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Combatting long recovery times during cold weather with a heat-pump water heater

CMOD | Posted in Mechanicals on

we moved into a new home (border of CZ 3&4) this past July, and one of the first improvements we made was to junk the 20yr old, poorly vented gas water heater (it was in the hall coat closet!) and replace it with a new state 50gal heat pump hot water heater located in the vented garage attic. We would have preferred the 66 or 80 gal, but due to space constraints and the fact that we’ve been fine in the past on a 40 electric, I figured 50 heat pump would be good enough.

Through the summer and fall it has performed very well, in hot weather, when the attic is over 100degF, we have very nearly unlimited hot water. As the weather has gotten colder, we’ve begun to have issues with long recovery times, with the heat pump running for up to 2-3 hours after someone has taken a shower, which can make the morning rush somewhat unpleasant. I’ve already bumped up the output temp from 120 to 125, and I’m hesitant to go higher without creating a scald risk.

I thought I’d cast for opinions on the ways to combat this, in general order of preference:

1) pipe insulation – there are some long copper runs through a vented crawlspace that need to be insulated in any case, but I don’t expect this to solve the issue entirely. ~$30-40 in pipe foam

2) Electric booster – rheem makes one for about $300 and they claim it gives a 50 gal the capacity of an 80gal. Only kicks on when the temp coming out of the main WH drops below a set point, so it likely wouldn’t run in the summer at all. I like this option because it’s simple to install, and $300 is about the price difference of the 50gal and 66gal heat pump models.

3) Air seal the garage. Currently it’s pretty well vented, with a gable vent, ridge vent, and soffit vent. Temps currently runs about 3-8 degrees above outside air, and the garage is not used for chemicals or vehicles so I’m not concerned about air quality, as long as it’s easy to open the vents up later if we move. This also probably doesn’t get all the way there, but may help if I can keep the garage 20deg or so above ambient.

4) Drainwater heat recovery – This was my first choice until I realized how much I’d have to re-route drain and supply pipes. Drain runs down an outside wall and supply is about 10 ft away through an interior wall. May have an option for horizontal install in the crawl, but still quite a bit of work. About $900 for all needed materials, and a lot of time, possibly including patching and painting. Probably too expensive and too much work.

5) passive booster – a little pipe setup that mixes 140degree hot with cold to output 120 Degree hot water. extends capacity but means the heat pump is making 140degree water even on days when 120 would suffice. Cheap and easy install.

Thoughts?

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Replies

  1. user-2310254 | | #1

    Chris,

    You definitely should insulate the lines in your vented crawlspace (and consider converting it to a seal space for better overall energy performance). You could also install ultra low flow heads on your sinks and showers if the current ones are builder grade water wasters.

  2. GBA Editor
    Martin Holladay | | #2

    Chris,
    1. Does your heat-pump water heater include an electric resistance element? If it does, is the heat-pump water heater set in the right mode? (You want to make sure that you haven't set the water heater to "heat pump only" mode.)

    2. Is the volume of the attic large enough to meet the minimum volume requirements established in the water heater manufacturer's installation instructions?

  3. CMOD | | #3

    Martin- yes and yes. It has trouble in 'hybrid' mode, and almost no trouble in 'electric' mode. I have not tried "efficiency" in cold weather but I'd expect it to be worse than hybrid as that's basically "heat pump only". The attic is vented and open to the workshop below, so there's plenty of volume.

    Steve- We will eventually encapsulate the crawlspace but we have other projects first on the list, and I don't think that will be enough to overcome the fact that the heat pump struggles to draw heat from colder air.

  4. Expert Member
    Dana Dorsett | | #4

    Drainwater heat recovery doesn't require that the heat exchanger & water heater be in close proximity to be effective, if that was the issue. Even a 50 foot round trip of 3/4" copper plumbing between the two has only about 1.25 gallons of water in it, or about a half-minute of shower flow.

  5. Jon_R | | #5

    Another vote for low (like 1 GPM) shower heads. So the heat pump water heater and pipes are in unconditioned space in a climate where temperatures go well below freezing?

  6. GBA Editor
    Martin Holladay | | #6

    Chris,
    Jon R has identified the problem, of course, and I'm sure you have figured it out, too. Operating a heat-pump water heater will lower the temperature of the room where the heater is located, anywhere from 2 F° to 6 F°. The aim is to locate the water heater in a room that doesn't drop below 50°F. I doubt if an unconditioned garage attic in Climate Zone 3 or 4 is warm enough to meet that goal.

