Boiler responds to demand for hot water but not to demand for heating

25 Jul 2012
Reaction score
West Midlands
United Kingdom
Hello, my boiler is a Potterton Supprima. It's about 19 years old. The original PCB was replaced some time ago by a Siemens one.

With heating demand from the thermostat, the pcb indicator light is steady orange, which means the pcb is not receiving a heating demand.

I wanted to check whether the problem was the thermostat. It's a Honeywell T6360B.

When the set temperature is above ambient the thermostat clicks.

The terminal to terminal AC voltages are as follows:

Set temp above ambient: 2-3 246V and 1-3 0V
Set temp below ambient: 2-3 20V and 1-3 220V

Looking at the circuit diagram of the thermstat it looks as if might be behaving as it should. Would anyone care to comment please?

If it's not the thermostat, I assume it's starting to look as if the part of the pcb dealing with heat demand has failed.

I would appreciate any comments or guidance.
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What system controls do you have, as in 1 three port or 2 two port valves?
Unlikely to be the boiler if hot water is OK.
Thanks Mick but the 3 port valve is behaving OK these days. The boiler indicator turns green and the boiler ignites if I turn up the temperature on the small thermostat that's attached to the hot water cylinder. The boiler doesn't repond at all when I turn up the Honeywll thermostat. I was hoping it might be defective. I am suspicious about the boiler pcb because the boiler indicator glows a steady orange and doesn't respond when I turn up the thermostat. Orange means no heat demand according to the diagnostics in the installation manual that was left after the Siemens pcb was fitted.
"No demand" means there is not a call for heat, which is sent to the boiler from the 3 port valve. What makes you think the 3 port is ok? (What make is it?)
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If the boiler works for hot water then it's not a boiler fault.

I'd say it's most likely your 3-port but without a multimeter and access to your wiring it's impossible to be certain
Just come back from work. Thanks for all the comments. I have tried a fix by turning hot water to continuous and moving the valve to the test position. Now the pipe out of Port A is hot and my radiators are heating up. Will it be safe to leave it like that for a few hours or am a likely to get more problems?

My 3 port valve is a Honeywell V4073A with a Sundial Y plan connection. The valve has stuck in the past but I managed to free it. Eventually the gearbox on the synchronous motor failed so I replaced the actuator unit a few years ago.

Thanks Mick I now realise I must have misunderstood how the 3 way port valve is supposed to work. I had previously thought it was supposed to respond to the boiler, not control it. It's all clear now

I have temporarily turned the valve to the test position and turned on hot water. With the cylinder thermostat set to 70C I am getting hot water through the radiators. Will it be safe to leave it like that for a couple of hours or should I turn it off again once the house has warmed up?

I was wondering whether remove the actuator, because then I could set the valve to the mid position manually. This would mean the boiler would stop heating the water when the cylinder temperature reached 70C.

The gearbox seems to be OK because if I pull the lever to the heating position and release it the springs pull the valve fully clockwise to the hot water only position. However when I set the programmer to continuous heating the motor stays put and doesn't move. It therefore seems possible the synchronous motor is defective after all.

I would appreciate any guidance.

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