Friday, August 28, 2009

Heat Pump Hot Water Heater

So, you're trying to decide how to heat the domestic hot water in the zero energy house you are designing for Denver.

Solar hot water heaters are an excellent choice. However, the logistics of a solar thermal installation are daunting compared to a typical appliance installation.

A logistically easier solution is to use a heat pump water heater (HPWH) in conjunction with PV solar.

Rheem has just released a new nice-looking unit.

The jury is still out as to which configuration will be best. Recent test results and common sense are beginning to favor HPWH with PV. The reasons for this are that solar DHW is insufficient and inefficient in the winter, and underutilized in the summer. A grid-tied PV system has the advantage of constant efficiency and 100% utilization year round. In addition, large PV cost reductions are coming, but none are expected in solar thermal. State-of-the-art solar thermal collectors still use supply-restricted copper, aluminum, and glass. The labor content of solar thermal installations is also quite high.

Some puzzles remain with the implementation of HPWH systems. Per the manufacturer's recommendations: "Because the heat pump’s exhaust air is cooler than the surrounding atmosphere, these water heaters generally do not belong in or near busy living areas of the home." Since they pull the heat out of the surrounding air, a small, airtight mechanical room isn't the best place for it either.

Discussion of HPWH locations:

1. The garage. A new, well insulated garage might work. An uninsulated garage slab gives off enough low temperature free geothermal heat* to prevent freezing of the pipes to the heater. There are at least two problems with the garage, however: 1. In summer, the HPWH gives off cold air that would be more desirable inside the living space than in the garage. Ducting air from the garage to the house is a CO hazard, and not recommended. 2. If the (insulated) garage door is inadvertently left open in the winter for an extended period, the water lines to the HPWH are at risk of freezing.

2. Mechanical room with forced ventilation. The control scheme and ducting layout for this ventilation can get complicated and costly.

3. Basement. This is where you usually find the water heater in new construction, and will work just fine if you don't mind a cold basement in the winter. If you finish the basement and add heat to it, that heating system will be feeding the HPWH the heat it needs, and this will spoil the overall efficiency of your system.

"The House as a System" philosophy of home design will get a real workout on this one. Please enter a comment if you've heard of any other solutions.

*A perimeter insulated slab in an insulated garage gives off 5-15 Btu/hr/ft2, based on preliminary research. This is a great match for a 7000 btuh HPWH.


  1. I like your comments about HPWHeaters. I'm a plumbing contractor in Western Oregon and can't wait to start installing the Rheem 50 gallon heat pump water heater. I hope they come out with an 80 gallon version soon. Because of the slower recovery rate of HPWH's it would make more sense to have more storage.

    I began installing solar thermal in northern California in the early 80's and have come to realize that heat pump technology is likely superior even if PV is not in the mix. Here on the west coast where we get lots of fog and clouds during the year, this is the way to go.

    Over the past year I have installed 6 E-Tech HPWH's and have not been happy with the reliability of these units. They were recently purchased by AO Smith so perhaps they might improve as time goes by.

    -Paul Cooke

  2. Paul,

    I'm glad to find a contractor with experience in HPWH. If the house has a finished basement or no basement, where do you put the thing? In the winter, it steals its heat from the heat in the house.

  3. With respect to a zero energy house, I am assuming we are talking new construction (at least my 50 year old house building anew would be a lot easier than a retrofit).

    If that is so, why not use a ground based heat pump and not have any of these concerns?

    You can use the ground based heat pump for your heating and cooling, and add on hot water for little extra effort, I am led to understand.

  4. A new, zero energy home, by definition will require very little heat. It does need some backup space heat, though, for those very cold times.

    The conventional wisdom of zero energy homebuilding has evolved to require electricity for this backup space heating. Gas, even if available, is out. (PV is the reason for this)

    To date, I have not found a true ground-coupled heat pump space heating and DHW system that can be installed for under $6000. Typically, they run $12,000 to $40,000.

    Therefore, by far the lowest life cycle cost for space heating is electric resistance, which can be up and running for as little as $200. The savings from a ground coupled heat pump won't be recouped before the equipment is worn out.

    Lower still might be a mini-split heat pump, or a PTAC, but it depends on the house.

    "Green Builders" still doing ground-coupled heat pumps for new superinsulated construction are doing the whole industry a disservice.


  5. I've read the 100khouse blog for a while (that's how I ended up here today, from a comment you left there on the pictures of the M&M house).

    And no, there is no "green builder" involved, just me.

    But I was thinking a GSHP feeding radiant heat for the house and driveway, and have it feed a minisplit type system for cooling. Total conditioned area, including basement and attic would be 4200 sqft. (Well, basement probably won't be directly conditioned, so maybe more like 3200 sqft).

    To make sure I am understanding you correctly, it isn't that the GSHP wouldn't be more efficient, it is that the up front outlay would be so much more that a homeowner would never recoup the costs within the lifetime of the equipment, right?


  6. If natural gas is available to you, it's probably the cheapest method.

    If you need a heat source for your old, sort of leaky house, GSHP is OK.

    It's just not a good choice for a new, tight house, because there aren't enough savings to justify it.

    With respect to your driveway, don't be surprised if heating that will soon be illegal. It's a gross waste of resources.

  7. This post is a rather good discussion, but misses the opportunity for air-to-water heat pumps, where the heat pump is seperate from the hot water tank. This application is not common yet, but is seen with the Daikin Altherma system, and a few others. I think it holds a lot of promise, for exactly the reasons noted above for why the HPWH are a good solution, but a problematic application.

    I am looking for an air-to-water system currently for my home that would have an outdoor heat pump, and the tank with a heat exchanger in the basement. The tank would feed my domestic hot water as well as a radiant floor system for the main floor and in the future a finished basement as well (after I dig it down for more headroom, insulate the slab and put some pex in it of course).

  8. Check
    They have mated a york 2 stage YZH heat pump with a heat exchanger which can provide hot water (120f) for underfloor radiant or chilled water for forced air hydronic cooling.
    The units can be installed by york dealers.

  9. GE tank-style HPWHs are down to $1400.

    The Daikin and the York cost a few times that much.

  10. I like the concept of HPWH, but not sure they make sense in most heating dominated climates.
    Here in Colorado, as you stated, there is the potential freeze problem in the garage and in the house during heating season, your just stealing btus from your space heating system.
    I guess an unoccupied basement works if it is weel insulated from the rest of the house.

    For a cooling dominated climate they make more sense.

  11. I agree with Richard Patterman. Concept-wise, HPWH would be best used on areas with cold weather but maybe not on warm areas. I think you really need to have a special area in the house where you can put this where it is warm during the summer and the air during winter won't seep in.

  12. Thank you for all the info and comments! I am new to the home buying and married life game so I need all the advice I can get, I have a three bedroom two bath one kitchen one laundry room home. Any suggestions on a water heater in Denver I should look for? The one we have needs to be replaced.

    1. In Denver, in an existing house, a tank style natural gas heater is still the way to go.

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  15. I think putting the water heater in the garage is a good idea because it gives you more room in your home. I would like to see it done this way in my next home. Ours is in a storage closet which I rather put storage in.

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