Over the past year or so I’ve read a bit about new heat pump water heaters (GE, Rheem, Whirlpool, and several others offer these now). These units pull heat out of the air and put it in your domestic hot water via the refrigeration cycle.  

Problem is the manufacturer and government rating for energy star and tax credit; unit C.O.P.,  ignores the impact of the heat being removed from the surrounding space.  The surrounding space will either need more heating capacity or use less cooling capacity.. depending on the climate.  One case where you could possibly ignore the heat taken from the space is if you happen to have an application where the average annual space heating load is equal to the average annual space cooling load.  Even then, you have to assume steady space and domestic hot water loads whereas in reality they will have lots of peaks and spikes that will not line-up.

I searched around a bit and couldn’t find any explanation on how to account for this in a cost/benefit (payback) analysis. Since I live in heating dominated, I came up with the following for effective “heating season” COP ……
Effective Overall HPWH COP = HPWH_Rated COP / (1 + (HPWH_Rated COP / Space Heat_COP) )

Example…
Hot Water: GE Model HPWH COP 2.35 has one of the highest published/rated COP.
Space: ClimateMaster Heat Pump has rated heating COP of 3.5 (derated to include pumping penalty for my open system)
 
Effective HPWH COP = 2.35 / (1+ (2.35 / 3.5 ) ) = ~ 1.4

If the above is correct, assuming a heating dominated climate and ignoring cooling side benefit and tank loses (due to similar tank insulation)… savings over a standard electric water heater is less than 30% (1 - ( 1 / Effective HPWH COP ) ).  Assuming average annual cost of $470 per year (electric resistance hot water heater DOE yellow sticker), savings is just over $11 a month.   Even after tax credits and rebates its going to take a long time for these savings to pay back the upfront premium.   If space is heated with anything that is less economical (say natural gas or propane), the payback is going to be worse (longer). 

Deep down the engineer in me wants to see this cool new technology in my basement (so hopefully I'm missing something! :) )...   but from all the above the current price premium and only modest savings make them sound like a poor investment for heating dominated climates.

Does this sound right?

Actually discussed at length previously.
For the most part some of us concluded "location, location, location" had much impact on ROI
Joe

As an extensive participant in some of the prior HPWH threads I agree with the foregoing analysis. I think it is worth noting that even in most heating-dominated climates there are several months where heating is not needed and that dehumidification, if not actual cooling, is genuinely helpful in many basements.

New DOE rules mandate that beginning in 2015 all electric storage water heaters of tank size >55 gallons will be required to be HPWHs. Given that fact and that the refrigeration components in a HPWH aren't much different from those found in a $200 portable dehumidifier, I think we can shortly expect substantial price drops for these units. As they become a commodity essentially combining components of electric storage water heaters (tank, resistive elements, insulation, fittings, controls) and a portable dehu (compressor, refrigerant to air heat exchanger, condensate plumbing, controls) I would expect HPWH prices to drop to near the sum of prices of conventional storage electric water heater plus portable dehumidifier (that is well below $1000) as economies of scale and wider competition kick in. I would also expect EFs to rise as compressors, fans, heat exchangers and pumps are optimized for this application.

Within 5 years we should see HPWHs costing ~$800with EFs approaching 3.0, so folks with less favorable ambient conditions such as exist in heating dominated climates would be well advised to hold off. Let we afflicted with 8 month cooling seasons serve as early adopters of this technology; we'll bear the initial higher prices needed to move HPWHs into the mainstream.

Joe/Curt-

Before I posted this thread, I did a forum wide search on heat pump water heaters and somehow must have missed the threads you are referring to above.

At any rate, you mention excellent points.

Right now, the benefits of reducing humidity and reducing air conditioning load, are hard to quantify with $$'s when I don't own or run a dedicated dehumidifier and run the A/C so liittle. I'm thinking this will change, when our basement is finished and closed up, and as parents/kids in our house get older and lest tollerant of climate variations in the house !

Curt - I'm glad to hear you folks down south are so willing to blaze the path and foot the bill !! Hopefully you are correct and soon prices for these will drop and effiiciencies will increase!

Fopr a spirited debate try:
http://greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/76818/afv/topic/afpgj/4/Default.aspx#59227
If that doesn't work search the thread title:
"Desuperheaters, buffer tanks, mixing valves, AirTaps, and more! Oh My."
j