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Community and Q&A

Heat-pump water heater vs. instantaneous water heater

knobhead | Posted in Energy Efficiency and Durability on

Hey Martin;
New member and my first question! I’ve been following blogs, become a fan, and I’m on board/planning to build with exterior rigid board, using a mini-split, and concrete floors for mass.

I’d like to use some in-floor heat in baths/kitchen ONLY. Was considering domestic hot water from instantaneous propane water heater, using loop off same for in-floor. Reading FHB 7/2010 article by Sean Groom on HPHW (heat pump hot water heaters), I’m wondering feasibility of using one of these, with projected C.O.P. of 3-5, as my domestic/in-floor heat supplier. Disregarding storage issue, it would seem that just a C.O.P. 2 would be a wash between propane/electric !?

If I ran a PEX loop to garage and installed HPWH there, it would keep it from freezing, save the 50 db compressor noise out of the house, and spare me a vent issue of the propane hwh from home’s insulated envelope.

Any thoughts? Thanks ahead of time!

Steve B.

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  1. GBA Editor
    Martin Holladay | | #1

    The usual question: What's your climate? Where are you located?

    Obviously, in northern areas of the U.S., you can't put a water heater in a garage, or the pipes will freeze, so I'm assuming you live in a mild climate.

    I'm not a fan of in-floor radiant heating systems connected to water heaters, for a variety of reasons. Instantaneous water heaters can be particularly problematic in such an application.

    If you are heating your house with a ductless minisplit system, the investment in hydronic heat distribution for a couple of rooms is a waste of money. Take the money you would have invested in PEX tubing and circulation pumps and invest the money in a better thermal envelope -- thicker insulation or better windows.

  2. knobhead | | #2

    Martin; I'm in zone 5, just south of Lake Ontario-in upstate N.Y. Headed towards mid-life and was hoping for a little sensible heat in couple areas of house with floor heat. Plans now are rigid as per Straube.........still trying to get walls r-40 and roof r-60 while leaving wall cavities empty for possible condensation. Wasn't gonna rely on in-floor and flywheel us into a sauna!! So, back to HPHW vs. , well, any other domestic hot water----. Is HPHW a better choice? And along the line OF better thermal envelope-----do you know about AND can you comment on ALSIDE window/door products reliability/factors? Thanks one more time!!
    Steve B.

  3. wjrobinson | | #3

    Daikin Altherma air to water heat pump

  4. knobhead | | #4

    I think after much head-banging, that I'm pretty much convinced towards a mini-split, but I'm still pondering just domestic water and feasibility of stand alone water heater using new heat pump technology sitting on top. Or are you saying use Daikin to also heat domestic, instead mini-split?
    Steve B.

  5. GBA Editor
    Martin Holladay | | #5

    1. If you live in upstate New York, don't put your water heater in the garage.

    2. Remember that heat-pump water heaters are most efficient when they are installed in a warm spot. Remember also that if you live in a heating climate (upstate New York), your heating system will need to provide enough heat to counteract the cooling effect of the heat-pump water heater. The heat isn't free -- the water heater grabs the heat from the room where it is located. That's why I think that heat-pump water heaters make much more sense in a hot climate than in a cold climate.

  6. Riversong | | #6

    It sounds like you're talking about heating the garage with a loop off your heat pump water heater. But if the HPWH is also heating part of the house and the domestic hot water, then it's pulling much more heat out of the garage than it is putting in - and everything will freeze.

    In a house that insulated and tight, I don't know why you think you need concrete thermal mass. Even if you want a couple of warm floors, you'll want quick-response radiant floors with low mass. The only place you might need some additional mass is behind passive solar gain windows.

    Unless you need AC (which you shouldn't with those R-values and good window shading), and you want both hydronic heat and domestic hot water, then you might be better off with either a small modulating-condensing boiler or a Polaris water heater and a mix of baseboard (or panels) and in-floor radiant.

