Have a GSHP with DSH and happy with it. My results have been so good that parents are thinking of going the same route. - Why should they pay an extra 1k for a DSH when they could pay 800 more for a heat pump water tank. The advantage to the latter would be that it can work year round even when the GSHP is not working - it would cost less - and seems according to the numbers to save more money. - http://www.aceee.org/consumer/water-heating#new - See "step 2" in above link - Given the above - why don't electric heat pump water tanks replace DSH in most applications? I know it robs heat from the house - but that helps in summer - is neutral in spring and fall and only cools basement in winter (we live in Cleveland). - Any thoughts appreciated.

Todd,
A DSH uses energy that's already available. HPWH takes energy from the home. Is it consequential, not much, but comparing the 2 is apples to oranges.
Have their geo sales company itemize the savings either provide and pick the one that fits best. BTW is HPWH only $800 more with the extra electric circuit installed?
J

I haven't seen HPWH's much lower than $1500.

# of full time occupants has a major impact on water heating investment / payback duration.

Thanks for the input.

Joe - what extra circuit? Do you mean there is one for the coil and one for the heat pump?

The extra 800 is the 1100 extra for the HPWH minus the 300 tax credit. The 2 I priced out were the AO smith conservationist 80 gallon all electric (700 USD) and the AO smith Voltex HPWH (1800 USD).

So the price difference seems to work out to 800 but I didn't know there would be an extra circuit involved.

Scratch the extra circuit if you are already electric hot water.
Curts concern about recovery time is part of the downside of HPWH.
Your math is flawed if you are taking tax credit off waterheater but not DSH.
j

You could do both - DSH and a HPWH - you need a tank anyway.

I put in the GE HPWH and used my old electric tank for the buffer. Works great so far. Makes the basement utility room a bit colder than it was, but not too much. The Geo DSH is providing most of the Hot Water during the winter, so it does not run much in the winter. It dehumidifies as well, so I can turn off our basment dehumidifier in the summer months. With the tax incentives and rebates, it was a good deal and I needed a second tank anyway. Not sure the true ROI, but the rabates made up my mind.

OK Joe - I see your point about getting the tax credit for both the DSH and the HPWH ------ and getting the number from the Geo company sounds good. So here they are.

They said that baseline hot water cost for a family of four for one year with an electric tank (EF .96 or so) would be about 500. They told me my DSH would take hot water energy costs down to 350. The ACEEE website link in my original post above says that for a typical family of four that a HPWH will have yearly costs of about 190.

This is a pretty typical family of four.

So heads up comparison would seem to favor going with the HPWH (they both cost about 1000 more and the HPWH saves a good bit more money.)

Jonr - your point about doing both is a very interesting thought. There are no plans to sell the house so I think we can look at a long term payback.

Let me take a crack at mashing the numbers together and then you can provide critique:

Baseline hot water use with electric tank will be 500. The DSH will take it down to 350. Since a HPWH will take a 500 dollar bill and make it 190 (a 62% reduction) ---- then the HPWH should also take the 350 remaining bill that we would have to electrically heat after the DSH and crunch it down by the same ratio (350 x (1 - 0.62) = 133.

So the 190 bill with HPWH would go down to 133 with the combination (if I did get that right). That is a 57 dollar savings. Since we'll be spending an extra 800 to get the HPWH (1100 extra minus the 300 tax credit) - then the payback would be 14 years (800 / 57 = 14).

Joe - I think I can factor in the tax credit here but not in the original calc where I was looking at one versus the other because here I am looking at the combination.

So since they are going to be there a long time = maybe jonr is right that we should look at both.

Or did I step off the path of good math and sound logic somewhere along the way?

Nothing wrong with your logic. I would suggest that their $190/yr operating cost may not be accurate but it's certainly less expensive to run than other things.
j

Better look into the cop as the temperature drops and as your basement comfort level drops. The maintenance costs have not been calculated into your equation.

AC Wizard -

I assume you mean that the COP of the HPWH will drop as the basement temp drops? The basement is well insulated both in the walls and with underslab insulation so I think this should not be too bad.

What maintenance costs do you see? I guess you mean with having 2 tanks to take care of? I think one is going to have that anyway with a 2 tank DSH setup.

If there are costs I am not seeing that need to be factored in please let me know.

A little off topic but, I just finished installing a Navien tankless water heater. Total cost of install was 1500 and my time. It delivers what I am calling smoking hot water 130f that is unlimited with no cost of storage. If the heat pump water heater is 1500 and robs energy you allready paid for and have to replace in the living space I am not seeing the value? I will post back about propane consumption. For me and mrs. pirate it seemed a no brainer to pay for any storage for intermittant usage during the day unless we are filling the jaccussi.
Eric

Posted By ANdad on 06 Mar 2011 06:42 AM
AC Wizard -

I assume you mean that the COP of the HPWH will drop as the basement temp drops? The basement is well insulated both in the walls and with underslab insulation so I think this should not be too bad.

What maintenance costs do you see? I guess you mean with having 2 tanks to take care of? I think one is going to have that anyway with a 2 tank DSH setup.

If there are costs I am not seeing that need to be factored in please let me know.

Todd, you misunderstand. The HPWH takes heat from your basement. Insulation will not prevent that. A point we have all discussed is that if that heat needs to be replaced, then you are adding to the burdens of your HVAC system.

I don't anticipate extra maintenance.

j

Joe - Curt - Eric - ACwizard -

Thank you all for very helpful input.

I called the installer to get a quote for the second tank install. He said that Water Furnace now has a stand alone unit that is tied into the ground loop (but in the basement). He thinks this will be better than either of the above set ups.

This stand alone unit (the NSW 018 is what he told me) is better than a DSH in that it can call for hot water anytime - not just when the unit is running so we can now get very cheap hot water all the time. He thinks the year round how water costs would be around 75 bucks. The cost is apparently 3000 but there is the 30% credit.

In addition he said that the HPWH will chill of the basement a lot (such that the occupants might not be happy) - I figured it would drop the temp only 2 - 3 degrees but he seemed to think more.

So the HPWH would cost 1600 (1900 - 300 tax credit) plus 600 for the install (2200 total) or the NSW018 will cost 3000 plus 600 install (3600 - 1200 tax credit = 2400).

The difference of 200 would be made up in less than 4 years so this seems like a good idea.

Hmmmmmmmmmm.......

HPWH or NSW 018 water to water heater.

Am I thinking straight that the NSW would save more money and have a lower carbon footprint?

Also the NSW would not chill off the basement or add to the HVAC load.

You earlier wrote that heating water resistively costs $500 / yr. A water to water heat pump using your loop should operate at a COP of 3-4, so I don't see how the cost drops to $75 / year.

COPs shown in specification tables are steady state - in the real world there are cycling losses to consider.

Good point -

The manual states that the COP will be 3.7 at 50 EWT. The ACEEE website says high efficiency electric heater will cost 439/yr for family of four.

439/3.7 = 118 $ per year.

So maybe 75 was a little optimistic of him.

Unless you think that cooling mode when we use the AC in the summer would make the summer water that much cheaper (or almost free).

So maybe if you take the 118 and subtract 25% (for 3 months of almost free hot water) then the cost would be 118 * 0.75 = 88 $/year.

So 75 might still be a little optimistic - but apparently the water to water heat pump would save 133 - 88 = 45 per year. So for an extra several hundred dollars this still seems like a good payback.

Or is there something else I am overlooking?