I received an email from a homeowner in MI asking about deploying an HPWH in MI. Rather than responding directly I've elected to post the question here for all to see and reply:

New construction in Michigan, 4-person family, W to W Waterfurnace
> w/desuperheater, radiant infloor heat in basement and 1st floor,
> walkout ranch, 2,400 sq.ft.
>
> Does a HPWH make any sense in the unfinished basement near the WtoW
> geo/desuperheater?  Any "heated air" created by the geo/desuperheater?
> ROI?  Husband thinks "yes" - I'm not sure...have to make decision
> asap.


My recommendation is first to ensure the desuperheater has its own unpowered buffer tank.

For an HPWH to work, it needs a fairly sizeable room to work in...10x10 or better. In summer it may produce useful cooling and dehumidification in a basement. However in winter, it may add a bit to the heating load of the house since it cools the surrounding air. In extreme weather, summer and winter, an HPWH shouldn't need to run much since the desuper (with its own buffer tank, of course) should produce the majority of hot water. During mild weather HPWH will be meeting most or all of domestic water heating load. I would expect its 10x10 room to become markedly cooler (5-10 degrees) during such times.

There is some heat thrown off by operation of a heat pump and desuper, but it likely isn't much relative to heat required by an HPWH in full cry. An HPWH does have the effect of recycling its own standby losses, essentially "pumping" them back into its tank.

Owner of an HPWH has to be willing to tolerate noise similar to a small room airconditioner or dehumidifier...not generally an issue in a basement mechanical room. HPWH does not recover as quickly as a resistive water heater, so upsizing might be needed / desireable to maximize efficiency.

ROI may be quite long. A family of 4 heating water resistively only at national average electric rates might spend about $500 per year, possibly a bit more in MI owing to colder than average incoming domestic water. A Desuper should halve that cost. An HPWH would likely halve it again. Therefore, the HPWH annual savings will be on the order of half of half, IOW, about $125. This will result in a long ROI time for an HPWH likely to cost $2k installed.

I do expect HPWH prices to fall after the tax credit shrinks or goes away in 2011.

You might add that in a family of 4 it'll take running the internal resistance elements to have any chance of keeping up with serial showering in winter. They all have very sucky first-hour ratings on most- even the bigger ones), further reducing the ROI.

In families of 4 that shower as opposed to tub-bathing the extra money would be better spent on a drainwater heat recovery heat exchanger, that would double the first-hour rating of the system while in showering mode. But even then you'd have to do the financial analysis with a sharp pencil in hand to determine it's cost-effectiveness with a desuper providing such a large fraction of the annual load.

This is why I like to publicly discuss questions I get here rather than responding privately.

I had in mind adding remarks suggesting that a load adequately met by a 50 gallon storage electric water heater might need an 80 gallon HPWH to ensure adequate delivery while preserving the touted efficiency, in essence allowing operation solely in 'eco' (heat pump) mode or at least minimizing resistance element operating time. An 80 gallon unit commands a premium of $500+, driving up ROI.

I had forgotten about a drainwater heat recovery exchanger.


That the drainwater device only works for showers is an important consideration - the hot water use has to be of a nature that warm water is running down a drain at the exact same time as replacement cold water is entering the water heater. This pretty much only applies during showering. If showering constitutes half a home's hot water use and a drainwater system recovers 1/3 of the heat otherwise lost, then a drainwater system could shave 1/6th off an annual water heating bill.

Dana - does 1/3 seem reasonable to you, taking into account cycling losses, mixing and cooling as drainwater makes its way to the unit, etc? An example on a drainwater website shows 104*F water entering the unit...I think that's overly optimistic at best - most folks shower at less than 104, and water leaving a shower head at 104 isn't anywhere near that warm by the time it hits the shower drain, much less when it gets to a recovery unit.

Depending on the utility company in MI payback horizon can vary dramatically as well. Consumers energy for instance charges nearly 1/3 more per kwh for electric heating than does DTE. You definately want to contact whowever your elec company is to explore discount rates.
We have not installed any residential HPWHs in MI to date as I don't see a great ROI on them in resi settings.
I concur with Curt that unit pricing should drop dramatically in next year or 2 (I'm guessing units to cost 1/3 less).
Joe