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How hard should I work to retrofit a gas water heater to electric heat pump?

khpot | Posted in Green Products and Materials on

Since our natural-gas 50-gal tank water heater is a little over 15 years old, I’m researching replacement options and would like to prioritize lowering our carbon footprint – if it can be done without an outrageously long payback time. Heat pump water heaters appear to be current obvious energy-efficiency choice. However, we have a number of seemingly challenging constraints, and thus I would appreciate some advice from this great community.

First, our water heater is currently installed in a pretty small closet (25 3/4″ wide x 23″ deep x 8′ tall, with a 21 1/8″-wide door jamb) with a fully-louvered door. Basically it just accommodates our current 20″-diameter tank. Are there even any HPWHs that would fit this space?

Next, we’d need to add the required electrical service to this upstairs closet. Unfortunately our breaker panel is two floors away in our basement/garage. The panel is at least mounted on an internal wall that extends all the way to our attic, so it seems possible to route new wiring up to the attic and then across to the closet – but I don’t have a good sense of how much I’d expect to pay for such a project.

Finally, our current tank’s inlet and outlet are teed to send a loop up to the air handler in our attic for our hydronic furnace system. So any new water heater would need enough supply to allow for that as well. In fact, our current water heater definitely has trouble providing for a single hot shower while, or shortly after, our furnace runs. On the plus side, if we could eliminate the natural gas use from our water heater, it simultaneously eliminates our gas use for the furnace.

Some other parameters:
 – we are a 2-person household, trying to be relatively water conscious
 – we live in Portland, OR (climate zone 4), so never dealing with really cold temps; and we have the option to buy 100% renewable-source electricity
 – the extra ‘free’ cooling sounds nice for our bedroom level during warm months; during heating season, we also prefer our bedrooms cooler than our main lower level (and have just a single thermostat on that main level), so letting the heat pump pull some extra heat out of the upstairs doesn’t seem that bad to me; though perhaps it could be vented to the attic just above instead.

 

So, what do you think? Is it possible to put in a heat-pump water heater without tearing apart the whole house? Is it worth it? Is there an alternative we should consider instead?

Thanks!

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Replies

  1. Expert Member
    NICK KEENAN | | #1

    Is the hydronic loop the only heat source for the house or just to heat a particular area?

    1. khpot | | #5

      Yes it is the only heat source for the house. But we find it quite sufficient to maintain the 65 deg F we generally set throughout our 1200-ish sq ft.

      1. Expert Member
        NICK KEENAN | | #9

        OK, since that's the case, the heat pump water heater isn't going to work in that role. The HPWH takes heat out of the interior and puts it into the water. You'd be asking it to be a perpetual motion machine, providing its own heat.

        So things just got more complicated. You need to replace the gas water heater as a heat source too. If you want to go all-electric I suspect your best bet is a mini-split heat pump.

  2. JZohio | | #2

    In order to really justify this amount of work to be done to lower your carbon footprint, I would look at the rest of your commodities. Do you buy gasoline for your car or did you spend the extra money required to find a vehicle that leaves less of a carbon footprint? Do you buy certain foods that allow for less carbon to be emitted? Are you educating others of the need of lowering carbon emission? All these go into whether or not we believe we can make a difference and that is what you have to find out. Are you willing to spend the difference?

    In any regard most of the 50 gallon hybrid tanks are 22-23" in diameter. The frame to your closet will likely have to be removed to allow for installation. Then there will need to be a louvered door built and installed to allow proper ventilation or ventilation out of the unit to a more largely ventilated space.

    Your next hurdle will be the electric which can either be a 30 amp circuit or 15 amp depending on the model you use. If you were to have the wire installed by a professional look to spend upwards of at least $1200 or more.

    I understand their efficiency and operation is more desirable. Now to a main hurdle is you'll need to fork out at least $1700 for a decent unit. There are cheaper ones, but hey you get what you pay for.

    Removal and proper installation will depend on difficulty but I dont foresee you spending any less than $750 on the actual set up/install.

