Does a heat-pump water heater make any sense in this situation?
I’m looking for water heater replacement options. Currently unit is a Takagi propane tankless. It’s installed in the unconditioned attic due to space constraints inside the house. As part of insulating the attic with much thicker insulation, I need to move it from its current location and would need to re-run the vent if the unit is reinstalled, necessitating some new parts (this is stainless flue material, and fairly expensive).
Initial plan was to simply replace the tankless with a standard electric tank, but I got into a conversation with another contractor who is involved in a multi-family development nearby, and they are spec’ing heat pump water heaters installed in attached garages. I was surprised by this, as I have only seen two of those in the area, and think of them as warm-climate appliances. But, the fact of the attic location intrigues me–much of the year the attic is probably fairly warm, as the house is out in the open and has a charcoal-gray roof.
If I understand correctly, the unit would use typical heating coils when the ambient temp was too low to keep up using only the compressor, so at least efficiency shouldn’t be any worse that a standard electric unit.
Can someone who is familiar with these units offer some guidance? This is Zone 4 marine… winter temps in the mid 30s to 40s, and winter is short. Electric is 7.5 cents and propane is >$3.00.
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David,
Assuming that you can keep the pipes from freezing -- and it sounds like you can -- I think that a heat-pump water heater probably makes sense in this case. You have a mild climate, expensive propane, and cheap electricity.
David, you mention an unconditioned attic and adding much thicker insulation. Is this a vented attic (soffit/ridge) with insulation on the floor, or is it unvented, with insulation under the roof deck? Air moving through a vented attic space will provide much of the heat pumped into the water tank. If the attic is unvented, the heat pump will cool off the air space around it quite a bit, perhaps more than will allow the heat pump to operate efficiently.
The attic has four gable and four gable vents, no eave/ridge venting. The attic floor is being insulated.
Any comments on devices like this? http://www.airgenerate.com/retrofit.php
One thing I don't necessarily like about heat pump water heaters is the fact that an expensive appliance is attached to a water tank that might rust out or otherwise fail. Separating the heating system from the storage makes some sense to me.
That roof configuration would bother me, as far as locating an ASWH up there on the floor. I wouldn't think that you'd get much air flowing through those vents, up high, while you are producing cool air down low, unless the wind is blowing. When the sun shines (in the PNW?), sure, the warm air in the attic will be sufficient. It's the cold, rainy days when I would expect trouble. Imagine an A/C unit in the attic space in that weather. How cold could you get the air up there over a couple of hours of running the A/C, if all you have are four small open "windows" up high to bring air in to keep the A/C unit loaded? I would think that the combination of very cool air and high relative humidity would give you frosting conditions on the evaporator coil easily. Mind you, I have no experience with ASWH. I'm just playing devil's advocate here.