modulating valve 42a is closed a reduced flow of gas is permitted through restriction 42b thermostat 34 deactivates the pump 18 in response to sufficiently high water temperature sensed at the accumulator thermostat 34 and also deactivates heater 14 by closing primary valve 42c the heater also includes a conventional pressure and temperature safety relief valve 58 which may be located in the heater output line 30 and or the accumulator tank 12 there are two fundamental modes of operation of the system in both cases however it is important that water at a predetermined temperature be available for instantaneous use at the use point 10 in the first mode of operation which may be termed the 34 no demand 34 mode there is little or no demand at the use point 10 and the system must by recirculation maintain the temperature of the water in the accumulator tank 12 so as to be ready for instantaneous use in the second or 34 demand 34 mode of operation there is constant or intermittent demand at the use point 10 and there is little or no recirculation in the accumulator tank 12 in either case the thermostat 34 is set to actuate pump 18 at a predetermined minimum temperature lower than the desired temperature of the water to be delivered to the use point 10 this is because of the low position of the thermostat 34 in the tank and the fact that there will always be stratification of the temperature of the water held within the accumulator tank when the thermostat 34 senses the accumulator water temperature dropping below the predetermined minimum either due to normal accumulator heat loss or due to demand at the use point 10 while the pump 18 is deactivated the latter drawing cooler.