Confused about COM on Smart Thermostat

alex_r

Need to replace old timer to a new smart yoke Meross MTS200 to control oil boiler (Warmflow Agentis with 4 terminal connectors Switched L and Permanent L). Thermostat yoke has regular N, L, NO, NC and COM terminals but I got confused with how COM works. Why does the thermostat need second constant phase feeding COM if the unit already has power ?

Anyways, I drew up a wiring diagram, does it look right? Cheers

10-10-20

It's called a dry-contact, or a zero-voltage contact.

Running L into COM externally supplies L to the output of the relay NO or NC and it's just done like that on some systems but on others they simply loop L into COM internally. You can't switch N on those systems if it's already setup with a connection to L.

Hope that explains it in two sentences. 😀

alex_r

https://www.boards.ie/discussion/comment/123216030#Comment_123216030

yea it does. cheers. It would help if I could see their internal diagram. But your reply confirms my theory.

Funny thing, the circuit loop seems managed digitally internally depending on what setup you choose in the menu upon installation either Boiler or UFH. And for UFH it seems like they loop L to COM.

Lenar3556

The trigger to start some boilers is low voltage or voltage free (thermostat just joins to wires fed from the boiler).

Dry contacts like the one you have make the thermostat more universally compatible with lots of different applications