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

11 years ago

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. :)

Sort by:Oldest

Comments (7)

Adam F
Original Author
11 years ago
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?
Brian_Knight
11 years ago
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
Related Discussions
Hot water on demand?
Q
Comments (28)
Kelntx- this subject has been discussed ad nauseum on the plumbing board. The most infamous thread just was resurrected (and filled to capacity) within the past few days. FWIW, I think solarpowered is mistaken in several comments. "In fact, the efficiency of a tankless heater is somewhat less than the efficiency of a good tank-type heater. " I'd like to see some numbers to back this up. Standby losses in tank heaters are what everyone talks about (and what tankless salesmen hype). But just the heating part is less efficient, too. Firing a burner under a tank heater is not very efficient for heat transfer, unless you go to the more expensive direct-vent condensing type heaters, because a chunk of that heat is going up the flue. Tankless heaters (I'm considering gas only) are 80+ percent efficient at that. On recirculation systems: "However, if installed correctly with properly-insulated pipes, they can actually save a significant amount of energy, because you aren't losing all the heated water in the pipes between the heater and the tap every time you run hot water. " I just don't see how this is the case. The pipes ought to be insulated regardless. So, if I put in a recirculating system, I'm going to have not just a pipe to the point of use, but also a return pipe back to the heater. Worst case, that pipe runs all the way to the point of use. And once you turn off the faucet, it's sitting with heated or partially heated water that then cools off. I just don't see how that saves energy. The bottom line is that having the point of use close to the heater saves both water and energy, regardless of which type heater you use. On the original subject, we put in a tankless during a remodel. But we didn't do it to save money. That's it's more energy efficient was a great selling point. Being able to put it outside, however, was the big factor that allowed us to put it near the bathrooms it serves. And we avoided the expensive vent pipe that way. If you have hard water in central TX, I might stick w/ a tank and put a lot of thought into where it's stashed. Scaling is definitely a problem for tankless, and you'll have to have it flushed w/ an acid periodically to prevent buildup.
...See More
hot water radiant heat( boiler vs on demand )
Q
Comments (6)
Just some thoughts on improving your existing setup (it's probably got 20+ years' service life left): 1. Don't use a heating boiler for domestic hot water, especially in the summer. 2. Convert to gas if you're currently burning oil (and be sure to carefully clean heat transfer surfaces). 3. Provide as many zones as you can afford, schedule the temperatures with programmable thermostats; use one circulator with multiple zone valves instead of multiple circulators. 4. Arrange for fire on demand (if you have a cast iron boiler - don't do this with a steel boiler). 5. Provide a reset control to limit boiler temperature when it fires (I always liked Honeywell T475A); install the water sensor in the pipe, don't just strap it to the pipe surface. (This is another one not to do if you have a steel boiler). 6. Put in a stack damper (make sure it works properly). Had a good time with this stuff in my first home, a 1930's semidetached; started with a new Weil-McLain oil fired and went from there, over a 24 year period.
...See More
circulating hot water heat
Q
Comments (1)
The radiators won't freeze unless the temp of the room falls below 32F. But the pipes can freeze if they are buried in the wall and you have any holes or leaks in your insulation. If you run the radiators upstairs full on, then the reguarly circulating hot water be enough to overcome any small breaches in the insulation, but if you turn the radiator down reducing the flow, you could have a problem. Of course, maybe your pipes are entirely inside the heated space, so this doesn't matter. Lucky you to have h/w radiators, I long for them! Your unequal heat thing maybe correctable by bleeding the system, or it may be inherent in the way it is. A good heating contractor could tell you. In some cases it is possible to remodel the the system and get zoned circulation, but this can get expensive. You might ask your question over on the heating forum. How did you get the strikethrough's? Good luck, MOlly~
...See More
gas hot water in series with geothermal
Q
Comments (4)
The biggest efficiency loss in a gas heater is the heat that goes up the flue in order to achieve atmospheric venting. I don't know that having one or two heaters matters with that as long as they aren't both firing at once. The second greatest heat loss is the flue up the center of the tank. The third source of heat loss is the remaining tank surface area that is insulated. I would have to guess that two tanks would have more flue heat loss and more external tank surface area. Are you using these tanks as storage for the geo-thermal? Can you just use a well insulated storage tank and couple that with a tankless heater that makes up the difference? That would seem to be your best bet.
...See More
ILoveRed
11 years ago
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.
dekeoboe
11 years ago
We have a 40 gallon storage tank off the desuperheater and that feeds into our heat pump water heater.
momto3kiddos
11 years ago
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!
robin0919
11 years ago
adamdoc.....you might want to post this question over on greenbuildingtalk.com They have alot of professionals over there that might be able to give you the best option.
Annie Deighnaugh
11 years ago
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 tankless...one 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....

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

Comments (7)

Adam F
Original Author

Brian_Knight

Related Discussions

ILoveRed

dekeoboe

momto3kiddos

robin0919

Annie Deighnaugh

COMPANY

BUSINESS SERVICES

GET HELP

CONNECT WITH US

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

Comments (7)

Adam FOriginal Author

Brian_Knight

Related Discussions

ILoveRed

dekeoboe

momto3kiddos

robin0919

Annie Deighnaugh

COMPANY

BUSINESS SERVICES

GET HELP

CONNECT WITH US

Adam F
Original Author