Service manual
06/2010
Section 2. Description of Operation RV500
With gas and power supplied and no water flow, the RV500 will appear dormant except for the green
LED flashing once each second. However, the microprocessor in the RV500 is actively monitoring
the three temperature sensors, the set temperature request, and the water flow each half second in
anticipation of a request for hot water.
When a hot water faucet is opened and flow is 0.5 gpm, the microprocessor sends ignition voltage
to the modulating valve and powers the ignition relay on the board which provides 12 vdc to the DSI
board.
The (Direct Spark Ignition) DSI:
Sends 12 vdc to the gas solenoid to open and supply gas to the burner
Starts sparking the igniter
Monitors the flame presence. When the DSI senses a flame it grounds the feedback to the
circuit board. If the flame is not sensed during a 3.5 second ignition attempt the DSI will turn
off gas and maintain the feedback signal to the circuit board open (12 vdc) for a 3.0 second
purge and repeat the 3.5 second ignition attempt. If there is no flame sensed during the third
attempt, the DSI will lockout and feedback will remain open until the software resets it 10
minutes later and the ignition process is repeated. If the flame is lost during normal burn, the
DSI will attempt a re-ignition.
When the feedback signal is grounded there is flame. Knowing T-in, T-set, and water flow,
the software calculates the gas required for the desired set temperature. Stored in a chip on
the circuit board is a calibration of the modulating valve (AutoCal). The microprocessor
accesses the calibration to convert the gas flow to a voltage to send to the valve. Variations
in the heat exchanger and the sensors cause the T-out to be slightly different in reality. To
compensate for the variations the microprocessor adjusts the gas flow every 0.5 seconds
based on the error between T-out and the T-set.
Every 0.5 seconds the microprocessor reads the temperatures and water flow and
recalculates the gas flow required. If the water flow changes the gas flow is reset within one
second to a new value. Anytime water flow is turned off, the software will turn the gas off
within 2 seconds.
If you are operating within the capacity of the unit at steady state (see figure-1), you will see
small changes in the flame height. These are the small automatic adjustments the software
makes to maintain the set temperature. If you operate the RV500 above the capacity, you
will see the burner at the maximum burn, however, the outlet water temperature will be below
the T-set. Conversely, if you are operating below the capacity then the software cannot
lower the burner lower than the minimum burn and the water will heat above T-set. The
software will shut off the burner when T-out rises to 16F greater than T-set. It will remain off
until T-out < T-set. At the faucet this is seen as fluctuating temperature from very hot to cold.
The characteristics above and below the operating range are typical of all instantaneous
water heaters.