How to Make Hot Water With a Geothermal System

Many know that geothermal systems can make domestic hot water but no one seems to understand how it works. Very simply – geothermal systems have a compressor. Whenever the compressor is operating, there is waste heat produced in the refrigeration cycle.  That waste heat (superheat) can be used to produce domestic hot water. This is a nice
video that explains how superheat is formed.

The hot water generating component of a geothermal system is called a desuperheater. It is merely a pump and a heat exchanger that is plumbed in such a way so as to carry the waste heat away from the refrigeration cycle and into your water heater. Geothermal systems make hot water in both heating mode and air conditioning mode. A desuperheater does not affect the heat output of the geothermal system in the winter. Geothermal systems have safeguards that prevent the desuperheater from operating when that heat is needed for heating the building.

A little circulator pump pulls water out of the water heater and pumps it through a heat exchanger inside the geothermal unit where it picks up heat. The warmer water is then pumped back into the water heater.

When operating, a desuperheater is capable of producing hot water with a temperature of 130F+.  When the compressor is not operating, the geothermal system doesn’t make any hot water. The balance of the hot water needs must be made up with gas, electric, or some other fuel.  A geothermal desuperheater will produce about 50% of a home’s annual hot water needs resulting in significant savings over traditional water heating fuels.

The thermostat on the water heater should be set to about 120F. The desuperheater will draw 120F water out of the water heater and heat it to a higher temperature (125F to 130F+). When the desuperheater is not running, the water heater will still remain at 120F by using the conventional energy source.

A desupeheater does not make hot water ‘on demand’ or ‘as needed’. It should be thought of as bonus hot water. It should not be counted on as a sole hot water source. It is not meant to make hot water for radiant floors.