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Musings of an Energy Nerd

Attaching Ducts to a Heat-Pump Water Heater

If you install this type of water heater in a small room, you’ll need to provide ductwork

It's impossible to install a heat-pump water heater in a confined space unless the appliance is equipped with ducts. [Image credit: A.O. Smith]

An increasing percentage of green builders are choosing to install heat-pump water heaters — appliances that use an air-source heat pump to extract heat from the air, transferring that heat to water stored in an insulated tank. Most heat-pump water heaters incorporate both of these components — the heat pump and the insulated tank — into a single unit, with the heat pump compressor mounted above the insulated tank. This type of water heater is typically installed in a basement, mechanical room, or (in warm climates) in an attached garage.

If you look at the heat pump compressor mounted on top of a typical heat-pump water heater, you’ll notice that there are two openings for air flow. The unit has a fan that pulls ambient air through the intake. Inside the unit, this air passes over a copper heat-exchange coil and transfers some of its heat to the refrigerant flowing through the coil. Then the air, now cooled, is expelled from the exhaust side of the appliance.

This process raises the temperature of the refrigerant and, eventually, the water in the tank. The process also cools the air in the room where the heat-pump water heater is located. If a heat-pump water heater is installed in a very small room — a closet or a tiny mechanical room, for example — the room can get so cold that the heat pump can’t operate properly. That’s why the manufacturers of heat-pump water heaters dictate the minimum volume (generally, 1,000 cubic feet) of the room in which this type of appliance can be installed.

The larger the room, the lower the chance that the room’s air temperature will sink to a level that is too low for efficient operation of the water heater.

Some manufacturers of heat-pump water heaters allow installers to connect these…

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20 Comments

  1. Sean Conta | | #1

    Thanks for the great info Martin.

    In the Pacific Northwest we've advised builders that if the unit is located inside the heated space, ducting both intake and exhaust to the outside is a good option for 2 reasons - 1) you avoid pressurize/depressurize issues on the building envelope in a tight house and 2) the outside temperature is above 35 F most of the year up here and I believe that's the cutoff point where most of these units switch to electric resistance heat. Even though the heat pump will run less efficiently at colder air intake temperatures isn't that still better than ER mode?

    Oh and the other assumption would be this is a small home without a good spot to dump cold air into...

  2. Andy Kosick | | #2

    A Habitat project I'm working with will be installing a Rheem and ducting the exhaust into a conditioned crawl. The sole criteria is really comfort, it seems like a win/win to let the cool air temper in the crawlspace before returning to the house and also keep the intake temps a little higher throughout the cycle. This article is reassuring in that the ducting should have a negligible effect on performance.

    I'd like to second the factory filter not filtering very well. The one on my GE at home doesn't filter well and seems overly restrictive at the same time. Removing it is worth a good 5 db of noise when it's running.

    I realize this is a bit of topic, but John Semmelhack's comments about upgrading the filter for the HPWH touch on a concern I have about the Habitat home I mentioned above and others like it. That home will have 5 filters in it that need maintenance, 2 mini-split heads, 2 in the ERV, and the HPWH. If many of those are replaceable the cost could add up, and if they're washable the reality is that many homeowners aren't always that good at keeping up with this stuff. Is anyone out there doing long term maintenance contracts to make sure this stuff is getting done? Reminder systems at least. Ideas along these lines for affordable housing? As someone who does a lot of existing home analysis, maintenance is a serious issue. Dealing with the long term operation of residential high performance homes seems like a good topic unto itself. I'm curious what others are doing.

    1. Sean Conta | | #3

      Great point about maintenance Andy. My feeling is that roughly 0% of homeowners can be trusted to clean/change filters. I'm about to build a small home for a client and plan to "strongly suggest" they pay me a small fee to come do periodic ERV / mini-split / HPWH filter cleaning. Also gives me a chance to make sure equipment is running smoothly. Now, how many years can I realistically do that? Not really sure...

  3. Michael Bluejay | | #4

    I ducted my new heat pump water heater a couple years ago, with baffles, so in the summer I draw from my hot room and exhaust cooler air into the same room (free air conditioning), and in the winter (climate zone 2, always above freezing), I draw from the outside and vent to the outside.

