one outdoors.
    3 Variable speed indoor blower motor.
    4 Communicating thermostat and equipment.

    I have a Rheem RP17 heat pump most brands will have a model or 2 that have the suggested features, find the right installer use his brand.

    For the last 2 weeks we have had some very cold weather down to 0° and I have had 7 hours of electric heat operation I think most of that was during the 49 defrost cycles. My electric heat only operates when outdoor temp is below 7° or when defrosting the outdoor coils.


  4. GBA Editor
    Martin Holladay | | #4

    If your house has a 10-kW photovoltaic system, there is no reason to invest in gas-powered appliances (as Dana noted). You want an all-electric solution (and you don't want to pay any monthly fees for gas service).

    Use some type of air-source heat pump -- ductless minisplits, ducted minisplits, or a conventional forced-air system designed for cold climates. Just make sure that the system can handle your minimum outdoor temperatures.

  5. Stockwell | | #5

    Working on the Manual J now. I would go all electric but for one reason. The area I am building in is prone to frequent and extended power outages. A natural gas powered generator is an economical solution vs. battery backup for my solar. I don't want to install an LP tank, which would have to be buried and refilled, when gas is available. I will also look into alternatives to the Trane line of heat pumps. I'll be back with the Manual J figures when I get them.

  6. Stockwell | | #6

    If I look at the top of the line Lennox unit, this is what it says:

    Outdoor unit will not operate in the heating mode when
    the outdoor temperature is at or below –4°F. If the unit
    is operating and the outdoor temperature drops below
    –4°F, the unit will continue to operate until the room
    thermostat is satisfied or the outdoor temperature drops
    to –15°F

    I assume this is almost useless info unless they tell you how many BTU's the unit will produce at a specific temp? Is that information readily available?

  7. Expert Member
    Dana Dorsett | | #7

    Most manufacturers will have "extended temperature capacity tables" indicating the heating & cooling capacity at a range of outdoor temperatures, not just the HSPF and SEER test condition temperatures. If the Lennox unit has -4F as the lowest specified operating temperature , that means there will be a specificed capacity in the capacity tables for at -4F, but will not specify a capacity below that temperature even though it's still working.

    At NC type 99% outside design temperatures it hardly matters- as long as you have sufficient capacity at design temperature with bit of margin, heat strips can cover the difference when it's chugging along at outdoor temps below which there is a specified output. The number of hours per decade it's below -4F even in the mountains of NC is at most a few dozen, and for optimal efficiency the heat pump should be designed around the 99th percentile temperature, not the 99.97th percentile, otherwise it will be oversized and lose substantial efficiency to cycling on & off. Even the best modulating or mult-stage split system heat pumps have only about at 2.5:1 turn-down ratio. Many mini-splits have turn down ratios more that 2x that much.

    Carrier's modulating "GreenSpeed" technology heat pumps have a handy online tool that shows the capacity graphically across temperature, and allows the user to select different compressor and air handler options. It only covers their 2 ton and 3 ton compressors but a good dozen or more air handlers, and it can generate a PDF report:

    (click on the "HEATING CAPACITIES" tab)

    At 3 ton GreenSpeed can deliver 30,000 BTU/hr or so @ +15F outdoors, 20,000 BTU/hr @ -5F with several of their air-handler options, and could very well be in the right capacity range for a well insulated better than code 4000' house with triple pane windows.

    I believe (but don't know for 100% sure) the Bryant Evolution® Extreme Variable-Speed heat pumps are essentially the same under the sheet metal as the Carrier INFINITY® 20 HEAT PUMP WITH GREENSPEED® INTELLIGENCE heat pumps.

  8. Stockwell | | #8

    Thank you Dana. That is exactly the kind of info I was looking for! That is an interesting tool. I will contact the Carrier dealer in the area and see what he has to say. By the way, the 99% design temp is 19.

  9. Stockwell | | #9

    I am back with a Manual J in hand. Our heating load is 23,389 Btuh and cooling load of 28147 Btuh. I'd like to use that Carrier/Bryant with Greenspeed. The Manual J calculates a 2.8 ton unit would work, so 3-ton seems just perfect. According to the Carrier tool, the heat pump should be sufficient 99% of time. Heat strips can kick in for the rare times it is too cold. Does this all sound right? The house is 2 floors, 4000sf, over a crawl, with 3162sf finished and conditioned.

  10. Expert Member
    Dana Dorsett | | #10

    That sounds about right to me on a gut-feel basis, albeit without doing a full review of the construction details and Manual-J my gut feeling COULD be just about the pizza I just had for lunch. :-)

  11. walta100 | | #11

    When you go one system in a two story home getting a system that will provide the same temp up stairs and down both winter and summer is a big challenge not that it can’t be done just understand you are asking a lot and will have no guaranty of success. Consider a few options

    1 A thermostat down stairs with a remote sensor up stairs that will control the system based on the average temp from both sensors.

    2 A zoned system actively controlling up stairs and down.

    3 Separate systems up stairs and down.


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