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Combi boiler stat not turning off heating

View the thread, titled "Combi boiler stat not turning off heating" which is posted in Boiler Advice Forum on UK Plumbers Forums.

Matt0029

Gas Engineer
If the stat is not shutting off a combi boiler (analogue stat Honeywell) even though the stat is clicking when turned up above the room temperature. What are the likely causes. Unless it's never been wired right from the start up. Thanks.
 
I just wondered if the stat can still be the fault if it is clicking when turned above and below room temperature? This is the stat?
 

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Check to make sure there’s not a link in the boiler end - either a 24v one, or linked the stat out.
 
Just going to download the instructions. But to test would I be looking to see if I get 240v between live and switch live?
No mate, testing that you’ll probably get 120vac.

Test Neutral to live in (should be terminal 1) - 240vac
Test Neutral to Switch live - on demand - 240vac
Test Neutral to switch live - on satisfied (or stat at lowest) - should be 0 Vac
 
If you test between live and switch live with the stat contacts closed, you would get a reading of 0 volts as there's no difference in potential between the two. As above you need to test between live and neutral and switch live and neutral.
 
Does testing neutral to switch live on demand with a reading of 240ac show that switch live is sending power to the boiler. Making a circuit when there is a call for heat?
 
Thanks so testing neutral to live in. With a reading of 240vac between the ports. Shows I am getting 240vac to the stat.
Yes correct, but doesn’t prove it works properly, hence the additional testing and don’t forget the link.
 
Thanks just trying to find the wiring diagram. I think Live is 1. Neutral 3. Switch live 2. ?
No:

1 should be live in
3 should be switched
2 should be Neutral

Wiring diagram usually on the reverse of stat cover.
 
Test terminal 3 to earth or neutral with stat off should be no voltage

If there is 240v check 1 if that’s got 240v aswell stats dead
To to check I'm on the right track. Testing 3 (switched) to earth or neutral with the stat off. A reading of 0v. Shows that no volts are passing though the circuit i.e no call for heat at the boiler?
 
Correct but I have seen them wired backwards eg 3 live 1 switch
Yep, me too. Had one recently, set to a mid point, so in winter it’s fine. The way it was wired max was off and low (5 degrees) on the VRT30 was off.
 

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