Worcester 24i Junior Boiler no longer communicates with thermostats

18 Oct 2020
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United Kingdom
Hi there,

our boiler has stopped responding to thermostats.
We have tried a few different thermostats, the relays click away and it sends the signal to ignite but the boiler doesn't do as its asked. I checked the condition of the wiring thinking something might have came loose but everything is uncorroded and in the terminals good and tight.

I also checked the fuses on board and they fine.

Does anyone here know how to fix this or are there any tests I can do to narrow it down?

Cheers and thanks for a great forum!
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If you own a multimeter you can check if there is voltage along the circuit

How do you know it sends a "signal" to ignite?

Does your hot water work as normal?

Is your timer calling for heat and is the fuse in the plug/fcu ok?

Is the circuit that powers the boiler not tripped at the consumer unit?
The NO contact in the thermostat reads 0v when not calling for heat then when you increase the temperature to higher than the current room temp the relay closes and reads 20v, I guessed that is supposed to be telling the boiler to ignite but now I'm wondering why it isn't 250V?
Everything else about the boiler works as normal and all the fuses are in place and conducting and no MCB's are tripped.
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Then it should be 240v going out , do you have the other thermostats you have changed ?

Have you bypassed the thermostat to see if the boiler will fire?
Broken neutral? Carried out any work recently, eg wiring, hanging a mirror/picture/shelf?
Post a pic of your thermostat and boiler and dont put 240v on to the 20v wire, tillwe see what you have it could be low voltage switching and you will damage the boiler if you get it wrong

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