Over the last few years, I’ve written several columns describing how air-to-water heat pumps can provide heating, cooling and domestic hot water for homes. Many of the systems involve buffer tanks to help stabilize heat transfer from the heat pump to a zoned distribution system. Those tanks are especially important with a single-speed heat pump supplying a highly zoned distribution system. Without the thermal mass they provide, the single-speed heat pump would undergo frequent on/off cycling that ultimately leads to premature failure of components, such as compressor contactors or start capacitors. Short cycling also lowers the overall efficiency of the heat pump.
The emergence of air-to-water heat pumps with inverter driven variable speed heat pumps allows heat output to be reduced down to about 30% of the heat pump’s rated capacity. This can reduce the size of the buffer tank — perhaps into the range of 25 to 40 gallons in typical residential systems, depending on the extent of independent zoning.