Thermostat and Boiler not communicating

2 Mar 2018
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United Kingdom

Over the past couple of days I have noticed there seems to be an issue with my central heating.

The boiler appears to be working normally, it is firing up heating up the hot water but it isn't consistently firing up for the central heating.

I think the issue is something to do with the thermostat.

I've tried resetting the controls, checked the pressure, pressed the reset button on the boiler itself, bled the system at the boiler and the rads.

Any help would be greatly appreciated.



Boiler: Potterton Promax SL
Thermostat: Myson MRT1
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Hi I'm not aware of a Myson MTR1. Do you mean a Myson MRT1? If so, what made you come to the conclusion that the thermostat may be at fault?

Also, what type of system is the boiler connected to? For example a Y-Plan with a single 3-Port motorised valve, or an S-Plan, with two (or more) 2-Port motorised valves.
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Hi Stem,

Apologies yes it is MRT1 - edited now.

It is a little difficult to explain but when I got home today I turned the dial to approx 15degC near where it clicked. The boiler came on for a bit. It has not gone off, so I've turned the dial to 20 - 30 (I've tested various temperatures) and it hasn't come back on and the room definitely isn't over 20. Thermostat is an un-educated guess really, mainly through process of elimination.

Thanks again
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It looks like an S-Plan. With two 2-Port motorised valves. Typically like this.

One is for the hot water, the other is for the central heating. When the room thermostat calls for heat, the central heating motorised valve will open and when it is fully open it will trigger a small internal switch that operates the boiler.

This means; that the problem you are encountering could be, that as you thought, that the thermostat is not operating correctly and not opening the motorised valve, but more often, the thermostat is OK, and opens the motorised valve, but the switch inside the motorised valve is not operating correctly and so not switching the boiler on.

Unfortunately what is causing the fault can only be determined by electrical testing with a multimeter when the fault is present.
Hi again Stem, there is a part which looks exactly like that.

Thanks for the advice, we'll get somebody in asap and wrap up warm in the meantime.

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