Wire letter guide
Rh and Rc Thermostat Wires Explained (Dual Transformer)
Rh is power from the heating transformer and Rc is power from the cooling transformer. If both terminals have their own wire from the wall, your home has two separate 24 V circuits, often a boiler plus a separate AC air handler.
Never connect Rh and Rc together on a system like this; tying two transformers together can damage the equipment. If there is only a short jumper between Rh and Rc and one wire from the wall, you have a single transformer: treat it as R.
How to check whether you really have two transformers
- Look at the old thermostat: two separate wires from the wall (one on Rh, one on Rc) usually means two transformers. One wire plus a short metal or wire jumper means one transformer.
- At the equipment, follow each wire. If the Rh wire goes to a boiler or zone control and the Rc wire goes to the air handler or AC, they are separate systems.
- With power on, a meter between the Rh and Rc wires (disconnected from the thermostat) can read anywhere from 0 to about 48 V AC when they come from different transformers. Any reading there is a sign to keep them apart.
Nest, ecobee and Honeywell T9 all have separate Rh and Rc connectors. On most of them, inserting wires into both tells the thermostat to keep the circuits isolated; follow the brand’s installation guide for any jumper tab or setting.