The following sequence is for the Rinnai tankless water heater. If you can learn this sequence, you will have a good understanding across the board for all brands out there as they all follow a similar sequence of operation.

Step 1. Upon a call for hot water, the water flow sensor will sense water flow and send a pulsing signal to the PCB (control board). The gallons per minute minimum to start the unit start trial for ignition will differ across models and brands, but 0.4 gpm is common

Step 2. The combustion fan starts. The positive pressure created at the outlet of the combustion fan to force out the exhaust in-turn creates a negative pressure at the inlet of the combustion fan which draws air in for combustion.

To simplify what is said in step 2 a bit further:

1. As the fan is turning, it’s pushing air through the heat exchanger and out the exhaust
2. Air moves through the heat exchanger for the combustion process and out of the exhaust venting
3. As the fan is pushing air out (positive pressure exhaust side) the fan is pulling in the air at the same time (negative pressure combustion airside)

Step 3. The sparker starts, the gas valve opens to low fire, flame rod proves flame, and then the sparker stops.

Step 1, 2, and 3 happens almost instantly from the time the user turns on a hot water faucet to the flame sensing. However, it can take some time to move the hot water from the tankless unit to the user depending on the distance.

Step 4. The PCB (control board) is continually watching and adjusting the fan speed, water flow rate, outlet water temperature, user water temperature setpoint, and flame signal.

As the user has the demand for water, the fan speed will adjust along with the modulating gas valve input to meet the demand for heating water. There’d be no point in firing all gas valve solenoids (SV 1/2/3/4) and monitoring with both flame rods (FR1/2) for a small request for hot water when just SV1/SV2 and FR1 would meet the hot water request.

Water is heated above setpoint and tempered down using the bypass. The water leaving the water heater is monitored by the outgoing water temperature thermistors at all times to make sure it’s about +/- 1 or 2°F of the unit setpoint. Heating higher and tempering down allows for a higher flow rate.

Example: User has the setpoint of the water heater to 120°F. Actual water is heated to 140°F and tempered down to 121°F measured on the outlet of the tankless water heater using the outgoing water temperature thermistor.

Step 5. Once flow rate drops to about 0.26gpm, the shutdown process begins. The gas control valve closes. The water flow control valve switches to a standby position. The combustion fan continues to run for a little bit to make sure the combustion chamber is free of fumes.

The PCB (control board) watches all components while the unit sits waiting for the next call for heat, so it’s essential not to remove power from these units when going away for an extended time especially in freezing conditions. These units are usually equipt with freeze protection and removing power causes freeze protection to be disabled.