  7. steve bumpus | | #7

    The concrete mass is two fold.....I really want to use concrete in main room for asthetics and some solar south facing windows. But mostly asthetics! Re: in-floor.....yeah, I've been juggling the whole controls/bells and whistles aspect of just in-floor in baths/kitchen. Sounds like electric resis. mats under tile with t-stats may be easier and maybe even quicker response/better if zoned/etc......I have access to nat. gas for hot water, but struggle with $18.00 maint. charge right out of the gate every month. It's like another TAX, so maybe instantaneous propane taking combustion air from outside is best avenue for tight envelope home. So my next battle is deciding best windows for my application and if I truly need/justify cost payback on double vs.triple windows. And does anyone know if using foil polyiso sheets on ext. sheathing will be a concern with 4 planes of foil. any bennys to inside sheet being non-foil and ext sheet foil/ I undertand R-factor is less, but what exactly are tradeoffs? THANX!!

  8. user-659915 | | #8

    Steve, I hate to say this but it looks like you're making the common mistake of assembling a bunch of poorly understood 'green' features without paying attention to how they all play together. The foil for example adds nothing to the R value unless it's facing air space. GBA's Q&A is a valuable resource for individual questions of detail but I get the impression you're missing some key parts of the big picture. I suggest you pause, take a deep breath, and do some heavy reading on the general theory of high-performance building systems before you go any further. There's plenty of good and comprehensive information across this site to help you. That, or hire a knowledgeable consultant and pay attention to what they tell you.

  9. GBA Editor
    Martin Holladay | | #9

    Propane usually costs 2 -- in some cases even 3 -- times as much per BTU as natural gas. If you have access to natural gas, don't consider propane.

  10. steve bumpus | | #10

    I'll always have questions, which is good, and I've poured hours over this site, years of Fine Home Building, and other sources, but the end result, after many varied opinions, it it ultimately comes down to my best guess. A perfect world gives us the "right" way each and every time.But I don't care to live there! My foil question harkens to Buildings Science's Betsy Pettit's design of foil faced polyiso sheets on exterior of wall sheathing. With all that I've tried to digest, a red flag comes up in regards to the foil or possible barrier/barriers there which I'm guessing might sandwich in moisture/condensation between sheets. Maybe thats not a concern, I hope, because that seems to be today's best option for heading close to Passive house criteria. I will not build until I'm convinced I have the envelope/home that I'm willing to gamble my wallet on. I'm guessing the foil facing has little or no R-factor, just concerned about condensation/mold/alien get my point!!! Thanks for any/all input.

    Steve B.

  11. steve bumpus | | #11

    To the propane question. I understand price difference and if I use a lot of a fuel type. But if my calculations work out as stuff I've boned up on. And if I can get the envelope good enough so that very low btus/$$$$ are required from a mini-split, wouldn't that $18.00/mo. charge be a deciding factor between running nat .gas, and hiding a propane tank? I'm basing this thought somewhat on Marc Rosenberg's presentation @ DOE this past spring?? where he said" At what point is the gas meter dis-connected to save the monthly meter charges .After a deep retrofit, gas consumption may drop to the point that the meter-reading charge exceeds the charge for gas usage. At that point, an all-electric house makes a lot of sense". And I would use propane for hwh/range/generator. Mini-split, of course, would be electric. Thots??

  12. Riversong | | #12

    Steve B,

    The foil facing on polyisocyanurate mostly serves to limit thermal drift - the gradual outgassing of low-conductive blowing gasses - and maintain its R-value edge for a longer time.

    But the foil also virtually eliminates water vapor drift. This could create a problem of moisture trapped between foam board layers if adequate water sealing measures were not taken, where it could, in the right conditions, freeze and cause mechanical damage. More importantly, however, is that this creates a completely impermeable outer skin which works fine only as long as interior relative humidity and air movement is strictly controlled and there is never an exterior leak through the foam layers - creating a saturation condition in the structural sheathing or framing which will not easily dry by normal diffusion and convection processes.

    An advantage of propane over natural gas, perhaps, is that the supply cannot be interrupted by "grid failure" as long as there is some reserve in the tank.

