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Posted by Robert Gammon on Wednesday, 30 April 2008, at 2:26 p.m.,
in response to Re: geothermal heating/radiant flooring/domestic hot water, posted by Brad

A desuperheater is a SMALL heat exchanger inside the heat pump. It is a commonly installed option in almost every geothermal heat pump. It works best in cooling dominated climates as it takes energy that would otherwise be transferred to the ground loop from the air in the house and sends it to the DHW tank. In heating season, it sucks some energy that would otherwise go to the radiant loop and puts it to the DHW tank.

However, it is a very low flow system. It produces 0.4 GPM per ton of heat pump (3T = 1.2GPM). Another way to look at it is GPH, and 1.2GPM is 72GPH. The water temp of the desuperheater jumps to about 15F greater than its input temp, except when input temp reaches 130F and stays there for more than 30 seconds.

With gas fired DHW tanks, the mfgs recommend using a buffer tank between the desuperheater and the DHW tank. Let the desuperheater heat up the buffer as high as it can while the heat pumps are running. Then when the occupants make a demand on DHW, the DHW draws off water from the buffer at 100-120F temps. The DHW tank has only a very minor amount of energy to supply to get its water back up to 120F or whatever your setting may be.

On average, difficult as this concept is as we have such radically different climates in the USA (from almost no heat needed along much of the Gulf Coast coastal communites - to almost no need for AC in many northern communities), a desuperheater will provide about 1/3 of the energy needed by a DHW for a typical family.

I have seen calculators that suggest that a single 4T geothermal heat pump will supply as much as 65% of the total DHW need for a family that lives in the coastal regions of the Gulf States. At that level, total DHW costs for a year may drop to under $50!!!

One caveat that arises here is that the desuperheater commonly circulates your tap water. The mineral content of that water is enough to hurt the performance of the desuperheater. So much so that the US DOE ways that typical performance of desuperheaters is about 1/3 of manufacturer claims. One manufacturer, water furnace, says that scaling of lime inside the desuperheater is responsible for the decreased performance.

This scaling problem will be true for almost ANY open loop geothermal heat pump. Its just because the nature of the heat exchanger in the desuperheater, it is more susceptable to scaling than the main w-w heat exchanger is (its actually water to refrigerant to compressor to refrigerant to water - shortened to w-w). Open loop heat pumps almost always carry a recommendation that descaling of the primary heat exchanger be performed as a part of nearly every annual service call.

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