Will the new Nest thermostat work if my old thermostat wiring looks like this?

[Tadeas]

I'd like to get a new thermostat, but the option to check for compatibility doesn't work on the Google site. From what I understand it probably won't work, but I'd like to get other people's opinions. There are 2 wires connected to the system ( Black and Brown) There is a grey wire still unconnected at the back.There is room for 3 cables in total. The minimum output here is 24-250 VAC. And there is a marking 1, 2,3. Anyone got any advice?

[David_K]

Wire labels such as 1 2 3 4 indicate a proprietary system. Proprietary systems usually only work with thermostats made by the same manufacturer. But some proprietary systems may also have the option to use standard heating system wires. If you really want to investigate replacing it, I'd recommend contacting a local professional and have them check if your proprietary system can be rewired to be Nest compatible.