As discussed in my previous article, geothermal systems are highly efficient in multiple ways. One specific way they add efficiency, is by adding heat to a domestic hot water supply, which lowers energy usage on the water heater itself. This feature is known as the domestic hot water generator (HWG for short), and it utilizes the compressor’s discharge gas for this purpose.

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Before the primary refrigerant/water coaxial coil loop, there is a secondary heat exchanger for the purpose of the HWG. This heat exchanger is where domestic water is present. The domestic water is moved via an internal circulator within the system when the HWG is enabled, and is heated by the discharge gas. The heated water is then moved into the bottom of your electric water heater or a separate storage tank if you have a fossil fuel water heater. In essence, instead of having to heat cold water from your main supply, the HWG helps preheat the water to a setpoint of either 125 or 150 degrees fahrenheit, depending on the setting.

This diagram courtesy of ClimateMaster

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This setting does affect the system performance slightly, and performance charts for most manufacturers state that performance readings are done with the HWG off.  This effect is seen more in the heating mode than in the cooling mode, as the heat generated by the system, during cooling operation is moved into the ground loop to be rejected, and that same heat is sent to the air coil during heating mode.  Essentially, with HWG enabled, the domestic hot water loop is robbing some heat from the geothermal system that is heating the space.  Personally, I leave my HWG enabled year round and have done the same for customers with geothermal heating. 

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Matthew Showers

Matthew Showers is an HVAC service technician, performing work in both the residential and commercial fields.  His specialty is in inverter and communicating systems.  Follow Matt on Instagram

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