below a selected value or when it is determined that more space heating energy is needed for proper operation of the system during a heating mode it may provide a signal via line 78 to the control circuit unit 68 to again actuate the solenoid valves 70 and 72 and shunt the water circulating through the desuperheater 20 through the bypass tubing 46 and prevent further discharge into the water heater tank this effectively prevents further drain of the heat value within the desuperheater 20 from the exhaust gases passing through the conduit 18 to the heat exchange unit 22 illustrated in fig1 this effectively maximizes the heat energy available for heating the air returning through air return 28 and being passed out by blower 30 through the forced air outlet 32 containment of all of the elements or components of the system within the single cabinet likewise means that bypass tubing 46 will be enclosed within that same cabinet this will minimize the ambient or stand by heat loss through the bypass tubing 46 during the system heating mode thereby minimizing the heat loss through desuperheater 20 during the shunting operation aforedescribed while still allowing water circulation to minimize desuperheater tubing maintenance alternatively the control circuit unit 68 may be triggered by either the third water temperature layer sensor 64 or the room sensor 76 to discontinue operation of the pump 42 this also will shut off further circulation of heated water when desired in which case valve 70 72 and bypass tubing 46 will not be required in instances where water characteristics e g soft water are such that mineral.