Freeze risk for a heat pump water heater in an unconditioned garage
I have an unconditioned garage with a gas water heater in climate zone 5B. The garage is unconditioned, but insulated (R-15 whole wall, no slab edge or sub-slab insulation), and obviously has some exposed plumbing in it. Right now, the temperature in the garage doesn’t get low enough to present a freeze risk, probably aided by the water heater’s standing pilot light and hot metal flue. But if I replace it with a heat pump water heater, the unit’s going to be continuously cooling the room, right? I keep thinking about how this can only possibly increase the risk that the pipes will freeze in winter. I do understand that the uninsulated slab is a moderating influence, almost a poor man’s ground-source heat pump–but how can I know if that will be enough besides doing it and being sad if my pipes burst and happy if they don’t?
Is the heat-pump-water-heater-in-an-unconditioned-garage technique limited to non-freezing climates?
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Replies
Nathaniel: check with the manufacturer to be sure, but I think HPWHs are supposed to be used in warmish locations, i.e. above 45°F. In addition, the lower the ambient temperature, the lower the efficiency. Couple that with the fact that the heat pump will cool the air even more and it sounds like a risky idea. If your WH runs on natural gas, you might not save any money by switching.
My gas water heater is on its last legs, so I have to replace it with something. I am aware that I will probably not save any money no matter what I replace it with since my whole family's monthly hot water expense is just $5 or so right now. I only want to switch to something electric so I can seal up a roof penetration and offset it 100% with solar PV in an upcoming project. My thought was that a HPWH would let me get away with have fewer panels, but the only location I could put the unit is my unconditioned garage. I keep reading people extolling the virtues of HPWHs in unconditioned garages with uninsulated slabs, but it seems like that's really only viable in places where there's never any freeze risk when you include the cooling effect of the unit itself.
Hard to see much freeze risk as long as the power is on.The garage is attached to the house and is ground-coupled, right? The tank is full of hot water and will protect itself, and heat will travel some distance down the piping connected to it. Insulate all of the piping well and keep the runs short--use PEX and minimize the number of metal fittings in the garage space. I think it would have to get in the mid 20s in the garage before there would be any risk.
Nathaniel,
I have to disagree with David. I live in Climate Zone 6, which is colder than Climate Zone 5. (A couple of weeks ago, my thermometer hit -29°F. Yesterday morning it was warmer; it was -14°F.) But temperatures in Climate Zone 5 can easily hit 0°F.
Will plumbing pipes freeze in a garage when the outdoor temperature is 0°F? Of course they will. And if you install a heat-pump water heater in that space -- an appliance designed to lower the temperature of the room in which it is located -- you're just about guaranteeing frozen pipes.
David, you live in a nice, mild climate. Garages in the Pacific Northwest behave differently than they do in Massachusetts.
Most units probably have the option to disable to heat pump in winter and only use it in the summer (and in the winter just use it as a conventional electric heater.
As an electric, you have lower standby losses than with a gas-fired tank, but unlike with gas, all that loss stays in the room rather than going up the chimney.
Martin, OK, setting aside the question of outdoor temperature, do you think a change from a gas WH to a heat pump WH will have a significant impact on the temperature in the garage? My wild guess is that the pilot light is contributing only a tiny amount of heat. The flue does contribute some when the unit is firing. There will be jacket loss from either type of tank. When the temp is in freezing range a HPWH will probably be in resistance mode and not further cooling the garage much. Will a switch to HPWH push him into the danger zone?
David,
Q. "Will a switch to HPWH push him into the danger zone?"
A. Yes.