CH stays on

purpleglow · Post by **purpleglow** » Wed Jan 25, 2023 7:53 pm

Hello

I have NEST 3rd gen wireless thermostat and Viessmann boiler with a unvented cylinder.
The system has a honeywell mid position valve (v4073a1039) and a two position valve in Y configuration

On the app when if I switch on CH it comes on. However when I turn it off the boiler is still on.

I believe wiring is ok
On app
Both off 1,2,4,5 are live grey to mid posn live
CH on 2,3,4,5 live white/grey live
HW on 1,2,5,6 on none live
Both on 2,3,5,6 live white live

When Ch is switched off I noticed that white has 57 V. The orange wire in the wiring centre also have 57 so this is being fed to boiler. I understand this can happen with Honeywell valves but I don't get why the boiler does not switch off as it is not getting 240v

If I turn HW on and then turn it off the boiler then turn off as orange has zero

Any ideas?

Neelix · Post by **Neelix** » Wed Jan 25, 2023 8:36 pm

Has it ever worked properly?

aeromech3 · Post by **aeromech3** » Thu Jan 26, 2023 8:26 am

Thought I'd seen this problem before, dug around and this could be the reason:-
https://diy.stackexchange.com/questions ... -only-mode

The part XEB1201 shows avail pack of 5 for about £7 from RS.