Geothermal Hot Water +/- On Demand +/- Circulation System

adamdocFebruary 7, 2013

Hi all - as we progress in the actual building of our dream house on a nightmare lot (anyone want about 500 landscaping boulders?), we are nearing a decision point about something I've been obsessively reading about for months...

We are definitely doing geothermal, and the installer recommends, of course, taking advantage of the free hot water with a 100 gallon holding tank. I know that the actual temp of that water will vary (though should never be cold) by sesaon, so we planned on adding an on demand heater (Rinnai, maybe) as well.

I seem to think that putting the on demand heater in series after the holding tank would make most sense, since it won't kick on (I think) if the incoming water is already hot, and even if it isn't hot enough, the temp change (delta) will be less than a cold feed, making the unit work more efficiently...

The issue we worry about is adding a circulating system to the house, and where to pull that feed from. I don't want the on demand heater running constantly just to circulate water, but can I have a second feed coming from the holding tank, so that we circulate at least warm water, in the worst case scenario?

I've asked around, and every professional installer has a different opinion of what "should work."

Another issue is that we have an in-law suite (+1 small bedroom/bathroom, 2 total BA) set up over the garage, which would have a very long run. We've considered putting a second on demand unit (lower capacity) right under it in the garage, but then using that unit for the in law suite, as well as the washer and dishwasher... My only concern there is that we'd then be using instant hot for the washing machine and dishwasher, even when we could be using the free hot water from the holding tank during part of the year. At the same time, if we have only the holding tank and one on demand unit for 4 1/2 baths and the washers and dishwashers, we may max out capacity...

Our plumber also told us that we'd need larger than standard pipes if we don't want to add the second heater under the suite, which would kill any savings of not buying the second heater.

We have 6 1/2 baths, 2 dishwashers, and a washer. We may add a second washer in the basement down the line.

Any thoughts on solutions? I just can't find any good resources that aren't selling their solution! Imagine that. :)

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An additional thought - the geothermal guy recommends installing this insulated holding tank, by just using a standard tank and not turning it on...

Would I be better off just using a high efficiency regular hot water heater, and filling it with the geothermal hot water, and entirely foregoing the on demand, except for the in law suite?

    Bookmark   February 7, 2013 at 1:22PM
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Who thought hot water could be so complicated huh? Iam doing a similar set up right now except we are using a heat pump water heater as the main/back up. Highly recommend you look into them if you have the appropriate climate and space.

Be aware that the Desuperheaters usually only produce measurable hot water during the cooling cycle which could be seldom depending on climate and efficiency of the building envelope. If you go tankless (personally not a fan in most situations) be sure its capable of handling pre-heated water.

Holding or Buffering tank is a good idea no matter what the back up tank is. A cheap electric storage without the power hooked up is fine. I would add insulation to the tank(s) and absolutely insulate ALL hot water lines as much as possible.

Obviously, energy use matters to you so be very careful with how you set up a circulating pump. A manual or occupancy switch located in the farthest bathroom is the best bet. Continuous circulating pumps could easily end up burning more energy than what youre saving with the Geo.

I would also recommend monitoring the energy use of the circulating pump for the Geo. COPs usually ignore this part of the equation. Check out the below link and do someother keyword searches.

Here is a link that might be useful: GBA link geo + tank Q &A

    Bookmark   February 7, 2013 at 3:30PM
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I may not be much help here because I may not totally recall how our system works.

Our house is 7-8 yrs old and we have geothermal. The 2 units are in our basement, one in each end of the house. We have 5 zones. We have 2- 80 gallon water heaters right next to eachother. One stores water. If I have a lot of guests I can turn on the breaker for the second water heater so that both water heaters are producing hot water and I have plenty of hot water.

I also have a pump that runs hot water to the opposite end of the house so that the water doesn't have to run forever to produce hot water. I know it's not an on demand water heater.

This system has worked well so far.

    Bookmark   February 7, 2013 at 4:05PM
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We have a 40 gallon storage tank off the desuperheater and that feeds into our heat pump water heater.

    Bookmark   February 7, 2013 at 7:07PM
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We are building right now and we have geothermal with 2 units, one on each end of the house. The master (small laundry) and kitchen are on one side and kids bedrooms and main laundry are on the other side. Two hvac contractors we spoke to and our plumber suggested having the desuperheater on each unit preheat a tank and then feed into an electric hot water heater tank (no tankless- we didn't look into it due to the lag time). This way we will not need a circulating pump (expensive per the contractor) because the hot water will be near point of use on each side. We also expect to turn off the hot water heater on the kids side when they are grown and gone. We may be overdoing it, but we wanted plenty of hot water available for us, 3 kids, possibly a parent or two one day.
Good luck with your decision making!

    Bookmark   February 7, 2013 at 9:27PM
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robin0919 might want to post this question over on They have alot of professionals over there that might be able to give you the best option.

    Bookmark   February 7, 2013 at 11:19PM
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Annie Deighnaugh

We have geothermal...did not ask for the desuperheater, but they gave it to us anyway by mistake, so we hooked up a tank to it for preheated hot water. But we don't have heat or a/c about 6 mos of the year as the weather is decent, so we have tankless too. We have 2 on the west side of the house which does just the master bath and one on the east side of the house which does the PR, laundry, kitchen and guest bath. We did not do recirc, so we do have to wait a bit for the hot water to come because the recirc systems just chafed my desire to go green....

And keep in mind that the larger the water pipes, the longer you have to wait for the hot water to come unless you do the recirc pump.

I don't understand a recirc pump with a tankless as the whole point of tankless is that you heat only the water you need. The one for our master bath probably heats water for only about 30 min a day....

    Bookmark   February 8, 2013 at 10:14AM
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