I currently have a very old thermostat, White-Rodgers 135-415, which I'd like to convert to a Honeywell RTH230B.

Currently there are only 3 wires that are connected to the thermostat (which is only for my apartment's heat.

White wire is connected to what looks to be labeled "Common"
Black wire is connected to what looks to be labeled "Heating"
Red wire is connected to what looks to be labeled "C"

Does anyone know to which slot each wire should be connected, when installing to the new Honeywell RTH230B?

Any help would be greatly appreciated.

Thank you

I believe the black goes to the W terminal, the red goes on the Rh, and this stat does not need a common, so should be able to cap up the white wire.

I have the diagram for the Honeywell stat, and cannot find the diagram for the WR unit.

Well, I finally got a response from White-Rodgers... apparently my old unit (which my apartment complex still uses) became obsolete in 1967.

"After great research", the White-Rodgers help center "found a suggestible definition of the wiring system".

In case anyone happens to find themselves in the same boat...

White "common" wire is the power input (which goes to Rh or Rc on RTH model).
Black "heating" wire is wire that the thermostat throws power to, when it wants to turn on the heat. (which goes to W on RTH model)
Red blank wire is the wire that the thermostat throws power to, when it wants to stop the heat (not necessarily turn on the cooling). (which goes to Y on RTH model)

... so far it seems to be working the way it should :-D

Fantastic.