Abstract: A combination water heating and space heating apparatus includes a water heating unit and a space heating unit releasably coupled with the water heating unit. The water heating unit has a water storage tank and a helically wound tubular heat exchanger inside the tank for exhausting products of combustion from a combustion chamber located in a top part of the tank and for transferring heat from the products of combustion to the surrounding water. The space heating unit is an air handler with an hydronic heat exchanger coil and a blower for blowing air over the coil. Hot water is supplied from the tank to the coil and is returned to the tank by means of a water circulation pump located in the space heating unit. Air blown over the coil is heated by the hot water flowing over the coil.