How to bypass (or replace) a CH room thermostat

M

Mickwuk

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• 19 Oct 2019

• #1

I want to bypass a room thermostat so that it is effectively permanently "on". I think I just need to jump two wires, but which two? Or am I wrong anyway?

I have four wires labelled NO (normally open), COM (common), L (live) and N (neutral).

Jumping NO and COM seems to do nothing (that was my first attempt - my simplistic view was that this was the thermostat "switch"). Jumping L and NO turns on the CH but it still runs even when the controller heating is off.

Any advice gratefully received!

P.S. Or, how do I wire a Salus RT100 thermostat which I have bought as a simple replacement.

 

JohnD

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• 19 Oct 2019

• #2

Mickwuk said:

a room thermostat so that it is effectively permanently "on"

Click to expand...

grasp the thermostat knob and turn it all the way to its highest setting.

Or, photograph the thermost front and back, showing the wires, and the terminals they are connected to, and the letter or number beside each.

You may find a label glued to the inside of the casing. Photograph that too.

 

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• 19 Oct 2019

• #3

Thank you JohnD. Old thermostat as it was...

...and wiring diagram...

I thought that simply connecting COM and NO would bypass the stat. Doesn't seem to, unless I have a different problem elsewhere.

New stat wiring diagram...

...I connected COM to COM and NO to NO. Turning it right up does nothing.

Either I misunderstand something fundamental or there is a bigger problem elsewhere.

BTW, the original problem was the the old stat stopped working. I thought a bypass would prove that. It is a wireless receiver and one of the resistors on the circuit board appear scorched.

 

JohnD

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• 19 Oct 2019

• #4

no pictures visible.

you can copy and paste pics into your post.

 

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Mickwuk

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• 19 Oct 2019

• #5

Hmm, not sure my images are working?

 

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• 20 Oct 2019

• #6

Take two. Old wiring...

Old wiring diagram...

New wiring diagram...

 

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Madrab

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• 20 Oct 2019

• #7

Yes, COM and NO should be the volt free contact for the boiler and connecting them should just act as a link. You need to check back to the boiler to ensure it's wired the same way.

What boiler is it?

 

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Mickwuk

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• 20 Oct 2019

• #8

Thanks. It's a Potterton Kingfisher 2. It was all working well until the thermostat seemed to fail. I guess there is either a bad connection or the thermostat failure has caused a problem.

 

Madrab

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• 20 Oct 2019

• #9

If I remember rightly the Kingfisher 2 needs a switched live, it doesn't use a volt free contact. Which is why when you connect the L to the NO it runs.

I would be careful though, you really need someone with boiler wiring experience to sort it out. If you send 240V down to the boiler you could easily fry something in the boiler (or yourself) and that won't be easy to replace ..... in such an old boiler.

 

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• 20 Oct 2019

• #10

Thank you Madrab. Yes, I think I'm at the end of my DIY ability. Thanks for the help.

 

ianmcd

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• 20 Oct 2019

• #11

The pic you have posted is for a combi boiler, the Kingfisher 2 is not a combi boiler, you possibly have a y plan or s plan system and the thermostat will send power to these valves and not the boiler, joining COM and NO bypasses the stat, so if that is not bringing the boiler on it is not the stat that is faulty

 

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• 20 Oct 2019

• #12

Thank you ianmcd. Maybe the controller is the problem then. Think I definitely need a professional.

BTW, the photograph is how it was before I did anything and it has been working ok for years.

 

Last edited: 20 Oct 2019

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• 25 Oct 2019

• #13

Just to close this off. I replaced the controller and all was well. It seems that the wireless thermostat receiver had blown and perhaps damaged the controller in the process. COM and NO were the correct terminals for the thermostat. All working well now with new controller and (wired) thermostat. Thanks for the various replies.