What is a 'COM' wire?

[2point8ghiax]

Just wiring up my new heating thermostat. I'm not having any problems, but wanted to know what 'com' means.

Presumably its common, but what exactly does that mean? Common to what?

Is a common wire positive or negative?

 

[seco services]

if it's a heating stat that'll be the live from the programmer.

 

[DetlefSchmitz]

Common to both sides of the switch, which are Normally Open and Normally Closed. Normally Open would close (be connected to Common) on a demand for heat, and of course Normally closed would open at the same time.

The second part of the question therefore makes no sense. It wouldn't anyway, as we tend to use alternating current supplies for power distribution these days.

 

[Lectrician]

Positive and Negative are terms used with a DC supply. We use Live and Neutral for AC supplies. (and even that is open to a bit of debate).

You may connect a 'black or blue' conductor to the room stats com, call or sat terminals, but if it was a neutral you would end up with a bang and a blown fuse.....

More info required TBH.