Thermostat E Wiring

[Ferg67]

I’ve just bought a new Nest Thermostat E to replace our wired Honeywell DT90e wired thermostat to control a fairly old Worcester combi boiler. Having removed the front of the D90 I can see that only two terminals are connected - brown to A and grey to B (see photo). There appears to be an unused black wire and unused  bare earth wire (in grey PVC cover) which have simply been trimmed back. Any ideas on whether this is compatible with the Nest E and which wires go to which terminals in the Nest?

 

[David_K]

Not compatible, I'm afraid. If you find wires in your thermostat’s connector that have non-standard labels (for instance 1,2,3, or A, B, C), your system is proprietary. If your system only has two wires (like yours), but you have both heating and cooling, your system is also likely proprietary.

It is possible for some proprietary systems to be rewired by a pro installer to be compatible.