HPWH as hydronic supplement for A2AHP
Out of nerdly boredom, I have been thinking about trying to supplement my underpowered A2AHP (always disappointed and poorer when the heat strips run for hours on end) in zone 7b with a homebrew hydronic system consisting of a hydronic fan coil, a wee water pump and a hphwh (I can find these in working order for <$500 on marketplace). My thought is to put the water heater in my carport shed (right next to house) where the carport/shed roof has a ton of solar gain. (House roof would be better for larger solar gain but I don’t want to put a water heater in my attic). Heat water up daytime when the shed roof is warm drawing hphwh supply air from the shed sealed roofspace, maybe store more heated water in a spare (available for <$200) water heater tank or two in my coal cellar. Then, run stored hot h2o through the fan coil on cold nights/mornings when a2ahp cannot keep up or recover temp after nighttime setback. Obvi won’t work if I let snow accumulate on shed roof, but this is rare. Shed roof insolation in winter is likely better than 40 kWh/day, so could bank some substantial portion of 10 ton-hrs of heating as mcdeltaT? I wanna say the compressors on those things are just under a ton so that might limit daily accumulation more than insolation? Other than being a waste of money why won’t this work?
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You'll run into two things.
HPWH don't like to run bellow 50F and they don't like to run for long periods of time. When you stick them in a small space like that, they will quickly cool the air and I doubt you'll get enough solar gain in there to keep up.
A 60 gallon tank heated to 140F and cooled to say 90F is only 25000BTU (or 7.5kWh), so not a whole lot of heat.
In zone 7, you'll have to run some resistance heat on cold days, if you want lower cost, PV is the better answer.
Thanks, Akos. I understand that they don't generally work well at low temps (high lifts)... I would have to investigate the temps that the shed roof achieves. However, I was unaware of the "don't like to run for long periods of time" issue in hpwh. I've always understood that it is short runs that are harder on heat pumps (that big startup inrush current jostling the barely-insulated windings via lorentz force and the general thermal cycling of all of the brazed refrigerant circuit joints) and that running for a long time without shutoff is no prob and even preferrable. Can you help me understand the issue with HPWH running for long periods (or point me to a resource?). Admittedly, they were clearly not designed for what I am proposing
If you look at the literature of most HPWH, they don't allow you to use constant recirculation. The reason is that the constant recirc causes the heat pump to run too much.
My guess the compressor is designed only for intermittent operation, run it too long and something melts.
My impression is that this is a loss-of-efficiency issue, not a wear/damage issue with the recirc. If I try it, I will let you know
My understanding is that the heat pump expects the tank to be striated, with cold water at the bottom and warmer water at the top. The refrigerant runs from top to bottom so that it always exits at the coldest temperature in the tank, giving the highest temperature differential and maximizing efficiency. Running a circulator causes the tank to mix and upsets this assumption.
so then you are saying that the "no recirc" commandment doesn't preclude my abuse of their engineering for purposes of hydronic heat supplementation and nerdliness?