What a desuperheater does in a geothermal system

How does a geothermal desuperheater make hot water?

Inside a heat pump, the compressor squeezes refrigerant gas and heats it up. In a normal cycle that heat goes to the house air or to the buried loop. A desuperheater sits in the line between the compressor and the main coil. A small pump circulates water through it, the hot gas gives up part of its heat to that water, and the warmed water goes to a hot water tank. Some makers call the same part a hot water generator. The refrigerant then carries on into the main cycle a bit cooler than before.

The name comes from the fact that the refrigerant is superheated at that point, meaning hotter than it needs to be to condense. Taking some of that surplus heat costs the heat pump very little, which is why the hot water seems almost free. It is not entirely free, because the compressor still draws electricity, but you are using heat that would otherwise be dumped. For the wider system, see how ground-source heat pumps work.

When does a desuperheater work best?

It produces hot water only while the compressor is running. In summer cooling mode, the heat pump is removing heat from the house and would otherwise reject all of it to the ground, so the desuperheater can capture a good share of it for hot water. That is the strongest period. Our guide to geothermal cooling explains the cooling side.

In winter the heat pump runs often, and so the desuperheater has a lot of runtime. What it takes comes out of the heat that would have gone to the house, so the benefit is real but smaller. In spring and fall, when the compressor barely runs, it adds little. A house with a bigger load and more runtime gets more out of one, and a small, well-insulated house gets less.

Does a desuperheater replace the water heater?

No. Most installations add a separate hot water tank, used to pre-heat, in front of the existing water heater, which then tops up the temperature and covers the times the heat pump is off. Hot water demand does not follow heat pump runtime, so a backup is essential. Ask the installer how the tanks are piped, how the heat pump gives priority between space heating and water heating, and how the system handles a long run of showers.

Some people use a heat pump water heater or a solar thermal system instead, or in addition. The guide on heat pump water heaters in Canada compares that route, and how solar water heaters work covers another. The Drake Landing homes show another route to the same goal. Their solar hot water page describes how solar heat was used for water heating alongside the district space-heating loop.

What are the drawbacks and risks?

• Scaling. In hard-water areas, mineral buildup can foul the heat exchanger. Ask about its materials and cleaning.
• Water safety. The exchanger must separate refrigerant from potable water, typically with a double-wall design. Confirm the design meets local plumbing code.
• Limited output. It cannot make much hot water by itself, so the saving is modest in some homes.
• Added cost and complexity. Extra pump, tank and piping bring extra parts that could fail.

Savings depend on how much hot water you use, the cost of the fuel that heats it today and your heat pump's runtime. A rough way to check is to look at your current water heating cost and ask the installer to estimate the share the desuperheater would cover, in writing.

Is a desuperheater worth adding?

It tends to be worth considering when the heat pump is being installed anyway, because the part and the piping are a small add-on at that stage. Retrofitting later costs more. Households with high hot water use and a cooling season benefit most. If you are weighing a full system, see the geothermal heat pump cost guide for the larger cost drivers, and compare the desuperheater quote on its own line item so you can judge it separately.

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