    For more information on this issue, see Heat-Pump Water Heaters Come of Age.

    That article also emphasized the need for a large tank:

    "All of the researchers I talked to emphasized the benefits of a large tank size, so don’t buy the G.E. unit, the Rheem unit, or the smaller AirGenerate unit [that is, don't buy a unit with a 50-gallon tank]. “Bigger and hotter tanks [60 gallon to 80 gallon tanks] are better,” said Aldrich. “It’s counterintuitive.”

    "According to “Measure Guideline: Heat Pump Water Heaters in New and Existing Homes” by C. Shapiro, S. Puttagunta, and D. Owens, “The units with smaller tanks demonstrated difficulty in maintaining hot water delivery in high demand situations, even if their electric resistance elements are used. The units with larger tanks provide a buffer in times of high demand and therefore are expected to use their heat pump for recovery, rather than reverting to electric resistance heating to maintain outlet temperature. The result is more efficient operation and better performance in terms of availability of hot water. In households with more than two occupants, a HPWH with a larger tank will likely be a better option.”"

  7. CMOD | | #7

    Dana- As you speculated, the issue isn't the distance from the drain to the water heater. It's that the drain is in an awkward location and relocating the drain to be near the supply or the supply near the drain involves a lot of demolition and reconstruction I'd rather not do. There would be possible installation of a ~48in unit horizontally in the crawlspace, but that's still pretty high cost and would come with an efficiency reduction due to the horizontal configuration. I do like the idea of a passive solution though.

    Jon R - I had considered shower heads, but the existing ones are only 7-8 years old and aren't super high flow. One is a 2.2gpm and the other has no label but seems about the same. It is unconditioned space, but outside temps that go below freezing are rare, and crawl/garage temps that go below freezing are very rare, even exceptional, and tend to be short lived.

    Martin- I was aware of both of those recommendations before we installed the HPWH, and even called various manufacturer's technical staff to discuss. The primary reason we went with the State WH was that their manual and support staff assured me that it was fine in the garage and that if the environment was too cold for the Heat Pump to work effectively, it would switch over to the electric element. From what I can tell it does this around 35-40deg ambient. So the HP still technically "works" at 35-50, it just has to run a while to collect enough heat. As mentioned, we would have preferred the 66gal for a bigger safety margin, but the 50 was the only one that would fit between 24" o.c trusses to get it in place. Based on previous experiences we also concluded that 50 gal. was probably oversized enough, even accounting for longer recovery times. That's still technically true 97% of the time, with the exception of mornings that hit around winter averages (but not colder).

    I will probably see how things go once I get all the pipes insulated, and may try a different shower head as well. But I'm still leaning towards the electric booster as a failsafe. Cost and hassle of installation are pretty minimal, and should run pretty rarely.

  8. Expert Member
    Dana Dorsett | | #8

    Chris B: I'm not clear as to what is meant by "...the supply..." in the comments "...elocating the drain to be near the supply or the supply near the drain..."???

    As for other passive solutions...

    Take a serious look at your option #3, and consider insulating it as well. Does it have any south facing windows for solar gain?

    An insulated garage on a slab in zone 3 is at least somewhat earth coupled to deep subsoil well north of 55F (and could be more so with slab-edge insulation), and would probably average above 50F in winter if helped along with even a modest amount of solar gain as long as the garage door stays closed most of the time. Typical January mean outdoor temps in zone 3 are usually above 40F, and insulating it would increase the average indoor temperature in winter while decreasing the interior temperature swings, raising the overnight low indoor temperatures by quite a bit. Atlanta GA is on the cool edge of zone 3, close to zone 4, and has a mean January outdoor temp of about 45F with January daily highs normally higher than 50F.:

    https://weatherspark.com/m/15598/1/Average-Weather-in-January-in-Atlanta-Georgia-United-States#Sections-Temperature

  9. user-2310254 | | #9

    Chris,

    You might want to try a couple of inexpensive 1.25 gpm shower heads. If they help with your water heater performance, you could always move up to a better built and more aesthetically pleasing fixture (or not). I also would insulate the crawlspace lines. It's not unusual for Zone 3 to have a few days of single digit weather. It might not happen this year, but it will happen.

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