Hello,

I just moved into a new house and I am looking to upgrade to a programmable thermostat. My old thermostat is mercury based and has only two wires; the problem is that neither wire is labelled and they are both exactly the same colour. How do I tell which is W and which is Rh?

Ben

Follow up - the two copper wires themselves seem to be quite thick. It is not a high voltage thermostat (max 30V), just very old. Probably original to the house, which is about 50 years old.

Can I still install the programmable digital thermostat I bought?