As global energy planning moves away from fossil fuels and toward electricity, an increasing number of hydronic heating systems are being supplied by heat pumps. Some use water-to-water heat pumps supplied by geothermal earth loops. Others use air-to-water heat pumps. Both types can supply warm water for heating, chilled water for cooling and some or all of the domestic water heating load.
If the heat pump with a fixed speed compressor supplies a zoned distribution system, it is essential to use a buffer tank in both heating and cooling mode. If the heat pump has a variable speed, (e.g., “inverter”) compressor, a buffer tank may or may not be required. If the latter heat pump can reduce both its heating capacity and cooling capacity to approximately the design load of the smallest zone — or less — a buffer tank is not required. However, this scenario is currently the exception rather than the rule. This is especially true for highly zoned systems or system with more heating zones than cooling zones.