Protherm 40-50 Ci Overheating

I had to look at a Protherm 40-50 Ci boiler which was regularly overheating. The button on the overheat thermostat had clicked out a few times over the last few days.

This can be quite common on this boiler especially if the boiler thermostat is in it's maximum position as I expect the temperature difference between the overheat thermostat and boiler thermostat maximum is not that great. Reducing the burner pressure from it's maximum can help to reduce the overheating as can turning down the thermostat from it's maximum. Sometimes it can be caused by interruptions to the power supply whilst the boiler is running,

In the case of this system the fault was due to the hot water zone valve. The microswitch had failed and was permanently demanding heat from the boiler even when the controls were turned off. Because of this the boiler was coming on and there was nowhere for the heat to go as a bypass was not installed. The zone valve opens when power is supplied to to brown wire of the valve. There is power permanently applied to the grey wire which is connected to the microswitch and when the valve opens the microswitch 'makes' and power comes out through the orange wire to power the boiler.

I diagnosed which of the zone valves was causing the issue by removing both of the orange wires with the power off and testing for continuity between the 'grey' terminal and each of the orange wires. The hot water zone valve had continuity across the grey and orange and deemed the faulty valve. Replacing the hot water zone valve solved the issue.

Disclaimer: The works described in this blog are a record of work undertaken and should not be taken as guaranteed to solve your problem. Work should only be carried out by persons qualified and competent to do so.