  13. user-659915 | | #13

    "I will not build until I'm convinced I have the envelope/home that I'm willing to gamble my wallet on."
    I feel your passion, Steve. You want to build a low-energy home that you can be proud of, which makes you want to push the envelope (pun intended). On the other hand you don't want to make expensive mistakes - and there are plenty available out there on the bleeding edge. Many of the contributors on this site have been doing this stuff for decades, and are still learning. Don't expect to mix your own perfect cocktail first time around. Your lead question in which you propose a heat-pump water heater in your garage suggests you are not well-versed in basic principles. Get a consultant.

  14. wjrobinson | | #14

    Daikin ALTHERMA all heat and domestic hot water

    Propane dryer.. stove to design your home

  15. steve bumpus | | #15

    Thanks, I think you've put my mind at ease as long as the shell/envelope is put together right, saturation won't be a problem in regards to foil layers. I have 32 years employed with gas side of local utility and the writings on the wall when it comes to weak infrastructure and the potential for limiting all of our carbon footprint...READ controlling OUR homes gas/electric usage via "smart meters!

    I've searched for years for someone savvy in financial matters AND low energy homes. May as well be speaking a different language. FYI, I also built my own log home 23 years ago, and believe it or not, it's still standing. It was a learning curve, then, as now. I ran across Ted Inouie who operates Energy testing and Consulting in New Hope, Pa., who is a wealth of knowledge and then there is GBA!! Wish I had both years ago! I'm versed in principles of appliances, but the newest animal out is this Heat Pump Hot Water heater that I'm betting a lot of us are wondering about and of course if I'm asking, it's because I don't know. Who ever heard of putting a heat pump INSIDE a home, that would discharge the cool air INSIDE the house! But thats the trade off, isn't it .So I ask.......why not loop heat (from same tank) the confined space of the tank, but do it outside the envelope of the home (maybe in garage) to save re-heating the condenser air, keep 50 db compressor noise from house, AND save venting a gas water heater in bsmt. Sounded possibly logical with the data I had, but I needed input, so I asked. Call me "not well versed", but I've excelled in my life BY asking and making a decision, and doing. Mistakes??? You bet, but I would change nothing. And I move on....maybe I go with a sealed vented instantaneous hwh. That, or a high-efficient polaris, as Robert recommended. I can't exhaust the possibilities until I've asked the questions. And I've seen some pretty smart folks go back and forth here for the straight skinny. Wheres their consultant? If inquiring ideas from my fertile mind require me to hire a consultant, then of what use is GBA? I submit this site, and probably others, IS the consultant some of us need to come to our own conclusions. No "pay for performance answers" whether right or wrong, but honest, straight-shooters, right or wrong, who are giving you their best guess, but which, once again, brings it back in my court. You generous folks keep giving us YOUR best, and I'll give it my best!! Thank you!!!!

  16. user-659915 | | #16

    Steve - perhaps I should rephrase. While it's true that any reasonably smart person can design a house y themselves if their expectations of function and performance are not high, those who expect the best from their projects will do well to get professional help. If you try to complete this project without a professional partner, someone who has at least a decade or two of experience working in your climate and locale on structures similar in concept to the one you wish to build, you will almost certainly repeat the mistakes from which they have already learned. This would be fine if the purpose of your project is purely educational and self-developmental, but not so good if you have the objective of actually ending up with a well-functioning home.

    This partner might be your contractor, your architect or home designer, or an environmental engineer/energy consultant - perhaps all of the above depending on the skills available in your community. GBA is a wonderful resource. Answers you get here will be interesting, informed, passionately felt, and limited in usefulness without the filter of local experience.

  17. wjrobinson | | #17

    Heat pumps..... pump heat.

    Your water heater heat pump idea is a no go.

    There is no excess heat in your garage to pump into your home. Only in the summer when you don't want it.

    Water heater heat pumps are for warm climates.

    They switch to resistance when the air is too cold to take heat from.

