Nest wiring issue (I think!)

AndyH83 · Dec 17, 2022

Hi all,
I have an issue which i assume is due to incorrect wiring, but hoping someone might be able help me to fix it myself.

My setup is a Nest 3rd Gen running downstairs heating and water, and a Nest 'E' running the upstairs heating.

The problem is that the downstairs heating won't turn off while the upstairs is on.
So a daily scenario is that the downstairs is already up to temp, but will keep blasting heat until the upstairs gets up to temp, at which point they'll both turn off.

My work around currently is to temporarily drop the temp on the E upstairs, wait 30 secs for both the upstairs and downstairs to turn off, then put the upstairs temp back up.

I've created a wiring diagram for the current setup, and hoping it's a simple change needed - presumably on the U/F pump/valve. Any thoughts of what to try?
(And apologies for any incorrect notation, i labelled things how they made sense to me!)

[ElectriciansForums.net] Nest wiring issue (I think!)

davesparks · Dec 17, 2022

Yes that is wired incorrectly, you have a backfeed occurring due to incorrect wiring of the UFH valve.

In the diagram you have the permanent live (L on your diagram) linked to the motor (sw on your diagram) this allows a backfeed to occur through the microswitch and effectively latch that valve open as long as any other zone is calling for heat.

To solve this the UFH valve should have a permanent live connected to it so that it is the same as the other two valves in your diagram.

AndyH83 · Dec 17, 2022

davesparks said:
Yes that is wired incorrectly, you have a backfeed occurring due to incorrect wiring of the UFH valve.

In the diagram you have the permanent live (L on your diagram) linked to the motor (sw on your diagram) this allows a backfeed to occur through the microswitch and effectively latch that valve open as long as any other zone is calling for heat.

To solve this the UFH valve should have a permanent live connected to it so that it is the same as the other two valves in your diagram.

Thanks Dave, I thought that looked a bit odd!
So take a feed from the red live used by the other valves, into UFH valve L, and leave the Sw going into 3/NO on the stat.
What about the UFH pump, should i change that live feed as well?

davesparks · Dec 17, 2022

AndyH83 said:
What about the UFH pump, should i change that live feed as well?

No, not unless you want it to be running 24/7

AndyH83 · Dec 17, 2022

davesparks said:
Yes that is wired incorrectly, you have a backfeed occurring due to incorrect wiring of the UFH valve.

In the diagram you have the permanent live (L on your diagram) linked to the motor (sw on your diagram) this allows a backfeed to occur through the microswitch and effectively latch that valve open as long as any other zone is calling for heat.

To solve this the UFH valve should have a permanent live connected to it so that it is the same as the other two valves in your diagram.

Hmm... I've now tried this but with some odd results:
1) With all stats turned off, the UFH valve is staying open. I.e. it opens as soon as the central heating power is turned back on. But isn't calling for heat until the stat is on.
2) The UFH pump isn't coming on at any point.

Have updated the diagram to show the new live feed to the valve, that's been the only change.
Any ideas?

davesparks · Dec 17, 2022

AndyH83 said:
1) With all stats turned off, the UFH valve is staying open. I.e. it opens as soon as the central heating power is turned back on. But isn't calling for heat until the stat is on.

That suggests you've got the L and Sw of the valve swapped.

If the valve you have follows the conventional colour scheme then what you have labelled L would be a grey wire and Sw would be a brown wire.

It would help if you can post some pictures of the wiring and connections

AndyH83 · Dec 18, 2022

davesparks said:
That suggests you've got the L and Sw of the valve swapped.

If the valve you have follows the conventional colour scheme then what you have labelled L would be a grey wire and Sw would be a brown wire.

It would help if you can post some pictures of the wiring and connections

Ok, I'm in a process of elimination. Removed pump, so just testing that H2 valve. With only the brown live and blue neutral connected, and grey switch going nowhere, it's open all the time. (And the two other valves are not). Suggests a problem with that valve maybe?

If i swap the brown and grey, so with the grey wire into permanent live and brown going nowhere, it won't open at all.

The wiring is a basket case, so i don't think a photo will help much (see below!). That's why i decided to spend a day mapping it all out!
Also added photo of the valve.

AndyH83 · Dec 18, 2022

Ah ok, so if i carry on with my second option, grey into perm live and brown into stat switched live then the valve does what I'd expect. (I thought brown would always go into perm live!?)

Just need to get the pump behaving now!

davesparks · Dec 18, 2022

AndyH83 said:
. With only the brown live and blue neutral connected, and grey switch going nowhere, it's open all the time. (And the two other valves are not). Suggests a problem with that valve maybe?

That does not suggest a problem with that valve, that is exactly how that valve should behave.

AndyH83 said:
If i swap the brown and grey, so with the grey wire into permanent live and brown going nowhere, it won't open at all.

That is correct, the brown and blue are the live an neutral to the motor, when there connected and power is applied the valve opens.

davesparks · Dec 18, 2022

AndyH83 said:
Ah ok, so if i carry on with my second option, grey into perm live and brown into stat switched live then the valve does what I'd expect. (I thought brown would always go into perm live!?)

Just need to get the pump behaving now!

No brown is not permanent live.

Brown and blue are the valve motor, connect them to live and neutral and the motor turns to open the valve.

Grey and orange are a switch which is operated by the valve reaching its fully open position. They could be connected to any control voltage or signal type.

Nest wiring issue (I think!)

AndyH83

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AndyH83

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