  4. Jon A | | #5

    I just installed a Rheem 65 Gallon HPWH in a 500 SF unfinished, unconditioned basement with no other heat source. I'm in Massachusetts so the basement room usually bottoms out around 45 degrees which is where it sits now. During extended operation of the water heater the room will drop to about 43 degrees temporarily. Rheem says this unit can run in heat pump mode down to 37 degrees but I'm noticing the recovery time is very slow, which is expected. My question is this...does it make sense to duct the intake side of the water heater into the adjoining finished 600 SF basement room in order to steal the warmer air thus increasing efficiency and recovery time? I would plan on leaving the unit exhausting into the unfinished basement which would benefit from the drier air, but I could return the exhaust back into the finished room if needed. It seems to me that this would help with efficiency but would it be significant?

    1. User avater GBA Editor
      Martin Holladay | | #6

      Jon,
      Your suggested ducting experiment is certainly worth a try.

  5. Kevin Camfield | | #7

    We are thinking about installing a HPWH in our laundry/mechanical room. The laundry room is 850 cubic feet. The laundry room will also house the HRV system for the house. If we put a supply and return vent from the HRV into that room, will we be able to exchange enough air to negate the cooling effect of the HPWH? The internal ducted multi-position unit for the multi-split will also be in that room. Would another option be to do a significant enough air exchange through that unit to distribute the cooled air throughout the house? I think this latter option would have to be mainly on the supply side to prevent overheating of the room when the HPWH is not running. The multi-position unit will be run in an always-on type set-up in low speed.

  6. User avater GBA Editor
    Martin Holladay | | #8

    Kevin,
    Your HRV should be set up to provide the desired cfm of supply air as specified by ASHRAE 62.2 or to meet your family's needs. It seems a shame to devote a portion of your HRV's air flow to the mechanical room, where fresh outdoor air isn't usually required. Of course, an exhaust grille (connected to the HRV system) makes sense -- and if you have an undercut on the door, that exhaust grille will help with air movement into the laundry room.

    If this were my house, I'd probably live with the situation and see how cold the room gets. Of course, there is a downside to installing a heat-pump water heater that is smaller than the minimum recommendations set by the HPWH manufacturer -- you may void the warranty if you don't follow instructions. Options include a louvered door on the laundry room (a method approved by some HPWH manufacturers) or ducts connected to the HPWH as advised in the article on this page.

  7. Kevin Camfield | | #9

    Martin,
    Great advice as always. Based on the information in the article and the comments here I decided to go with the Rheem unit in a big part due to noise. It is only 49dB which compares to my current fridge at 44dB (according to the iphone noise meter app). The Rheem calls for a minimum room size of 700 cubic feet (thanks for the link). So I think I should be okay with an HRV exhaust as one might normally do with a laundry room and balanced HVAC connections for normal room conditioning. If I have to duct HPWH I can go out into the hall discharging right above where the main air intake is for the HVAC ducted unit.

  8. user-7665154 | | #10

    Thanks for this Martin. I'm about to install a HPWH on Long Island--likely the Rheem or Ruud. I had been planning on using an intake duct that would draw air from outside in the summer. In the winter, I would disconnect and cap it. The discharge duct would go into the basement year-round for dehumidification.

    I know that you strongly discourage a seasonal approach. But I'm one of the zero percent that would remember. I've worked on every inch of this house and pay a lot of attention to my heating/cooling! I'll have to remove it prior to dying, however.

    Considering the outside air in the summer could be a bit less than 20 degrees F warmer than the basement air, I'm still tempted to use a duct on the intake side during the summer. Even though our unconditioned basement is huge (perhaps 3-4000 sf) I can't help but think that the differential would provide some benefit. Do I need to be talked down from the cliff?

    Actually, the more I think of it, especially considering the cost of running the duct through the exterior wall ($250, unless I do the install myself), it's seeming less and less attractive. I'm being my own therapist here :-)

    Any thoughts? Thanks!