  18. Tom G. | | #18

    I live in the desert SW so my water heating needs are easily met with a 6,000 BTU HPWH. However I was born and raised in MN so finding any excess heat somewhere is highly unlikely but not impossible. Maybe if you had a full unheated basement insulated from the rest of the house you would get enough heat from the earth - don't know. I would check with my architect first.

    I know you have probably considered this, but have you considered a ground source or water source heap pump for your whole home domestic heat, air conditioning and hot water? They are very economical to operate. This is the way I would go if I were building a new home and had the space for the layout of ground system or a water source. Any type of fossil fuel source, natural gas, propane, oil, is according to our president "necessarily going to skyrocket.". Of course he was talking about electricity but if it goes up so will all other source.

    If it were me and I lived back in cold country I would go with a ground/water source heat pump for my domestic heating and hot water. Next I would go with structural insulated panels [SIP] construction with triple pane windows, Would also have interior to exterior heat recovery system for fresh air. Would have roof trusses designed to permit full thickness insulation all the way to the exterior walls. Would use blown in fiberglass for the first R38 of insulation followed up with 3" of cellulose insulation over that. Cellulose will cut down on air losses through the fiberglass. Better yet go with about a foot of foam if you can afford it. Would also orient my home south facing. Solar gain in winter is free heat, solar air heaters also work well in some areas if you don't have shading issues. One of my friends in MN shuts off their furnace on most sunny days even if it is 0 decrees outside. I would also plan for solar PV by doing the pre-wiring to the roof and breaker panel if building new. Otherwise it costs about $500.00 to add it later. We are only about 3-5 years away from 30% efficient solar PV panels at under $1.00/watt. At that price it could as least provide all of your daytime needs and maybe your evening needs if grid tied.

    O.K. way too long of a post. Oh and I forgot to mention. Consider looking for a new dryer to be on the market soon. The new models should soon start to use outside air intake for the air they need. Our current dryers take all our heated and/or cooled air [up to 500 cubic feet per minute] and blow it outside which is really stupid. And you thought America was serious about energy conservation, LOL

    Good luck and enjoy your new home.
    Tom G.
    [email protected]

  19. GBA Editor
    Martin Holladay | | #19

    You wrote, "There is no excess heat in your garage to pump into your home." Actually, there is.

    Assuming that the air in the garage is not at absolute zero -- a physical impossibility, by the way -- there is heat in the garage that can be scavenged by a heat pump.

    Heat pumps can operate very well at temperatures below indoor room temperature. The heat pump will simply lower the temperature of the garage -- for example, from 25 degrees F to 22 degrees F. The heat can be used to raise the temperature of water (for example, from 105 degrees F to 112 degrees F) or to raise the temperature of the indoor air (for example, from 65 degrees F to 70 degrees F).

  20. steve bumpus | | #20

    That was kinda the direction I was initially headed in. My garage, the uninsulated part seems at least 5-10 degrees warmer in winter, most of it likely due to wind and possibly wind chill factoring. My thot was if I was utilizing part of HPHW tank as in-floor, then it would make sense to loop floor area of water heater stall ON WAY to baths/kitchen, and for that matter, why couldn"t a house heat pump inside garage?? Barring noise/vibration, it might keep coils free from of pollens/debris. One might need to have room on outside wall and louver it, esp. due to hot days when reversing for A/C. But yes,I can see potential shortcomings from my northeast zone 5 area.
    I have retained Energy Consultant in last 2 months, and in process of retaining architect ...gotta go by the book here in N.Y.!
    Tom G.
    I was 110% on board for GSHP for 23 years until my mind was changed rcently by insulating. Building with double the 2009 code, I would be blowing myself out of the park trying in floor heating and the flywheel effect and although the C.O.P. payback is attractive, I'd still need someone who knows installation/will maintain/ and to get A/C would be even more controls. Sure, I could use air handlers vs. in-floor, but that was the whole attraction, heat to the feet!! Research has shown me that the training/technology still needs to be sharpened more, but I'm 85% convinced that for me, a mini-split will do all of the above, plus a whole lot cheaper. THANK YOU Ted Inouie!!

