DHW Domestic Hot Water with Geothermal

When adding domestic hot water to your geoexchange system, be careful. Most clients opt for the desuperheater method which works well enough with a forced air system but remember it only works when the heat pump is running.

It can be made better by adding a preheat tank that has no other heat going to it.

That way the desuperheater can usually be useful. What I mean is, what if the system is off for a few hours (mild weather) and someone has a bath? The hot water coming into to the DHW tank is cold and the heat pump won’t do a thing. In a few hours the DHW will be hot from the gas or electric in the tank.

The heat pump will start at some point that day and the desuperheater will shut off because the DHW is already hot. Great! If you have a preheat tank, then at least when the heat pump starts the desuperheater will have something to do.

The only good thing about desuperheater systems is that they are inexpensive compared to DHW on demand.

DHW Priority is more complex and expensive but gives you the entire heat pump capacity when you need it (instead of the 10-15% from desuperheater systems). The heat pump can either be ordered with it built-in, or it can be field-installed much like a typical “indirect fired” tank in a boiler system would be. The indirect tank becomes the preheat tank (up to max 120 degrees Fahrenheit) and a regular DHW tank is still needed to bring the dhw up to 140 degrees Fahrenheit.

The pitfall in an indirect tank system is that you must use the proper type of tank or use an external heat exchanger. The typical tank used for an indirect boiler application will lead to failure. These tanks use stainless steel coils inside. The coils are typically schedule 40 pipe. We must look up our thermal conductivity charts and see that stainless steel has abysmal heat transfer properties.

Because of the low temperatures that geoexchange systems supply the heat transfer will not be enough to get the heat out of the heat pump. Therefore we must use special tanks. The European tanks are good because they have figured this out long ago. They use exotic S.S. alloys and near paper-thin coils.

There are some North American tanks that may be good such as the ones with internal copper-finned coils or some models with tank in tank construction. However these manufacturers don’t care enough about our small segment of the tank industry and can’t be bothered to supply us with lower temperature specifications. We can’t afford to test them all ourselves.

An alternative is to use a brazed plate heat exchanger. This uses more piping, an extra pump as well as more labour.

The Controls: this can be as simple as an aquastat in the preheat tank that starts the heat pump and its circulators and drops out the heating system. If you have hydronic heating you should have some form of outdoor reset control which will likely have provisions for DHW priority.

If you don’t have outdoor reset on your hydronic heating system you should have it installed before the cost of heating goes any higher. But that’s a subject for another blog…