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Community and Q&A

Heat pump water heater for supplemental hydronic heating

NormanWB | Posted in Mechanicals on

I am in the process of designing a high performance home and am working on the HVAC system specs.

I am in mixed humid (zone 3 – Greenville, SC).

Duke Energy offers some reduced rates for Energy Star certified homes (I am waiting for Piedmont Natural Gas to get back with me on their rates). These rates are 5-6% below standard rates.

I am considering a heat pump system with gas backup, but Duke offers a further reduction if the home is all electric (HVAC and water heating) which is nearly 20% lower than their standard rate with Energy Star and I am not sure if they will allow the gas supplemental heat option (I have emailed them this). Also, I am not sure the gas company will be offering a whole lot if I am only using them for supplemental heat, plus they charge you $10 a month whether you use gas or not.

So, in order to get the all electric rate, I was considering using an heat pump water heater to supply the supplemental heat (hydronic coil in the AHU). A recirculating pump would move the hot water to the coil from the water heater only when supplemental heat was required.

Of course, the drawback of this is the water heater is pulling the heat from the conditioned space and cooling the are. But, since this will only happen when supplemental heat was required that may be acceptable. AHU and water heater are in a conditioned basement.

Your thoughts?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The amount of supplemental heat you'd get out of it is whatever the wattage is of the water heater when running, which may or may not be enough. In "hybrid" mode most HPWH water heaters can deliver 15,000 BTU/hr though, so it probably would be able to cover it. (15,000 BTU/hr is almost fully the design heat load of many modestly sized or higher-R homes in zone 3.)

    It's essentially the same efficiency as an electric boiler or electric baseboard, or supplementary heat strips on a heat pump, which might be cheaper to install than a hydronic loop to the water heater.

    What is the BTU rating of the coil (at 140F EWT, if it's in the spec)?

  2. GBA Editor
    Martin Holladay | | #2

    Pnjfksx,
    First, care to tell us your name?

    Dana is right, but it seems a little odd to try to heat a space by cooling off one corner of the basement and moving the heat to the air handler in the other corner of the basement. You're really using the electric resistance element in the heat pump water heater when you use it for space heat.

    You only need "supplemental heat" when the outdoor temperature is really low -- you probably want to use electric resistance elements rather than a heat pump water heater for this function.

    Then again, you could just use one or two ductless minisplits to heat your house -- they don't need any "supplemental heat" when the weather gets cold, if you specify the right minisplits.

  3. Jon_R | | #3

    Take a look at the Chiltrix heat pump. It can do space heating/cooling (even small, closed door rooms) and water heating. With a buffer tank, even some time-of-use optimization. On the other hand, cost or complexity issues might rule it out.

  4. NormanWB | | #4

    Norman is the name. Been lurking on this site for the last few months getting great information.

    It appears that the HPWH would need to use its elements to meet the heating need and would not be able to depend solely on the heat pump, so I might as well use the strips in the AHU, as I doubt gas would be cost effective since they charge $8-10 a month if I use none.

  5. Anon3 | | #5

    Run some BeOpt simulation, I think gas heating is still going to be cheaper for you. Go all electric if you can get net metering with solar panels, those got great payback if you can install it cheaply.

    In zone 3 heating cost exceeds cooling cost.

  6. Expert Member
    Dana Dorsett | | #6

    In a high performance house it's likely that even the smallest ducted heat pumps would have more than sufficient capacity for even the 99.6th percentile heat load without needing backup heat. A 2-ton Carrier Greenspeed (smallest in the series) is good for 25,000 BTU/hr @ 18F (Greenville's 99.6% outside design temp)

    Hit the Heating Capacities tab, and play around with some options.

    http://www.tools.carrier.com/greenspeed/

    The term "high performance house" doesn't have a standard definition, but even 2500' house with 2x6/R20 walls with U0.30 windows can easily come in under 25,000 BTU/hr of heating or cooling load with a bit of BeOpt simulation & tweaking.

  7. NormanWB | | #7

    I have toying with the numbers based on some calcs at HVAC-Talk.com. From what I can tell at my current electric and gas costs, the cost crossover point is at a COP of 2.6. Below that, gas is less expensive.

    However, given, as Dana noted, that I would seldom need the supplemental heat source, it might come down to the cost of the heat strips vs the cost of the furnace, which I suspect is much higher and would thus make the payback much further out.

    Next up is determining if the lower "all electric" rate makes the lower COP of the HPWH.cost competitive with a on demand gas water heater...

  8. Expert Member
    Dana Dorsett | | #8

    In most markets HWPH is competitive on an operating cost benefit with condensing tankless, even without factoring in the dehumidification & air conditioning benefit, even without being offered a lower rate for being an all-electric house. In your climate for at least half the year the cooling-conditioning benefit of the HWPH comes directly off the air conditioning power use (proportional to their differing COP efficiencies, of course.)

    The up-front installed costs will vary, but...

    In some cases an avoided fee for hooking up to the gas grid it would be enough to pay for the HPWH up front! This varies a lot by utility, and over time, but digging in a new gas service to a house isn't cheap, even if there is a main to tap into right out front. Most utilities in my area want to recover at least a decent share of that cost before they turn it on. YMMV.

    Also, installing gas distribution plumbing inside a house is a LOT more expensive than installing the additional wiring & breaker needed to support a HPWH.

  9. user-871508 | | #9

    Ed here. Early last fall we replaced a gas water heater with HPWH in the home of an employee that has a hydronic heating system. It's a complex system (home has bsmt apt on this unit, gas furnace for upstairs, shared water heater.) Worked fine in warm weather, ok in cold (had to up temp on HPWH, using high demand mode) until last month. Employee figured 30 year old thermostat failed, as heat intermittently stayed on for days at a time, overheating bsmt. Replacing thermostat worked for a few days, but now unit will not cut on to heat, all pipes to hydronic unit cold, all other domestic hot water ok. Wondering if coil has died, did HPWH in line effect lifespan, or if HPWH has burned out element from oversupplying? Calling in the HVAC and plumbing teams next. Y'all got any insight to share?

    1. Expert Member
      Dana Dorsett | | #10

      >" ...now unit will not cut on to heat, all pipes to hydronic unit cold, all other domestic hot water ok. Wondering if coil has died, did HPWH in line effect lifespan, or if HPWH has burned out element from oversupplying? "

      If the hydronic coil was somehow fried the pipes to it would still be hot. If the HPWH was fried the domestic hot water would be cold. A more likely scenario is a failed pump, bad relay in the controls, or hydronic zone valve, depending on how the thing was cobbled together.

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