  21. wjrobinson | | #21

    yes.. I know technically... you are right


    Practically you are wrong

    Without doing the math... there is just no heat in a garage of value during our winters. That is why the heat pumps switch to resistance.

    Also just do the math. Show me a water heater heat pump and a garage and temps and btus that would heat someone's home and domestic hot water. That's just ridiculous Martin.

    You're better than that.

    ie.. my garage would have to drop 5 times the delta t desired for my home which is 5 times larger than my garage. So on a 10 degree day my home would be 70 and my heat pump cooled garage would have to be 70-10=60x5=300 10-300=minus290degrees in the garage. the math is silly, so is the idea and all related posts.

  22. steve bumpus | | #22

    Sorry for the long-windedness, but I've got a year before ground-breaking, and I think I'm getting pumped!! I will choose carefully my carpenter, but I'm not sure anyone around here KNOWS importance of sealing a rigid/envelope, so I think I will oversee/do with friends SLOWLY and CORRECTLY. So I've absorbed window info, but like insulation, what IS the breaking point on glass....sorry! Like insulation, there is a diminishing return, unless one is independently wealthy. I understand potential liability of speaking out for/against different manufacturers. Surely all have complaints from customers. All have great sales pitches. Here's my big question/s for the day. Is there a huge difference going from good quality double to triple pane windows, across the board, from company to company???Or is it comparable to , say, having walls R-40 and going to R-50 ?

  23. GBA Editor
    Martin Holladay | | #23

    I never recommended putting a water heater in the garage. In fact I specifically wrote, "If you live in upstate New York, don't put your water heater in the garage."

    I was simply challenging your statement, "There is no excess heat in your garage to pump into your home."

  24. GBA Editor
    Martin Holladay | | #24

    You need to spend some time with energy modeling software -- especially if your construction project is a year away. Model your home both ways -- once with double glazing, and once with triple glazing. The software will answer your question.

  25. wjrobinson | | #25

    Martin.. had to read this again andpost again.

    You are absolutely wrong and I am right.

    There is no EXCESS heat in an upstate NY garage anywhere near the amount needed to heat a home that is 5 times larger. Crazy talk.

    Please post that you understand this as all your other posts and blogs do make good sense to me.

  26. wjrobinson | | #26

    Thank you for agreeing with my math.... there are no heat pumps that could get a garage to minus 290 degrees nor would any sane person put up with ridiculously cold garage space. Would be near impossible to start autos.... what would one wear to enter said garage... the vehicle if it started would get 2 mph... be an inbox... the wife would file for divorce... the kids would stop talking to you...

    Bad silly undoable idea.

    Sorry for posting with aj humor all you serious types.... too hot hear to not do so.

  27. Riversong | | #27


    There are cold-weather air-source heat pumps that are efficient even at sub-zero temperatures, including reverse-cycle chillers which are perfect for hydronic systems. But to be efficient, the outside coils need to be outside, not in a confined space, so that it can draw on ambient temperatures without changing them.

    Depending on your overall thermal envelope and passive solar strategy, high-gain lowE double-glazed windows are often the most cost-effective choice. I just ran a half dozen window options through my energy analysis spreadsheet for a superinsulated lake cottage I designed for Vermont, and the more expensive triple-glazed windows had a 40-70 year payback compared to the double-glazed option.

  28. wjrobinson | | #28

    Gotta love self correcting Droid phones....mph=mpg inbox=icebox

  29. Riversong | | #29


    You clearly do not understand how a heat pump works, or the laws of thermodynamics for that matter. So you would do well to stop broadcasting your ignorance.

  30. steve bumpus | | #30

    Thank You Robert, Martin, Tom, and James for your time,insight, wisdom and generosity. It is a refreshing comfort having a theater of ideas discussed in honesty. Stand-by for future questions, and stay cool, wherever you are!!
    steve b

  31. wjrobinson | | #31

    Reversing.. who asked you?

    You clearly just post insults instead of valid points.

    I dare you to post numbers for my house scenario that work you idiot.