  3. charles77 | | #3

    @khpot. 1) I recently bought a large house in Tacoma, Washington (similar climate to yours) and also wanted to electrify my house. Was keen to install heat pump water heaters (as well as air-to-air HVAC heat pumps). Due to building code requirements for "enough hot water & heating for the home" I needed to continue to have two water heaters and two HVAC furnaces (even though our actual usage is low). During my research, a number of contractors pointed out that heat pumps generally require more maintenance than gas-fired equipment, replacement parts are often hard to source (either quickly or at all), and I was only considering top-tier equipment (Trane/American Standard/Carrier/Bryant/Rheem Platinum, etc). Portland & Tacoma are both prosperous regions, so it's likely that installation and labor costs are *way* above the actual cost of the equipment (for any installed system). I was receiving $5,000~7,000 quotes for installing two 50 gallon heat pump water heaters in my big garage with tons of open space near the electric panel. The two Rheem Platinum water heaters themselves only cost $1,300 combined. Therefore any financial gains from energy savings will be quickly lost to labor. Mandatory building permits and inspections further weed out the more economical bids.
    2) HPWHs need enough air to work efficiently. Even with louvered doors, it'll be continually drawing in the colder air surrounding the tank, thus reducing energy efficiency vs open-plan systems. Adequately-sized condensate drain pans are also necessary to capture water condensing on the evaporator coils. Folks also complain about noise, though more very recent models are considerably more quiet than earlier ones. It would be best to visit an installed site to listen to it.
    3) HPWHs heat water slowly to maintain their energy efficiency. If they can't draw enough heat from the surrounding air then they will revert back to electric resistance heating which is very electricity-intensive. Standard shower-head flow rates are 2.5 gallons per minute, so unless you're taking very long showers or multiple back-to-back ones, your current one should be able to deal with it unless it has serious problems. You can easily increase its temperature in the winter and reduce it in the summer by turning the heating dial to provide comfortable year-round water heating.
    4) Separately, I've also heard warnings against tankless electric/gas heaters. Unit costs and installation are generally more expensive, they require more maintenance, and often require water filters. Tank-based systems consume more energy because they always heat water whether you consume it or not, but the tanks enable sediment to settle so they act as de facto filters.
    5) Ensure that whatever system you decide upon (HPWH, hybrid, gas, etc) has a multi-year labor warranty in writing, and that you will be serviced within a specified time window. I've read on many forums stories of folks waiting weeks to *months* for replacement parts or time slots. Contractors obviously make much more money selling new systems rather than maintaining old ones, so please keep this is mind.
    6) As more of these high-efficiency systems are mandated by governments, deployed commercially, etc, I'm sure a lot of these issues will eventually be resolved so don't feel let down if you can't implement them immediately.

    Best of luck and I'd love to hear how you get on!

    1. Expert Member
      Akos | | #7

      That is a strangely long anti HP rant from a new poster....

      1. charles77 | | #10

        @Akos. Sorry to hear that you think I'm anti-HP because my first post is long. I'm not. They obviously use less energy due to their efficiency, particularly ground or water-to-air ones. I lived in Tokyo for many years and most flats had ductless heat pumps built-in. Toshiba invented the inverters that make variable speed compressors so energy efficient in 1980. I just wanted to flag that device costs are often trivial compared to installation and maintenance costs, so it's best to ensure that after receiving multiple bids, one doesn't forget to confirm in writing that their installer will stand by it through the life of the product.

        1. vap0rtranz | | #12

          Labor is always expensive but what I paid to have our HPHW installed was 1/3 of what you've stated. Perhaps this is a case of West coast inflated construction prices and nothing to do with HPHW themselves.

          1. charles77 | | #14

            1) I absolutely concur with you there. I was trying to be helpful, but in hindsight, it's obvious that I should have been more clear that I'm not against HPWHs by any means.
            2) I used the Consumer Checkbook service (available in limited metro regions) to ultimately find a very competent HVAC specialist to install everything at one third of the average of the other bids. From my limited experience (with this and other work) I've noticed that small family-run businesses with many years of experience are far more economical (and often more competent) than sales-driven organizations that pay sales people indeed of their installers/technicians.

    2. khpot | | #8

      Thanks for all these good thoughts! And I do also agree that the landscape is likely to change more over the next several years as there is more emphasis on improving energy efficiency across the board.

  4. charlie_sullivan | | #4

    It won't work to use the water from a standard heat pump water heater for space heating. But you might be able to do that with a Sanden heat pump water heater, that has an outdoor unit with the heat pump and and indoor tank.

    https://www.eco2waterheater.com/

    But before going there, we should learn about how you heat the rest of the house.

  5. Expert Member
    Akos | | #6

    It won't work for two reasons.

    The heat pump in most HPWH is only 4000BTU, so about 10x less than the smallest gas water heater. That can barely heat a very small space even if very well insulated.

    The second reason is that the heat for he heat pump comes from the space it is installed in, so you would be taking heat from the place you are trying to heat, which won't work. There are some that can be ducted or the split one suggested above, but that won't change the fact that is way too small for the heat load of a typical house.

    Your best bet is replacing with a higher efficiency tank or a condensing tankless unit.

    You can go gas free with a heat pump but you would have to swap out your air handler and go with a standard heat pump furnace.

  6. charlie_sullivan | | #11

    The likely options to consider are:

    1. Get an air-to-water heat pump that sits outside and sends hot water to a new tank in the same closet, and to the air handler. Chiltrix is one example, although there are lots of others to consider now. I previously mentioned sander, but that's not likely to be good for the whole house.

    2. Replace the air handler with a heat pump system, and separately replace the water heater with a heat pump unit.

    I think option 1 is technically superior, but it is harder to find contractors competent to do that work, whereas option 2 is more straightforward.

    In either case, you can reduce losses in your ductwork by putting minisplit heads, hydronic equivalents, or simple radiators in some parts of the house and stop using some of those ducts.

    Probably the next step is to find a good HVAC contractor to work with.

  7. khpot | | #13

    The perpetual-motion aspect is a good observation. I had been thinking one tank of hot water is basically like another but you all are right to point out that the heating rate is also important. And while putting heat pumps inside does always have some efficiency impact on the rest of HVAC, when the same unit is also trying to space-heat, that really doesn't work out anymore.

