Vaillant VCW GB 242 and nest thermostat 3rd generation

8 Mar 2010
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United Kingdom
Hi everyone I have Vaillant VCW GB 242 boiler with switchmaster 300 time switch and no separate thermostat I know its old but still working good.

Install Diagram at VCW 242/282 ( for boiler.

I have installed a nest thermostat 3rd generation

Install Diagram 3rd-gen-Nest-Learning-Thermostat-Install-Guide-UK.pdf for nest.

The wiring with the switchmaster 300 was

Neutral on boiler to neutral on switchmaster 300 (blue)

Live on boiler to live on switchmaster 300 (brown)

Number 3 on boiler to number 4 on switchmaster 300 (black)

Number 4 on boiler to number 1 on switchmaster 300 (yellow/green)

This is what I have done

Neutral on boiler to neutral on nest (blue)

Live on boiler to live on nest (brown)

Number 3 on boiler to number 2 on nest (black)

Number 4 on boiler to number 3 on nest (yellow/green)

Can someone tell me if this is correct has the heating is on constant and I don’t know if this is to do with the install or if it is to do with the software setup.

Any help will be much appreciated thank you.
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Wiring sounds ok. Could be a duff nest. Any test equipment?
Sorry, I meant to ask, does it actually turn off whenever requested by the thermostat, or stay on regardless?
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Sorry it turns on and off when up to temp I have set up a Schedule to come on at 4am off at 6am and on at 5pm off at 10.30pm Monday to Sunday but between 6am and 5pm and 10.30pm and 4am it still calls for heat so the boiler is on 24 hours.​

I hope that makes sense thank you for your help.​

I’d it’s going off when temperature is reached, I would say it’s a fault with the thermostat itself. If when the temperature is reached, but boiler still firing, then test voltage AC between earth and normally open on the heat link.

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