Is Water Holding Tank for Geothermal Heat Pump Necessary?

Does a Water Holding Tank Provide any Value in a Geo Thermal Heat Pump System

By Mark J. Donovan

Question: Mark, I have had two open-loop water source heat pumps since 1986, and have been very happy with them. The water source is a well, and the water sink is a local stream.

I have a question that pertains to one detail. Today I use the well water only for the heat pump, but until ten years ago, I used it also for household use. Consequently the submerged well pump works with a 30/50 pressure switch and a pressure tank. The tank is now failing and I am wondering if I need to replace it.

The heat pump provides a 24-volt line that opens a solenoid valve that delivers water to the heat pump only when its compressor is on.

I would like to simply use this 24-volt line to turn on the well pump (via a relay of course); thereby eliminating the well pump’s pressure switch and pressure tank, and eliminating the heat pump’s solenoid value. Do you see any problems with this?

Answer: Joe, I don’t see a need to replace the water supply holding tank if you are only using the well now for your heat pump. In a house that doesn’t have a heat pump the water holding tank is used to provide a local reservoir for the potable water supply so that the well pump doesn’t have to turn on every time someone turns on a faucet or flushes a toilet.

A well pump’s lifetime is a function of how often it has to turned on and off. The holding tank also has an air bladder in it to help keep pressure in the house plumbing supply system and to some extent prevent water hammer effects.

The only possible value I could see with the water tank, relative to your heat pump, is that the water could potentially warm up a bit more prior to it being called into the heat pump. For example, if the holding tank was located in a relatively warm area of the home, e.g. such as a heated basement or utility room, and the water sat for an hour or two prior to being called upon by the heat pump, then more energy (heat) could be drawn out of it by the heat pump.

The only other consideration is how often the heat pump is calling for water.

Again, a holding tank helps to reduce the amount of time a well pump has to be turned on/off, thus extending the life of the well pump. If the heat pump requires the well pump to have to be turned on and off very frequently then there may be value in having the water holding tank. It also depends upon how much water needs to be drawn into the heat pump to provide one cycle of heat to the home.

Geothermal Heating and Cooling Systems

For example, if it draws in 40 gallons (and I am just making up this number) for one heat / water drawing cycle of the heat pump, and the water tank is only 30 gallons then the water holding is providing no value in reducing the number of on/off cycles of the well pump.

If on the other hand you had a 100G water holding tank, then you would reduce the number of on/off cycles of the well pump. Hence, in this latter example you would extend the life of the well pump.

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