Replacing Honeywell ST9420C and Wireless Thermostat with Google Nest

22 Nov 2021
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United Kingdom
UPDATE: Now resolved this myself, thanks to earlier post by Muggles. I just forgot to put the additional live connections to T2 and T5. Silly mistake on my part but all working fine now.


I’ve just bought a Google Nest Learning Thermostat v3 to replace our old setup. Sorry, I know there have been many other similar posts but I can’t see anything that seems to cover a similar setup and my knowledge of electrical wiring is very limited.

This is how the old programmer was wired:

We currently only have a Honeywell wireless thermostat:

Following another thread on here I attempted to wire up the Heat Link as in the photo below. It powered up fine and connected with the Nest Learning Thermostat but won’t actually switch either the heating or hot water on or off. I can hear a click but I assume this must be coming from the Heat Link only.

I’m not sure what our boiler set up is. it’s a Worcester Greenstar Ri. It has no pressure gauge and there is a separate hot water cylinder upstairs and a header tank above the main water tank in the attic. The last two pictures show the boiler and its external plumbing.

If necessary I’ll book an installer but I’m hoping I just need to change the wiring arrangement on the Heat Link. If anyone can help that would be much appreciated. Many thanks!
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