Domestic hot water from the heat pump
The same heat pump that heats the building also heats a hot water cylinder. It does this in turn, not at the same time: a diverting valve sends the water to one circuit or the other.
How it is connected
Hot water priority
When the tank sensor reports that the cylinder has cooled below its setpoint, the controller switches the diverting valve to the cylinder, heats it, then switches back to space heating or cooling. During that time the space circuit receives no heat. A well-insulated building barely notices a short interruption, which is why the cylinder coil and the unit's capacity must allow a reasonably quick reheat.
Tank sensor and setpoint
A temperature sensor in the cylinder tells the controller when to reheat. The RV DC inverter series delivers water up to 55 °C, so the stored water is always somewhat cooler than that. The lower the storage setpoint, the higher the heat pump's efficiency while heating it.
Coil sizing
A heat pump delivers its heat at a lower temperature than a boiler, so the cylinder needs a larger heat-exchange coil, specified for heat pump use. A boiler cylinder with a small coil will limit the heat the unit can transfer and extend the reheat time.
Hygiene
Stored hot water must be managed against Legionella according to local regulations. Where the regulations require periodic heating to 60 °C or above, that step is normally done with an immersion heater in the cylinder, because it is above the heat pump's water temperature limit.
Cooling with hot water
In summer the controller can still switch to the cylinder between cooling periods. Heating and cooling are not delivered simultaneously on a single diverting-valve system.
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