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Ground Source Heat Pump Domestic Hot Water efficiency

I understand that a desuperheater captures the heat that is naturally lost by running the heat pump. Is it capable of supplying all the domestic hot water in a typical home? Does this mean that it is essentially using 'free' energy and the Energy Factor of this DHW system is infinity (ignoring pump consumption)?

Asked by Nick Sisler
Posted Thu, 04/18/2013 - 15:23
Edited Thu, 04/18/2013 - 15:31

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4 Answers

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The hot water performance of desuperheaters really depends on the duty cycle & mode of the compressor. In cooling dominated climates in homes where it's running most of the year it can boost the net system efficiency.

Even when the fuel is free the EF is well under infinity, since there is at a minimum the additional pumping power required. It's no more "free" than a solar hot water heater, but it can be pretty cheap. But in other than for high duty-cycle primarily cooling GSHPs, it's not usually going to be the best way to go on a levelized cost of energy basis, since the first-cost of the equipment comes with a substantial price tag.

When the GSHP is in heating mode the desuperheater is just another load on the system, and in systems designed to the margin on heating capacity it increases the size of the ground heat exchange necessary to keep from over-cooling the loop and losing capacity. But when in cooling mode it's dumping a fraction of the heat that would otherwise need to be dissipated by the ground heat exchange, with a net-positive effect on system efficiency, and lowering the heat-saturation issues in the soil.

During the shoulder seasons (or in very low heating/cooling load homes) it's output is effectively zip, nada, niks.

Answered by Dana Dorsett
Posted Thu, 04/18/2013 - 15:43

2.
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Nick,
Any time that your ground-source heat pump is operating, it is using electricity. Therefore its COP or Energy Factor can never be infinity. You will get energy benefits -- either space heat, or cooling, or cooling plus domestic hot water -- but there will always be an energy bill to pay. It takes electricity to run your equipment.

Here is more information from the U.S. Dept. of Energy: "Homeowners primarily install geothermal heat pumps -- which draw heat from the ground during the winter and from the indoor air during the summer -- for heating and cooling their homes. For water heating, you can add a desuperheater to a geothermal heat pump system. A desuperheater is a small, auxiliary heat exchanger that uses superheated gases from the heat pump's compressor to heat water. This hot water then circulates through a pipe to the home's storage water heater tank. ... In the summer, the desuperheater uses the excess heat that would otherwise be expelled to the ground. Therefore, when the geothermal heat pump runs frequently during the summer, it can heat all of your water. During the fall, winter, and spring -- when the desuperheater isn't producing as much excess heat -- you'll need to rely more on your storage or demand water heater to heat the water. Some manufacturers also offer triple-function geothermal heat pump systems, which provide heating, cooling, and hot water. They use a separate heat exchanger to meet all of a household's hot water needs."

Answered by Martin Holladay, GBA Advisor
Posted Thu, 04/18/2013 - 15:49
Edited Thu, 04/18/2013 - 15:50.

3.
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Just the pumping power in the earth loops and firing up the compressor during the shoulder seasons to serve just the hot water loads seems like an incredible waste. I can't imagine it even meeting tank-top heat pump water heater EF performance under those conditions unless there's a real heating or cooling load on. At higher duty cycles on the GSHP it'll do pretty good, but again, at what levelized cost?

In big glass houses in FL it's a slam dunk, but in the northeast in a reasonably sized reasonably tight houses... get out a very sharp pencil on the accounting to see if it'll EVER break even.

Answered by Dana Dorsett
Posted Thu, 04/18/2013 - 16:11

4.
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Thanks Dana and Martin. This is very helpful.

Answered by Nick Sisler
Posted Thu, 04/18/2013 - 16:19

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