Abstract: A combination domestic hot water and space heating system has an instantaneous domestic hot water heat exchanger and a low mass boiler-type heat generator connected in a series by a boiler water flow circuit containing a circulating pump. The boiler-type heat generator has an internal reservoir containing between 10 and 20 U.S. gallons (37.85 to 75 liters) of boiler water, which is sufficient to enable the heat exchanger to supply normal short term demand, yet avoid excessive downtime thermal losses. A three-way diverter valve in the flow circuit diverts boiler water to a second heat exchanger in an air handler unit for space heating. Priority is given to domestic water heating.