Sequence Of Operation - Rheem 874W27NF Service Manual

SEQUENCE OF OPERATION

Refer to „Sequence of Operation Component Diagram‟ on page 9 to view components
shown in brackets e.g. (1)
1. When a hot water tap (1) is opened, cold water enters the water heater and passes
through the Water Flow Sensor (4), Heat Exchanger (20) and Transfer Pipe. A
temperature actuated Transfer Valve (28) located at the outlet of the transfer pipe
determines where to direct the water flow depending on the water temperature. When the
temperature of the water entering the transfer valve is below 42ºC the transfer valve will
direct water flow through the auxiliary hot water storage tank (29) and hot water stored at
70ºC is delivered to the hot tap from the auxiliary hot water storage tank.
2. As water flows through the unit it induces movement in the Water Flow Sensor (4) which
sends a pulse signal to the PCB (8). Once the pulse signal reaches a pre designated
frequency (at a min. flow rate of 2lpm) the PCB (8) activates the Fan Motor (26) and the
Fan (25) starts rotating.
3. After the Fan (25) completes a pre purge to ensure the vent is clear, the Proportional Gas
Flow Regulator (14) allows gas to flow to the Burner (16). At the same time the Igniter
(17) starts sparking continuously and ignites the gas at the Burner (16).
4. Hot water delivery from the auxiliary hot water storage tank will continue until the gas
heater has achieved ignition and the water being delivered from the gas heater reaches a
temperature above 42ºC at which point the transfer valve (28) will gradually bypass the
auxiliary hot water storage tank and hot water will be delivered directly from the gas
heater to the hot tap (1).
5. After ignition the Flame Rod (19) monitors the burner flame. The PCB (8) constantly
monitors all inputs to ensure the outlet water temperature is maintained within 1
thermostat setting. The Proportional Gas Flow Regulator (14) continuously adjusts the
gas volume and the water flow is also adjusted to assist in supplying a stable hot water
temperature at all times.
6. As the gas flow rate, controlled by the Proportional Gas Flow Regulating Valve (14)
changes, the PCB (8) varies the speed of the Fan Motor (26) to maintain the correct air
gas mix ratio.
7. When the hot tap (1) is closed, the pulse signal from the Water Flow Sensor (4) stops and
the burner flame is extinguished by closing the gas valve. The post purge operation then
commences to cool the heat exchanger (20).
8. Once the post-purge operation is completed power to the Fan Motor (26) is cut and the
Fan (25) stops.
9. When the water temperature within the auxiliary hot water storage tank drops below the
set point the surface mounted thermostat will provide power to the 500W booster
element (31) to boost the storage water temperature to 70ºC.
TM061 Rheem 27 Water Star Service Instructions – Issued: 11/10
REV A Issued 08/13
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