Service manual

MAIN COMPONENTS

1. Gas Control Unit

1.1 Modulating Valve

This device is used by the PCB to adjust the volume of gas to the burner in proportion to the volumetric

flow rate of water in order to maintain a supply of constant temperature hot water amid changes in water

flow rates and incoming temperatures.

1.2 Change-over Solenoid Valves

Additional solenoid valves are included to section the burner and stage the control in several steps. This

gives the burner more steady combustion at the required capacity and allows the water heater to

operate at very low flow rates and temperature rises.

2. Flame Rod

Monitors combustion characteristics inside the combustion chamber. If the flame fails, gas supply is

stopped. Works through rectification of the combustion flame. An AC voltage is supplied to the flame

rod. Electrons can only pass from the rod to the earthed burner through the flame, and never from the

burner to the rod, so the resultant DC current is used to prove combustion. When the DC current is

present the burner has normal combustion, if the DC current is not present (or an AC current is present)

the unit shuts the solenoid valve.

3. Thermal Fuse

The thermal fuse is an electric link which must be intact for the unit to operate. If the thermal fuse

reaches a set temperature it will melt and the unit will shut down. The thermal fuse must be replaced if it

melts. It is to protect against overheating and heat exchanger splits where water may leak out and be

superheated into steam.

4. Overheat Safety (Bi-metal Switch)

This Bi-metal Switch is fixed at the inlet bend of the Heat Exchangers final pass. If the temperature

outlet from the heat exchanger reaches 97°C the bi-metal switch will open and the solenoid valve circuit

is broken. This will cease combustion in case of overheat.

5. Combustion Fan

The combustion fan supplies primary air into the burner wings and secondary air up through the Bunsen

style burners. The fan is DC low voltage and the speed is controlled by the PCB depending on the hot

water supply and temperature. The fan speed is compared to the current required to attain that speed

for air proving. If the fan current is over or under the parameters for the given speed the unit will shut

down on air proving.

6. Water Flow Servo with Water Flow Sensor

6.1 Water Flow Sensor

Water flow sensing is done with a small turbine that spins when water travels through it in the correct

direction. Each of the four fins on the turbine has a small magnet on it. Outside of the valve there is a

magnetic sensor that detects the speed that the turbine is revolving. The revolution speed is input to the

PCB which relates this speed to the water flow volume and determines whether it is sufficient for

ignition.

6.2 Water Volume Flow Servo

Water flow control is achieved through the use of servo driven water flow and bypass valves. The servo

motor is controlled by the PCB. The ‘Water Flow Valve’ restricts the flow of water into the heat

exchanger assembly if the programmed temperature cannot be achieved. This will limit the maximum

water flow, and will limit the hot water flow further when the burner is at high fire to ensure the

temperature set point is met.