  32. Riversong | | #32

    Can some put a leash on ADJAC? He's single-handedly lowered the level of discourse on this site to the sandbox.

    He suffers from chronic verbal diarrhea, and - like a rabid - dog should be either muzzled or shot.

  33. wjrobinson | | #33

    Robert.. the insult dog.. .

    Back up your statement..

    Save your flaming for elsewhere.

  34. wjrobinson | | #34

    Scenario...100 degrees today. Water heater heat pump in garage... and now we want to cool the home.

    How? The WHIP cannot cool the home.

    Daikin ALTHERMA can and will do the job year round.

    Mr whine Riversong has posted no numbers or device to back up his need to flame

    Great flame though... and yes the whole family wears inappropriate footwear.

    Brooks. You're due next... what are your numbers?.. Martin, Robert, anyone...

    Show us differently or concede.

  35. Kevin Dickson, MSME | | #35

    A few months back I ran some calculations to see how much heat could be pumped out of a garage. The garage MUST be insulated, and the overhead door must be sealed as well as possible. The calculations were based on observed temperatures inside and outside the garage:

    "The heat loss on the garage at 12 degrees outside and 40F inside is about 8600btuh free geothermal from the slab.

    The heat flow through the slab is operating at a temperature difference of (55F-40F)=15F. If we can safely assume that the slab heat flow is linear with temperature, then how much does that heat flow increase at a lower indoor temperature? Well if we can take the indoor air temperature of the shop down to 25F, now the temp. diff. is 30F. Therefore the slab heat flow would double to 17,200 btuh.

    So, a rule of thumb, at least for the soil conditions at that particular site, is that a perimeter insulated slab can produce 11.1 Btuh/ft2 of low temperature geothermal heat for an air to air heat pump."

    The whole discussion is at:

  36. wjrobinson | | #36

    Kevin, interesting but not relevant to the site this post refers to or to the capabilities of a residential hybrid water heater heat pump.

    Read the specs on a hpwh and you will see that in northern ny they will switch to resistance 50% of the time annually.

    If you can make this work, I have millions of neighbors ready to purchase.

  37. wjrobinson | | #37
  38. Kevin Dickson, MSME | | #38

    Adjusting the calculation for 35F, (to prevent the pipes freezing) that's 7.4 btuh/ft2, or 3700 btuh for a 500 ft2 garage.

    That should be enough heat for DHW for many households, which makes the garage a great place for a hpwh.

    The caveats are that you must insulate and seal the garage, include the slab perimeter. Also that this rough calculation uses site-gathered data for one set of conditions. But everyone knows that heat will come up out of the earth (google FPSF), so here's a rough estimate of how much.

  39. user-659915 | | #39

    "That should be enough heat for DHW for many households"
    Maybe. However this thread began with a question about using such a setup for whole-house space-heating, which in most scenarios runs an order of magnitude greater than DHW demand.

  40. steve bumpus | | #40

    Actually my original question was hvac from mini-split, and using HPHW to supplement two bath and kitchen in-floors in addition to domestic hot water. Save all the infrastructure associated with that 'idea'. I MAY just lay elec wire mats under same with t-stats. If works out reasonable with electric rates, then i've saved installation of in-floor and any potential future maint charges. I remember a couple years ago seeing an article of a gent who years previous had run ductwork underground and was somehow geo-thermally converting that 45-55 degree air for his hvac. Maybe that's where I thot It could marginally work in garage. Ah, dreamers.......!

  41. wjrobinson | | #41

    It was just a dream Steve.

    Have you read up on the Daikin AC ALTHERMA yet?

    The unit would be used instead of mini split. It can heat air domestic water and in floor radiant.

    One unit does it all exactly what you are looking for.

    My own idea of a home is close to yours too. Superinsulate and use ALTHERMA to heat tile floors in baths kitchen.

  42. steve bumpus | | #42

    To: tom garvin:
    A month ago you mentioned a power venting/outside combustion air gas dryer coming soon to the marketplace. Do you know manuf. name or website info? Thanx!!!
    Steve B.

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