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Geothermal and water heater selection

Hi, I am in the process of building a new home that will have geothermal heating and cooling. Of course one of the benefits of geothermal is "free hot water." My geothermal installer is recommending two hot water heaters.

There will be no hot water on days the unit isn't running. My question is, should I buy two 85 gallon Marathon Water heaters from my local power utility for $918.06 (includes tax), or should I just buy standard water heaters?

My goal is to be as energy efficient as possible while not breaking my wallet.


Asked by Chris Johnston
Posted Aug 19, 2010 3:02 PM ET


10 Answers

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I should point out, the $918.06 is for EACH 85 gallon Marathon water heater. So, two would cost me $1836.12.

Answered by Chris Johnston
Posted Aug 19, 2010 3:03 PM ET


You have mutually exclusive goals.

If you don't want to break your wallet, then you certainly should NOT install a "geothermal" system (that is, a ground-source heat pump). What's it going to cost you? Maybe $18,000 to $30,000 ?

If you buy a more affordable heating system, you can take all of the money you saved and invest it in air-sealing work, more insulation, or better windows. You'll end up with a tighter house that uses much less energy.

If you want the most energy-efficient water heating system -- and I presume you mean, one that uses as little purchased energy as possible -- you should probably install a solar hot water system. But they aren't cheap.

If you don't want to break your wallet, drive down to Home Depot and get a gas water heater. Wrap it with insulation. And install low-flow fixtures so you use as little hot water as possible.

Answered by Martin Holladay
Posted Aug 19, 2010 3:14 PM ET


Good points Martin, and thank you for them. I am laughing out loud. I guess I lost site of the forest for the trees. It is easy to do with a construction loan when my wife is writing the construction checks and I am picking the items. She has told me, at this point in the budget, to minimize purchases now where ever possible, so that is why I made the break the wallet comment.

Here is what we have in place to date: house in mixed-humid zone (Southern Illinois). House has one-inch thick rigid foam on the outside. Inside has two by four walls which will be insulated with air-crete (buildingscience.com said 2 x 4's ok in this zone). Low E argon-filled windows and energy efficient exterior doors appropriate to this region are also in. Geothermal system with wells is in-place.

We need to decide on water heaters. I see what you mean - what's the point in saving a thousand dollars when we have spent $23,000 on the geothermal system, but if it is not efficient - both monetarily and green wise to buy the Marathon's - then why waste the money ($1,000 is still $1,000).

Answered by Chris
Posted Aug 19, 2010 3:28 PM ET


Ground-source heat pumps that provide domestic hot water include desuperheaters. I'm not sure why your plumber recommended two hot-water tanks -- is one going to be used as a passive tank through which the desuperheater circulates hot water? If so, this tank would basically be a preheater, and the second tank would include a resistance heater to bring the preheated water up to temperature (especially in the swing seasons, when the GSHP isn't operating).

If my guess is correct, the first tank doesn't need a heating element. Any old cheap water heater will work. Just disable the heating element and surround the tank with lots of supplemental insulation.

The second tank can be heated with electricity or gas. In most parts of the country, gas is cheaper than electricity.

If you are forced to heat your water with an electric resistance element, the Marathon heaters have a good reputation. However, the performance of a cheaper heater can be improved by wrapping it with extra insulation.

Answered by Martin Holladay
Posted Aug 19, 2010 3:40 PM ET


Good advice so far. I'd add that 85 gallons of hot water sounds like a lot unless you have several football players living there. We advise our customers that if they run out of hot water a couple times per year, then that is the perfect size, otherwise you are constantly heating a bunch of water that goes un-used. We have two athletic teenagers and have never run out of hot water with a 40-gallon gas heater. Also keep in mind that gas water heaters recover faster. Take care.

Answered by Keith Groninger
Posted Aug 19, 2010 4:07 PM ET


I think you might want to consider a heat pump water heater or a tankless water heater.

Answered by Roger Lin
Posted Aug 25, 2010 11:02 PM ET


If I do go with an electric "standard water heater(s)" and insulate them (wrapping in insulation), what type of insulation should I use?

Answered by Chris
Posted Aug 26, 2010 10:23 AM ET


Hardware stores and lumberyards sell kits to insulate the exterior of an electric water heater. All of the ones I've seen use fiberglass insulation with a polyethylene cover.

Answered by Martin Holladay
Posted Aug 26, 2010 10:25 AM ET


In reference to Martin's comment "In most parts of the country, gas is cheaper than electricity." I trust we are all aware that he is referring to piped natural gas. Though you'd never know it from listening to a tankless water heater or gas furnace salesman, LPG and propane do not generally share this cost advantage - quite the opposite in fact. Propane furnaces are currently about the most expensive possible way to heat a home here in NC.

Answered by James Morgan
Posted Aug 26, 2010 8:30 PM ET


To Roger: heat pump water heaters are of marginal benefit outside of cooling-dominant climates where they supplement a/c and dehumidification. In heating-dominant climates they steal the ambient household heat which you have previously paid for. Tankless water heaters show a poor ROI in any climate except where they are used to co-supply hydronic heating systems, as in common European practice. If fired by piped natural gas tankless heaters are economical when used in this way: if by propane, not.

Answered by James Morgan
Posted Aug 26, 2010 8:47 PM ET

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