How to Make Hot Water With a Geothermal System

Many know that geothermal systems can make 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 can be used to produce domestic hot water.

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 waste heat away from the refrigeration cycle and into your water heater. It 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.  (For a more vigorous description of superheat, you might enjoy this article.)

A little circulator pump pulls water out of the water heater and pumps it through a heat exchanger where it picks up heat. The warmer water is then pumped back into the water heater.  Here is a very basic sketch showing how a desuperheater works:

When operating, a desuperheater is capable of producing hot water with a temperature of 120F to 150F.  When the compressor is not operating, the 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.  Under typical conditions, a geothermal system will produce about 50% of a home’s annual hot water needs resulting in significant savings over traditional water heating fuels.

Still have questions about geothermal hot water production? Want to know what kind of water heater it will work with? Call or email us for more information.




  1. Gerard Westover says:

    What kind of water heater will a desuperheater work with?

    Thank you.

    1. EnergySmart says:

      It will work with any type of water heater. It is most easily connected with an electric or heat pump water heater.

  2. Johnnie Williams says:

    Directions for my geothermal systems says the lower thermostat should be set at 100 on my hot water tank. I set the top thermostat at 125. My wife complained there was not enough hot water to take a bath so I changed the lower back to 125.

    It would seem to me that using the desuperheater in winter would cool the water in the hot water tank that I am heating with electric power.

    1. Energy Smart Alternatives says:

      Hello Johnnie,
      We usually recommend that the lower element be set between 115 and 120F. That should help. Also, are you sure the top element is actually working?

      The desuperheater does not cool the water tank in the winter. Hot water is still produced by the desuperheater in the winter. When superheat is not available in the refrigeration cycle, the desuperheater function is disengaged so that all the heat can be diverted to the house. Your geothermal system has sensors that determines whether there is excess energy that can be diverted to the water heater. Google ‘superheat’ to learn more about the physics and how the heat is produced.

  3. Tom says:

    What is the best way to connect the hot water line from the desuperheater to the water tank. My original installation had a special fitting with a short dip tube. Hot water from the desuperheater would flow down the short dip tube into the hot water tank. Hot water from inside the tank to the house would flow around the short dip tube in the special fitting to the house. Now they tell me I should have the hot water from the desuperheater go into the drain fitting at the bottom of the hot water tank.

    1. Energy Smart Alternatives says:

      Don’t try to re-invent the wheel. You should plumb it the way the geothermal equipment installation manual says. That way you can’t go wrong. Installation manual are often available on-line on the manufacturers’ websites.

  4. Dale Rogan says:

    My Geo unit shows the cold water supply teed off to the Geo inlet and the return from the Geo Unit back to the HWT drain. In the heating cycle, the supply water temp is Tank temp and the return is about 10 F cooler. Is this due to the unit deciding what will be used for Home Heat or am I heating my house with the HWT. I have a 5 ton unit and the compressor cycles on and off rather quickly. 2100 sq ft rancher.

  5. Darin neugebauer says:

    My desuperheater does not seem to pump I’ve had repair people out here twice it has been replaced but water does not get hot is there a simple place to just replace the pump. Th out going pipe is warm but I remember when we first obtained it. It was too hot to touch.

    1. Energy Smart Alternatives says:

      If the pump has been replaced repeatedly, then there is something else going on. For example, if it isn’t plumbed correctly, this could be causing the pump to burn out. Review the plumbing diagram in the manual for your geothermal heat pump. It will provide instructions for pipe size, plumbing configuration, length of plumbing etc for proper installation. One other thing – make sure that the desuperheater is actually turned on and the pump has been burped (if required). There is usually a switch that need to be turned on or a wiring harness plugged in.

  6. Leighton Main says:

    Do I need to switch off the DHW switch in the summer and switch it back on during the winter. We have 2 water tanks. I assume one is a reserve overflow tank.

    1. Energy Smart Alternatives says:

      No, you should not ever change the DHW switch. Usually when you have 2 tanks, one is a pre-heat tank and the second is heated by gas, oil, or electric. The geothermal unit pre-heats the water before it goes into the 2nd tank thus saving energy.

  7. Steve says:

    What is the optimal way to connect to a hybrid heat pump water heater? Both my hot and cold tank connnections are warm, is something wrong?

    1. Energy Smart Alternatives says:

      No, nothing is necessarily wrong. Check the geothermal heat pump manual for the correct plumbing configuration. A hybrid heat pump water heater should be plumbed the same way as to an electric water heater.

      1. Steve says:

        Thank you. Not much of even hearsay on installing a hybrid heat pump to geo DSH. With the rebate it costs less than a decent electric water heater. I figured worst case scenario the heat pump has little work to do with the DSH and thus won’t contribute much to heating load

  8. Cary Budach says:

    I want to collect pre-heated water and store in a passive water tank (insulated). My main hot water is from a tankless unit. I assume that this will be compatible? I currenly heat my water to 115 degrees max. Should I just bypass the tankless and draw from the the tank, leaving the tankless as a backup?

    1. Energy Smart Alternatives says:

      It is unlikely that you will be able to make all of your hot water with a desuperheater. You can feed preheated water to the tankless (I’m assuming you mean gas or electric on-demand style?) but you will need to check the maximum incoming water temperature allowed to the tankless. If the preheated water is too hot, it can affect the operation of the tankless unit. Consult the tankless installation manual for any limitations on incoming water temperature.

  9. Steven Cole says:

    I have a Rudd hot water heater from 1993 that runs a geothermal heater that has rusted out. The diagram from above explains my system pretty good. This hot water heater has two extra pipes on the cold side that new hot water heaters do not have. The lower one is close to drain and is labeled on hot water heater cold outlet. The pipe closer to the top on hot water heater is labeled hot inlet. I have been told that you can put in a new hot water heater that do not have these side pipes. My questions are:

    1. I have been told that the cold inlet needs to be hooked to the drain? Is this correct?

    2. Where does the hot inlet need to be hooked? To the cold pipe on top or the hot pipe on top with a splitter? That’s the only two pipes less unless u count the pop off valve and the drain.

    3. I was also wondering there was a curved vinyl pipe connected to the hot inlet pipe that went about 6 inches into the hot water heater. What does this do? Does it have to be included in the new hot water heater?

    1. Energy Smart Alternatives says:

      Each manufacturer has their preferred method of hooking up the desuperheater but all you really need is one line into the water heater and one line out of the water heater. Don’t overthink it.

  10. Jim says:

    I have two hot water heater tanks. They can both be turned and off to heat water.
    My superdeheater will heat my water in my storage tank to 80 degrees when the power to the storage tank is off.
    During summer we turn on the air conditioning but the water in the storage tank does not heat,it seems to cool instead causing us to run out of hot water during the summer.
    If we turn on the power to the second tank to give us more hot water, we are then circulating hot water from the storage tank back through the superdeheater.
    In this case should we not turn off the superdeheater and stop the flow of hot water from going back into the heat exchanger?

    1. Energy Smart Alternatives says:

      This is really a plumbing issue. It sounds like you need a check valve or backflow preventor to stop cold water from short circuiting to the tap. If the desuperheater is working, it will make hot water. It cannot make cold water. You should keep the desuperheater on all the time.

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