Tankless + Geo/AC + Solar WH?

lynnisseFebruary 7, 2011

Anyone try combining water heating methods? We've had tankless (LP) for several years and just had geothermal heat pumps installed. We want to use the heat from cooling w/the geo pump when we can, at least to pre-heat the water before it goes to the tankless heater. We're also thinking about solar to do the same for the winter (and power the house). Can all three be effectively joined together? All hints/suggestions welcome.

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ionized_gw

You are talking about layering three of the most expensive water heating systems possible. Does the term "âÂÂdiminishing returnsâ mean anything to you? What you propose will never pay; it will be very costly.

Your HVAC installer should have known about heat pump water heating. With heat pump water heating, at the time you are using the AC, you won't need solar. Are you in a hot-humid environment? If so, a heat pump water heater is probably the best way to go, either freestanding or in conjunction with your HVAC system. Unfortunately, you already have one of the most expensive water heating systems possible with the tankless., but I suppose it does not pay to pull it out and sell it.

I think that solar can work in cold-climate, but I have not really looked into evacuated-tube heaters. I donâÂÂt have that worry. I think that will be costly as well.

    Bookmark   February 9, 2011 at 2:58PM
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lynnisse

Thanks for your reply, Ionized.

Actually, the tankless has been very cost-effective for us, cutting our use of LP by more than half. Sorry to hear that it may not be working out so well for others.

And we already have the geo/HVAC water heater coming as part of the installation of the heating system. It's not going to be an after-the-fact add-on. Of course, our HVAC contractor pitched it as part of the package, and of course, it won't do squat for us 5 or so months out of the year.

Hence the curiosity about solar and hooking them all together.

So, any suggestions out there about how best to tie them all together?

    Bookmark   February 10, 2011 at 12:06PM
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ionized_gw

I am sure that the demand water heater uses less gas. The trouble is that they cost way too much up front to make it a money-saving proposition for most people. That is according to the DOE's calculations with their assumptions. The latter probably include natural gas. Since propane is more expensive, that might put you in the reasonable category. How do your numbers work out for the present cost of LP; how long will it take you to make your investment back on the difference between the cost of a tank and the demand?

You already have the heat pump WH so you only need the solar in the winter. That means that you only get half the payback from that investment that people without the HP get. You just have to put the numbers together to see if it pays off for you.

How would it be put together if either you calculate it will be a reasonable investment or if you have more money than brains, is pretty easy. First you have to decide what kind of solar heater you are going to install.

    Bookmark   February 11, 2011 at 9:58PM
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lynnisse

Mmmm. Thanks, again, Ionized.

I think I have enough information/sources to figure out the money side.

Anyone have suggestions for how to connect it all? Plumbing, etc.?

    Bookmark   February 12, 2011 at 9:12PM
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Billl

"We want to use the heat from cooling w/the geo pump when we can, at least to pre-heat the water before it goes to the tankless heater."

This isn't going to be a DIY type setup. Heat pumps use very small changes in temperature to heat or cool. It isn't like there is some hot liquid that you will easily be able to use to heat water. Without some sort of heat exchanger, it isn't really a practical plan.

You could certainly use a solar hot water heater to preheat water for your tankless. It would be essentially the same as preheating for a conventional water heater. Any installer should be able to hook that up for you.

    Bookmark   February 14, 2011 at 1:10PM
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ionized_gw

Again, for the solar, it is impossible to say without knowing what kind of solar you will use, batch tank on the roof, thermo syphon, open loop, closed loop, in-tank exchanger, external exchanger, evacuated tubes,....

In order to make a complex system like this with any efficiency, you will likely need a solar heat exchanger, a geothermal heat pump heat exchanger and a storage tank. You will need controls, likely custom-designed, to control when and in what order the water goes through the individual heat exchangers. On a residential scale, you would have to pay someone too much to make it pay back.

I am sure that someone could do it as a hobby project in their own home and make it pay, but if you have to hire pros to design and install it, it just won't pay.

    Bookmark   February 14, 2011 at 9:39PM
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