Wiring terminology can appear daunting. You look at a circuit board or instruction manual and you see plenty of letters or numbers, but often no corresponding explanation of what they mean. Air conditioning thermostats are no exception; you can find plenty of letters on terminal connections, including RC and RH, but have no idea of what they mean, or what the difference is.
Combined air conditioning and heating units contain a thermostat that is powered by a transformer. This converts high voltage alternating current (AC) to low voltage direct current (DC). Some air conditioning units contain two separate transformers, one to control the cooling system and the other to control the heating system. Thermostats contain two sections, heating and cooling, determining when to turn on cool air and when to turn on warm air. All combined air conditioning and heating thermostats have RC and RH wiring.
Despite the many letters used to denote terminal connections and wiring most, if not all, are actually fairly logical. RC is no different and simply means red cooling. The wire is generally red, although there is no color standardization -- but more importantly, the "R" really means that the wire and terminal are hot; it's a live electrical wire. The RC wire connects to the RC terminal, which controls the cooling system.
Understanding what RC stands for means, you can probably take an educated guess as to the meaning of RH. The "R" still indicates it's a hot wire and terminal connection, and the "H" simply means heating. The RH wire connects to the RH terminal and controls the heating part of the thermostat.
RC and RH
If the air conditioning thermostat has one transformer to power both the heating and cooling system, then the wire from the transformer goes directly to the RC terminal and is called the RC wire. Effectively there isn't a second RH wire, although there is an RH terminal. However, the heating still needs to be controlled, so a wire known as a jumper is connected between the RC and the RH terminals so that power gets to the heating control part of the thermostat. This wire is also known as the RH wire.