These units move the heat out of the heat pump into a separate tank. This requires a circulator pump and a heat exchanger to transfer the heat to the tank. This additional hardware makes a stand-alone heat pump as expensive as an all-in-one unit, even though no tank is included.
The unit that has been on the market the longest is Nyle Systems’ Geyser. The Geyser R (about $900) is made to connect to a standard electric water-heater tank. It ties into the lower heating element of the electric water heater and uses that control to turn the heat pump on and off. The Geyser plumbing interface is usually accomplished by removing the tank drain at the bottom of the electric water heater tank and installing a unique concentric piping hookup. The drain is reinstalled in the end of this interface and the tank operates as before, but is primarily heated by the heat pump.
After installing the Geyser, the thermostat setting that regulates the upper electric element in a standard electric tank can be turned all the way down. This allows the heat pump to deliver all the hot water. The lower control is usually set to maintain the tank water at 120°F. If lots of hot water is required at once, the upper electric element can be left on for quicker recovery heating.
Nyle Systems also offers the Geyser RO. This unit is for indirect oil- or gas-fired DHW tanks. These DHW systems usually derive their hot water from a central heating system boiler. The Geyser RO is a much more efficient way to create DHW during nonheating times of the year. A thermostat built into the Geyser RO has a temperature sensor that installs on the tank to control the tank temperature. This unit has the same plumbing hookup as the Geyser R. The original heat source can still be used for quick recovery during times of extremely high demand.