Nest 3rd Gen - No Hot Water Control

22 Nov 2021
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United Kingdom

I managed to replace our old Honeywell ST9420C and wireless thermostat with the Nest Learning Thermostat V3 using the following wiring layout:

L-L, 2 & 5

Previously, on the old Honeywell programmer there were just 4 wires - N, L, 3, 4.

This is working fine for the heating but the hot water seems to be always on with the heating. We have a system boiler (2 valves so presumably S-Plan) and a separate hot water cylinder, which also has an electric immersion heating option which we rarely use, and a header tank in the loft. The old programmer had separate controls/timers for heating and hot water. On the Nest, it’s set to System Boiler with radiators but under the Pro settings it says hot water not connected, and there is no separate schedule/controls showing at all.

Please can someone help me resolve this. Could it in some way be due to me not properly disconnecting the old wireless thermostat? Not sure how to do that so I simply took the AA batteries out of the unit and put it in the cupboard.

Here’s a few photos that might help.

Many thanks.
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The wiring sounds OK from your description.

Does the button on the front of the Heat link turn the hot water on / off?

It does sound like there is a a problem with the set up especially as there isn't a schedule for the hot water showing. I can't remember the settings without having one in front of me to play with, but the Nest helpline are good with that sort of thing and will talk you through it. 0808 169 2307

Other than that, and assuming that the system was working properly before you installed the Nest, the only other thing I can think of was that there was an option for a wireless Honeywell hot water cylinder thermostat.


If you do have one, then it won't work with the Nest. Nest requires a wired cylinder thermostat.
sounds like you have the nest set to Gravity Hot water, you need to set it to fully pumped
Thanks very much Stem and Ian

I don’t think there’s a wireless thermostat for the HW cylinder. Looks like there’s just one strapped onto the cyclinder itself (Honeywell L641A1005 - grey box with a black temp dial).

Will look into the Nest settings to see how to change it to fully pumped. Don’t recall seeing that option previously but hopefully just missed it.

As for the button on the Heat Link, when I press it the hot water (drop) and heating (wavy lines) symbols both switch on and off simultaneously.


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if you sect HW only and just the HW comes on, but if you only select the heating but the HW also comes on, then it is indeed set to Gravity HW but if you are saying that if you select HW only then the heating also comes on then something is wrong
Feeling embarrassed now but I thought Hot Water “Not Connected” under Equipment - Pro Setup was a status rather than an option that could be changed. It seems to default on that but I’ve changed it to On/Off and now the HW Boost and schedule are showing up on the Nest app.

Thanks for your kind suggestions though.
Sorry Ian, just saw your post after my latest reply when I thought everything might be ok now. Can you tell me where the Gravity/Fully Pumped setting is found. I’ve been through all the Equipment and Pro Setup screens and can’t see it.

I think it might be working ok now. On the Nest App it’s showing Hot Water with a button that switches between ‘Off’ and ‘Press to Boost’ and there’s a separate schedule. However, even before I switched the HW setting from Not Connected to On/Off I was still getting hot water with the heating on. Not sure about now yet because there’s still plenty of hot water in the system.

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might be worth checking the legionairs setting for the HW as this could also cause the same problem you were having, it will activate every time power is cut and restored or on a fixed time basis
Robert, I'm looking to do the exact replacement as I have the same set up.
My st9420c is in the hallway and is fed from the boiler with regards to power L, N and the heat and water via the connectors 3 & 4.

Was it just try case of connecting honeywell 3 to nest 4 and honeywell 4 to nest 6 as you said earlier?

With regards to honeywell L and N just connect the same to Nest, was the power OK, no need to install anything else?

Nest power - just connect from nest link to Nest thermostat power?

What are nest 2 and 5 used for, I also have a system boiler with a separate tank

Many thanks
Robert, just watched the video and 2&5 is now clear for me.

As you had the wireless thermostat, same as me, it looks like we don't need to do any changes in the existing control box with regards to 4 & 5 - is this correct?
This "do nothing" seems to be true for any smart replacement


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