    1. User avater GBA Editor
      Martin Holladay | | #11

      User-7665154,
      If you pull in outdoor air for your heat-pump water heater, and discharge that air into the basement, you are in effect pressurizing your house. I don't recommend this, for a variety of reasons -- I'm in favor of houses that try to maintain a neutral pressure with respect to the outdoors. Pulling in outdoor air during the summer will also tend to increase your air conditioning costs.

      1. user-7665154 | | #12

        Thanks for the response Martin. Looks like I'll forgo the duct.

        You said "Pulling in outdoor air during the summer will also tend to increase your air conditioning costs." I was under the assumption that any warmth in the outside air pulled in would be extracted by the heat pump, sending cool, dry air to the unconditioned basement. What am I missing?

        1. User avater Expert Member
          Dana Dorsett | | #14

          >"What am I missing?"

          It may or may not be delivering cooler/drier air than the air that is being pushed out of the house by pressurizing the house, but it's definitely NOT lowering the cooling load by very much even in the best-case scenario.

          The humidity of the outdoor air is much higher than that of the indoor air (especially in the "-A" climate zones) in summer, so the temperature & humidity of the output of the heat pump water heater is guaranteed to be higher than if it were drawing from indoor air.

          1. user-7665154 | | #15

            Thank you Dana!

  9. User avater
    Peter Engle | | #13

    The HPWH reduces the temperature of the incoming air by about 20 degrees. If you're bringing 90 degree air inside, it'll only drop to around 70 degrees. That's not much cooling. And, the dewpoint of that air might not be much less than 70 degrees, making it pretty damp. if you want the HPWH to cool and dry the basement, you should just recirculate the basement air through it. Using outside air will increase the COP of the water heater a bit, but it will likely cause other issues that don't make the exercise worthwhile.

    1. user-7665154 | | #16

      That's really helpful Peter. To your credit, the two energy efficiency experts who came to do estimates didn't know what you guys know!

      Kudos to you. Thanks!

  10. nms43 | | #17

    I'm thinking to put the HPWH in my small laundry/mech room with a well louvred door.. and also locate an ERV exhaust point there.

    This way the ERV is sucking out the cold air from the room and constantly pulling warmer air into the room. Seems like a solid idea?

    1. User avater GBA Editor
      Martin Holladay | | #18

      NMS43,
      During the summer, the HPWH will be cooling your indoor air--a desirable feature. Instead of sucking the cold air out of the mechanical room and sending it out of the house, you might want the cool air to linger.

      During the winter, pulling cool air into the exhaust duct of your ERV will lower the effectiveness of the ventilation system's ability to warm up the incoming fresh air that is pulled from the outdoors into your ERV's core.

      In short, there can be undesirable consequences associated with your plan, and the energy implications are complicated.

      1. nms43 | | #19

        Hey Martin! Thanks for chiming in.

        For the summer issue you could place a damper on that exhaust point so it's not sucking out the cold air. Being in climate zone 5 I'm more concerned with the unwanted cooling during heating season.

        The HPWH has to steal it's heat from somewhere. What would be a better option? In my case there is no attic, crawlspace, or basement to utilize.

        1. Andy Kosick | | #20

          My increasing experience with these has led me to a couple conclusions since first reading this article.

          1. The HPWH has to steal heat from the house in a cold climate, don't pretend otherwise, just deal with possible comfort issues.

          2. In most cases the HPWH only runs a small portion of the day and you're actually a lot less likely to notice it than you think. The room it's in returns to normal quite quickly. This comes from customer feed back, but everyone is different.

          3. With the Rheem model you can schedule a tank temp increase in the night time hours, if you have a larger tank, a mixing valve, and run up tank temp, it can do most of it's pumping at night when no one will notice. (depends heavily of usage) It could also do this during the day if no one is home and sun is shining on some PV.

          4. I don't think this was mentioned in this article, and I have not had the oportunity to do this, but I believe John Semmelhack has ducted one into the heating/cooling duct work so the temperature is evenly dispersed, thermostat keeps things warm and no one notices a cool spot at all. Duct design must account for (or at least tolerate) the air flow from the HPWH.

          Good luck.

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