    I always worried this rabbit hole was just begging for massive scope creep!

    To focus on the efficiency aspect without necessarily jumping to heat pumps, then, it does seem like going electric just for the sake of it (ie, to a traditional electric water heater) is a losing proposition. If we stick with gas for now to avoid the full HVAC makeover, what are good features to look for?

    I've seen words like power-vent and condensing units thrown around for higher-efficiency gas tank water heaters. What do these do, and do they require any extra retrofitting to replace a conventional gas WH?

    Thanks!

    1. Expert Member
      NICK KEENAN | | #16

      Power vent: there's an electric fan that runs while the gas is burning, as opposed to natural vent which just relies on the tendency of hot gas to rise to get rid of combustion products. The big advantage is you can install it in spots where you couldn't run a chimney.

      Condensing: The cooler your exhaust gas, the more heat you're capturing and the less going up the chimney. Natural gas has a lot of hydrogen in it and one of the principal combustion products is water vapor. If your exhaust gas is cooler than the boiling point of water the water vapor in the exhaust gas will condense out. You need a specially designed burner that isn't corroded by the water, and captures it and disposes of it. Without condensing you can't get efficiency above about 85%, with condensing you can get up into the low nineties.

      A condensing burning will often have power vent because as the exhaust temperature drops it's harder to exhaust combustion products.

    2. mathiasx | | #17

      Either will need new (typically PVC) exhaust and perhaps air intake runs, as far as additional work. But the benefit is that you can seal up your chimney at that point, and if the water heater was the only thing that used the chimney, then the chimney could be brought down below the roof line and sealed if it runs through the interior of the house. Ideally you’d do this later when the roof is being replaced, and it doesn’t (in my experience) add too much to the labor or cost of a shingle job. The reason that bringing the chimney down into the attic or second floor is important is that the chimney is a tall thermal bridge that cuts through the structure, and allows in things like moisture when there’s no more exhaust gases drying it out.

      The power vent water heaters are way more efficient than the 15 year old natural gas water heater you’re replacing, and when I made a similar swap, I saw my natural gas bill drop in line with what the energy star sticker promised. The other benefit here to me is that these sealed combustion units eliminate backdraft possibilities whereas the old natural gas units have just an “open” hat and then run to the chimney — a very scary way to pull exhaust gases into your living space. (I replaced my water heater over 6 years ago, when there were no HPWHs available here.)

      The split air to water heat pumps like the Sanden may also be a good choice here. You’d eliminate combustion all together and could get it sized to space heat. But you may not be able to find someone who can install it.

      That leaves probably the easiest-to-source option: HPWP and any number of 1:1 minisplits sized to the heating load of the house. It sounds like this house has a rather low heating load, but you’d still want it calculated. The 1:1 minisplits are very efficient at heating and modulating down, and contractors are likely to be very experienced putting them in and getting them set up. Multi splits are less efficient, in general.

      1. mathiasx | | #18

        One other quick “hack” to the situation that I thought of is, if you’re replacing the ducted air handler, you could have the plumber use the hot/cold lines previously run to the air handler to plumb in hot water and get rid of those T’s up in the closet. It seems like you’d have more space for a HPWP wherever the air handler is now, than the closet/cabinet the water heater is in?

        1. khpot | | #20

          That's an interesting idea. There is more space in the attic near our air handler, though getting large items into that space is another bit of a challenge. If we go all in with a full HVAC project, it's worth considering. Thanks!

  8. charles77 | | #15

    Putting aside the caveats from my #4 point above and assuming you have clean water that doesn't need to be pre-filtered, then a tankless electric demand water heater may be a good option (assuming good installers/maintainers) if you can run the electric line to your closet.
    One benefit of these systems is that they provide continual hot water (so you don't need to worry about running out of water when guests are taking multiple back-to-back showers) and according to the below link they have a long lifespan.
    https://www.energy.gov/energysaver/heat-and-cool/water-heating/tankless-or-demand-type-water-heaters

  9. Expert Member
    Akos | | #19

    Go to here and sort by UEF. Generally higher UEF modesl will have higher efficency for space heating but not always.

    https://www.energystar.gov/productfinder/product/certified-water-heaters/results

    What you want to do is look for "Recovery Efficiency per UEF test method" value in the data for each unit. A decent unit should have recovery efficiency above 80%. This gives you the efficiency of the water heater when used for space heating.

    Once you find a couple of models, important to check the manufacturer's data sheet as not all are rated for combi heat. You might also want to check with your plumber to see which brands they can install.

    Since it is for combi heat, you generally want a unit with a larger burner. Something like this:

    https://www.energystar.gov/productfinder/product/certified-water-heaters/details/2317654

    If you are looking for even higher efficiency, there also a number of condensing tank water heaters (ie HTP Phenix) that will get you around 90% efficiency.

    If your water is not hard tankless units can be made to work, but as a retrofit it might be too expensive. You might need to upgrade the gas service (most need 3/4" line) and you need a higher pressure circulator pump for your air handler.

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