Replacing Honeywell ST9420C and Wireless Thermostat with Google Nest

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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.

Hello

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:

[GALLERY=media, 107281]7EBEC438-7E68-47A4-B702-E18B257DA4D0 by Robert de Castro posted 22 Nov 2021 at 7:23 PM[/GALLERY]

[GALLERY=media, 107276]197AB513-D0EA-4196-9CEB-C86B5CE6D1E7 by Robert de Castro posted 22 Nov 2021 at 7:23 PM[/GALLERY]

We currently only have a Honeywell wireless thermostat:

[GALLERY=media, 107277]E04C4ACE-4772-45F4-81DD-01B352CAEC13 by Robert de Castro posted 22 Nov 2021 at 7:23 PM[/GALLERY]

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.

[GALLERY=media, 107279]E0DAA577-1838-43C7-8C33-47B45B4B19D4 by Robert de Castro posted 22 Nov 2021 at 7:23 PM[/GALLERY]

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!
 
Last edited:
Sorry, The last couple of photos of the boiler wouldn’t upload before but here they are. Thanks

[GALLERY=media, 107280]71A2CBB0-67AC-460F-871A-A4018234D8DA by Robert de Castro posted 22 Nov 2021 at 7:23 PM[/GALLERY][GALLERY=media, 107278]B21E219D-F865-4112-BD61-20141EEC9469 by Robert de Castro posted 22 Nov 2021 at 7:23 PM[/GALLERY]
 
Hi, I have the same set up with honeywell and WiFi thermostat. I'm looking to swap with a hive though. Did you manage to swap them OK. I was told the hive is a straight swap with no changes. thanks
 
Hi, I have the same set up with honeywell and WiFi thermostat. I'm looking to swap with a hive though. Did you manage to swap them OK. I was told the hive is a straight swap with no changes. thanks
Nest and Hive are not the same , starta a new post with pics of your own setup, it is against site rules to Hi-Jack old posts for very good reason, you might think your system is the same as the origional OP, but it gets so confusing with mixed posts
 
Hi Robert, did you get this working as I'm leaning toward nest now
 
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