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I'm having a geothermal heating system installed in my home, and the contractor recommended heating my water using a buffer tank and desuperheater, which is a device that cools the refrigerant lines of the heat pump (when the heat pump is in heating mode) and uses the heat it captures for heating the drinking water in the buffer tank.

However, when the heat pump is in heating mode, the exiting refrigerant line is actually colder, not hotter, than when it enters. As a result, you cannot heat water during the winter.

I recently became aware of water-sourced heat pump water heaters, which basically act just like the heat pump for the home, except their only job is to heat a water tank. This seems to make a lot of sense for a house, however, because you could use the same geothermal water loop in the summer or winter to heat water; I would lose the "free" water during the summer and would have to pay for running the heat pump, but during the winter the price of hot water would remain the same low amount, instead of rising due to the need for full-time use of an electric or gas hot water heater.

Am I missing something important, or is there some reason why the internet and contractors seem to only suggest the desuperheater approach?

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I've been looking into a heat pump replacement.
You said "However, when the heat pump is in heating mode, the exiting refrigerant line is actually colder, not hotter, than when it enters. As a result, you cannot heat water during the winter."

That will depend on the location of the internal connections. The HP models I have seen with a desuperheater ( hot water generator ) place the HWG after the compressor before the reversing valve. That means the pump always runs forward, the HWG always has hot refrigerant to use and it works in summer and in winter. The reversing valve does just that - in summer it sends the leftover heat to the outside and extracts heat (cools) the inside. In winter it sends the heat to the inside and extracts heat from the outside. desuperheater connection

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A de-superheater works whenever the the pump is working regardless of season. It is just more effective in summer since it is transferring heat to the hot water system while it is trying to cool so your hot water is practically free. In the winter it heats water at a COP of the heat pump so about a 3 or 4 to one efficiency for a geothermal. Heat pump water heaters are like a refrigerator and draw heat from your basement to heat the water. Not as efficient as the geothermal. – ArchonOSX Jan 1 at 21:17

As far as I know, none of the "heat pump water heaters" commonly available are water source - they cool the air around the water heater - if you have heat that is less expensive than electric resistance, they are more efficient than an electric resistance water heater in winter (they add to the main heat source's heat load, though), and provide "free air conditioning" in the summer.

If your main geothermal heat pump is water to water, it should be a matter of having correct valves to either heat water and house, or cool house and heat water.

If your geothermal heat pump is water to air, you can use a heat pump water heater, where the heat is coming from the air heated by the geothermal heat pump, and perhaps that could be combined with a desuperheater in the summer months. It's somewhat less elegant than what should be possible with a water to water heat pump, since it involves two separate heat pumps.

A potential issue with the elegant approach is what happens when your water heating needs exceed your cooling needs. If the system can switch all valves fully automatically, it should be able to change from cooling to heating mode and back as needed to cover that. If not, it would be a pain, and it's easy to see why installers would prefer the simplicity of separate systems.

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I'm surprised no one makes a water source heat pump water heater. While the name sounds like a paradox, the science is sound. – David Pfeffer Jan 13 '14 at 12:59

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