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Community and Q&A

Heat-pump water heater for radiant floor heat?

Timsen | Posted in Mechanicals on

I’ve installed radiant floor heat in my northern California house and it’s working well so far. I’ve installed an electric boiler and am tempted to try to install a hybrid (heat pump) tank water heater in addition, in front of the boiler.
The current boiler is (except for startup) running at ~2kW so roughly 7,000BTU during the colder nights/mornings.
The heat pump water heater would be installed in my insulated garage, where the temperature is barely going below 50F. At that ambient temperature, a heat pump water heater with 400W compressor should produce around 4,000BTU. I was looking at the specs for the A.O. Smith Voltex HPTU-80N to come up with those numbers.

While it will not be able to replace the boiler, I’m hoping to significantly reduce the time the boiler will work. The heat pump thermostat would be set to 5F higher than the boiler. Also, boiler is coming after the heat pump in the primary loop.
As the heat pump should be 3-4 times as efficient compared to the boiler, I should be able to save a significant amount of electricity.

Are there any major problems with this idea?
Does someone know if heat pump water heaters have a minimum working water pressure? I can’t find this in the specs for any of them so assume there is no minimum water pressure. Am I right with that?

Thanks,
Tim

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Replies

  1. Jerry Liebler | | #1

    Timsen,
    I'd venture to say: It's only a matter of time until you'll be able to replace the boiler with an air source heat pump! They are available right now, in China, without mandatory "third party" testing to meet US requirements. Some also offer simultaneous air conditioning so you could possibly replace your AC outdoor unit and the boiler with one outdoor unit. They use enhanced vapor injection (EVI) scroll compressors driven by inverters. They'll produce water at 140f when it's -10f outside at a COP of 2+ and come in compressor power ratings of 4-10hp The COP should be around 4 where you are. The small ones look like big minisplit outdoor units,larger look like typical AC outdoor units. Stay tuned! What is the thermal output of your boiler? I'd say within a year someone will be offering a properly tested line of heat pump boilers in the US market, It may be prudent to wait before doing an upgrade you will regret. You might be able to speed the process of getting them into the US. The units I've seen pictures of are branded GREE and that brand is sold, exclusively, in the US by HVAC contractors. Please find an HVAC contractor who sells GREE, near you, and ask about the GMV-PDs series. I want one too! But for different reasons.

  2. GBA Editor
    Martin Holladay | | #2

    Tim,
    Unlike Jerry, I wouldn't base my decision on equipment being produced in China but not yet available in the U.S.

    Your first step should probably be to read this article: Air-to-Water Heat Pumps.

    I don't recommend your idea, for several reasons:

    1. It is an unusual setup that will confuse service personnel.

    2. You will be using an appliance in a manner for which it is not intended.

    3. During cold weather, when the heat is most needed, the efficiency of the heat-pump water heater will be low (because the garage will be cold -- even colder than it is now, because the heat-pump water heater will be lowering the temperature of your garage).

    4. Even if everything works, the water heater won't be able to supply your entire heating load in cold weather.

    Take the money you would have used to buy a heat-pump water heater (and the associated fittings needed to install it) and put that money toward a ductless minisplit. A ductless minisplit will provide both heating and air conditioning.

  3. Paul Kuenn | | #3

    Unless you have a super insulated house and need low temperature (90-120F) water to run through the pex, then I'd skip the heat pump idea. If the temp in the water heater drops too fast the heat pump will run constantly and possibly start to frost up. If they have an electrical heater attached to the coils to prevent this, then the power used will completely destroy your thoughts of efficiency.

    Our well insulated house does have a heat pump water heater ahead of the mini boiler but not directly in the water flow. The air pump water heater tank water heats water heading to the boiler through an exterior water/water exchanger. This takes the 60-90F water from the solar pre-warmed tank to 115F which is more than enough to heat our house through the floors. Keep in mind this is costly unless you are a DIY plumber.

  4. Timsen | | #4

    thanks all!

    Hearing what Gree is planning to launch is good. I'll keep that in mind.

    I'm a DIY plumber so serviceability or installation cost is not an issue. The boiler panel is very close to the garage where I already have a 240V service, so installation shouldn't take long.

    The water temperature for the radiant is set to 120F which is enough to heat the entire house even during the coldest nights we had last year. Also, our garage has vaulted ceiling which brings it to 8000cf so I'm not too concerned of ambient temp dropping too low.

    So based on all that, I'm still leaving towards trying that route. Especially as I can get a hybrid heater relatively cheap using local incentives. Also, this sounds like a fun project.

    Does anyone know if the low water pressure might be an issue?

    Thanks again,
    Tim

  5. Expert Member
    Dana Dorsett | | #5

    To be used for space heating CA Title 24 would require documentation of it's efficiency and capacity when operating in this mode, documentation which probably doesn't exist. If installed in a 50F garage, there is no way to guarantee that the garage will STAY anywhere near 50F during periods when the duty cycle on the heat pump water heater is high, during a cold snap.

    Air source heat pumps need access to their source (effectively infinite thermal mass of the air in the great outdoors), and even in N.CA the peak low outdoor temperatures are below the efficient operating range of heat pump water heaters designed for indoor use. A Japanese EcoCute consortium heat pump water heater would work in most CA locations, but they are not readily available in the US. (see: https://www.greenbuildingadvisor.com/articles/dept/musings/split-system-heat-pump-water-heaters )

    Most of northern CA has real sensible cooling loads, and a ductless mini-split heat pump could relieve the load of the electric boiler substantially, and provide high efficiency cooling in the summer.

  6. Timsen | | #6

    We have a central heat/cooling unit (Gas/Electric package) installed that works okay. The reason I want to use radiant floor heat for heating is it being so far more comfortable compared to forced air heat.

