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Community and Q&A

Air source heat pump water heater in a small mechanical room

user-1055444 | Posted in Mechanicals on

I am building a 2-unit addition to a duplex in Portland, OR, resulting in a 4-plex. The new units will be largely separate from the old, and will be built to PH-like standards (very tight air barrier, heavily insulated). With an eye toward economy, I am thinking for the 2 new residences to share a mechanical room. In this mechanical room would be water heater(s) for both new units, along with an HRV (likely Zehnder 550) serving both units.

Relevant plans and considerations:
– I am considering going with an ASHP water heater (GE Geospring) for efficiency’s sake, but understand that the ~50sqft room would not be voluminous enough to support an ASHP.
– I will possibly have a chest freezer stashed in this mechanical room, generating some amount of heat (i.e.
– I will likely have a through-wall air conditioner type unit cooling an adjacent insulated wine closet, and would vent this unit’s hot air into the mechanical room (i.e. ~2000BTU)

– I’m providing DHW for 2 units, with 5 bedrooms in total. Dishwashers and clothes washers to be fed cold water only, so DHW only providing for bathroom sinks and showers (4 baths total), and the 2 kitchen sinks. Could I economize by doing something like using one ASHP water heater in series with a HE resistance heater as a storage/backup? What would be the best way to go about this?
– Would the heat generated by the chest freezer and wine cooler be enough to supply the ASHP’s needs? Is there any good way to calculate for that? Or should I just think about ducts (below)
– GE has a ducting kit for their ASHP. Will this be too messy (leaky) for use in this type of construction (assuming vented to exterior)? Should I dare consider doing something creative like porting the cold air into the wine closet and taking heat from the ambient room air? Should I just duct in from the adjacent kitchen and out toward the adjacent hallway, etc.? Could I (gasp!) duct supply and return air for the ASHP using the HRV located in the same room?

I’m interested in hearing your thoughts on the matter. I like the idea of elegantly killing multiple birds with one stone, but don’t want to unnecessarily overcomplicate matters.


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  1. GBA Editor
    Martin Holladay | | #1

    Every manufacturer of heat-pump water heaters provides information on the minimum volume of the room where the water heater can be installed. If you choose to ignore these instructions, the water heater manufacturer is under no obligation to honor the equipment warranty. So I advise you to build a mechanical room that is large enough for your planned equipment.

    I wouldn't depend on having heat available from your freezer and your air conditioner when needed, because there is no guarantee that those two appliances will be producing heat at the time you need it.

    In general, using a small room as a heat dump for a freezer and an air conditioner is a recipe for disaster. What happens when your tenants go on vacation and there is no demand for hot water? The freezer and the air conditioner will both dump heat into the small room, and the air temperature in the room will rise to the point that these two appliances have trouble dumping heat.

    I'm not a fan of attempts to use ductwork to solve the problem of an undersized mechanical room. These ductwork schemes are over-complicated, and it doesn't take much of a problem with your equipment to have a big headache. Keep things simple.

  2. charlie_sullivan | | #2

    I generally agree with Martin, but not on the issue of timing of the heat generation and consumption. There will be enough thermal mass to keep the heat around for when you need it, and you can tolerate temperature swings in that room. The better argument is about overheating that room when the occupants are on vacation. Avoiding that problem might require a thermostat-controlled vent fan that would only turn on in that rare case.

    Figuring out the heat output of the freezer is easy---just look at the rated electric consumption. But for the site-built wine closet, you'd need to do a cooling load calculation and look at the COP of the A/C under the appropriate conditions, which might be outside its specified operation range.

  3. Expert Member
    Dana Dorsett | | #3

    In a Portland OR climate Sanden's EcoCute hot water heater would work just fine, at much higher efficiency than a GeoSpring, if you don't mind having a mini-split type compressor on the outside of the building. That gives you access to the great-outdoors as your thermal mass.

    They're not cheap, but the performance is pretty good, as tested in the Pacific Northwest by the NEEA consortium:

  4. user-1055444 | | #4

    Thank you for the replies. It sounds as though the waste heat might be doable, but isn't ideal. For what it's worth, this will be my residence for the foreseeable future, not a rental (though comments about future rentals are valid); plan for vacations would be to leave the room's door ajar (granted, easily-forgotten). Also, this room cannot grow any further. If ASHP water heater is nixed, I'll be going with electric resistance instead.

    What about the ducting accessory that GE sells to go with its Geospring? My backup plan is to duct to the exterior--is that ill-advised based on anecdotal evidence? And what of the idea to use two water heaters (one ASHP, one resistance)--would plumbing them in series be wise? Or would I better off doing just one ASHP, and one on-demand electric for backup?

    Dana, thanks for the mention of Sanden. I've long-since ruled them out due to price--it simply ends up being too slow of a payback, and too much capital for me on the front end...

    Thanks for all the help!

  5. Expert Member
    Dana Dorsett | | #5

    If you use the Sanden for both space heating and hot water heating the numbers work a bit better.

  6. mackstann | | #6

    There's another very simple solution, straight from the manual:

    Because this unit draws in air from the room to heat the water, the
    room must be at least 10’ x 10’ x 7’ (700 cubic feet) or larger. If the room is
    smaller, there must be a louvered door. Louvers should be 240 square inches
    (0.15m2) or greater. If two louvers are used, one should be near the top of
    the door.

  7. charlie_sullivan | | #7

    As I understand it, the ASHP water heater has the electric backup built in. So if it somehow makes the room to cold to work as an ASHP, it will switch over automatically.

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