    While I appreciate the suggestions to go with mini-split, My goal is to reduce electricity cost for the radiant heat system. Even if the heat pump is not working 100% efficient, it'll still be more efficient than the electric resistance boiler that's currently installed.
    The electric cost for the boiler is about $500 per winter season so even if the heat pump would only be at COP of 1.5, I'd save around $150 per season which would be enough for me to invest the $1,000 for the heat pump water heater.

    Does anyone have thoughts on the water pressure being an issue, as it's at 10PSI?

    Thanks,
    Tim

  7. Jerry Liebler | | #7

    Timsen,
    I did not mean to say that Gree is preparing to launch! I have no way of knowing that. I wish they would and it seems it should be easy because What I do know, for certain, is that Gree badge products , with UL,CE & AHRI+other certifications, are now being sold, by a major mini split manufacturer in China. There are 6 models ranging from 34KBTU to 100KBTU. They are really neat as you could heat/cool your floors while heating your domestic water and do conventional AC AND while cooling you can SIMULTAENOUSLY heat DHW transferring the heat from the area being cooled to the DHW.

    Meanwhile, What is the capacity of the boiler? There may be air source heat pumps that are designed as boiler replacements. They are widely used in the rest of the world but few have made it into the US.
    There are many air source heat pumps intended as swimming pool heaters available in the US and one of them might be a much better "alternative".
    EDIT:
    Perhaps the best "hack job", and what I'd do is" buy a suitable boiler replacement direct from China, via Aliexpress.com. It works! I confess I've done similar things. look for a DC inverter "monolock" style.
    With the monoblock you won't handle any refrigerant but freeze protection during a power failure is possible and you should deal with it. The easiest way is use swimming pool antifreeze in your heat exchange medium. The unit will look like the outdoor part of a minisplit and should be located outdoors, forget putting it in the garage. Good hunting!

  8. Expert Member
    Dana Dorsett | | #8

    Since the potable water would have to be separated from the heating system water with a heat exchanger, the pressure its not an issue. You'll have to use a potable water compatible pump (usually bronze, sometimes stainless) on the water heater side of the system.

    On the heating system side 10 psi could be cutting it close to the margin on pulling air into the system or pump cavitation especially if it's being overpumped, with even lower pressure on the intake side of the system pump. Most systems are set up at 12-15psi to prevent those issues, even a bit higher if the top of the system is more than a couple of floors up from where the pump, or where the pressure is being measured.

    If the goal is saving electricity, invest in ECM drive pumps for both the system and potable sides.

  9. Jerry Liebler | | #9

    Dana,
    Does your post belong in this thread???? What are you talking about? Re reading of the initial post I find no mention of potable water and have really find no clue of the pluming schematic.

  10. Timsen | | #10

    Jerry,
    Thanks for the tips! I looked on alibaba but it looks like almost all units are for 220V 50Hz so wouldn't work here.
    Also, I think the COP of current heat pump water heaters are comparable to those units for sale there and at same price level. I looked at the new Rheem hybrid. They have a COP of 3.5 and are tested for low climates, work even at 37F ambient. I'm planning to duct them so the cold air will be pushed outside.

    Dana,
    I'm also confused. The water tank should be within the closed radiant system, dedicated for that system only. It'll run with tap water but with only 10-15PSI. Note that no antifreeze is needed as temps doesn't go below 30F where I am.

    So the only remaining question is if anyone is aware of problems when the water pressure in the tank is at only 10PSI.

    -Tim

  11. Expert Member
    Dana Dorsett | | #11

    Jerry/Tim: I don't believe it's legal to use a water heater solely for space heating in California, but they may be used for combined space heating & water heating. Technically combi-systems also need to be AFUE-tested and meet CA standards. See page 253 (265 in PDF pagination):

    http://www.energy.ca.gov/2017publications/CEC-400-2017-002/CEC-400-2017-002.pdf

    I don't know with certainty whether California allows "open" systems, with potable water in the heating system plumbing, but since the pre-existing system is not using potable water it's a truly bad idea to make it into one.

    As long as it's serving at least a slop-sink with potable water in the garage you can argue that it's a combi system primarily used for heating potable water, and since you have an electric boiler for the radiant you can argue that it's only an auxilliary heater for the radiant floor, but some inspectors would be pretty skeptical of both arguments.

  12. MatthewSeattle | | #12

    Timsen,
    I am considering a similar set up, how did this work out for you?
    Thanks!

  13. Tom Woodson | | #13

    I’m curious about these options as well. The Fujitsu WaterStage, Daikin Altherma and Mitsubishi Ecodan provide heat, dhw and apparently have a cooling option. In colder climates, it seems like using a large storage tank as a battery could reduce make us more energy independent. It would be a great if these were available in the us market.

  14. Tom Bassett-Dilley | | #14

    Here’s a link to info on the Sanden HPWH we are in the process of using in an all-electric project. The house is mostly conditioned with air-to-air heat pumps (Mitsubishi, ducted), but about 700sf of basement habitable space will have radiant floor. It’s in Wisconsin with design temp of about -4F (away from office, guessing), so probably risky for the whole house. https://www.smallplanetsupply.com/sanden
    Curious how this approach is working for others in cold climates; generally we’re all air-to-air.

    1. Jon R | | #15

      Note that Sanden instructions say "For homes with a design temp of 12°F or higher, and a heat load of 8kbtu/hr or less". -4F isn't close to the former.

      1. Tom Bassett-Dilley | | #16

        Yes, we noticed, but we're only heating a very small area, so it's well below 8kBTUh; and we have air-to-air heat pumps for heating and cooling in the house as well.

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