Wire letter guide
R Wire on a Thermostat: What It Is and Where It Goes
R (sometimes RH, RC, V, or 4 on older units) is the 24 VAC "hot" side of the transformer in your furnace or air handler. The thermostat switches this power out to W, Y and G to turn equipment on.
The wire is usually red, but color is not a rule. Always go by the letter it was connected to. If your old thermostat had a short jumper between Rh and Rc, that is still a single R wire. Don’t connect the jumper to a smart thermostat.
Common R wire problems
- Thermostat is completely dead. With power on at the furnace, measure between R and C: you should see roughly 24 to 28 V AC. No voltage usually means the furnace door safety switch is open (door not fully closed), the service switch is off, or the 3 or 5 A fuse on the control board has blown.
- Fuse blows the moment power comes back. R is touching C, a grounded metal box, or a mislabeled B (common) wire on an O/B terminal. Turn power off, separate the bare ends, and check every wire before replacing the fuse.
- Loose R connection. An intermittent R wire makes the thermostat reboot or drop Wi-Fi. Strip about 6 mm (¼ inch) of fresh copper and seat it fully in the connector.
R is the only wire that is live all the time, so it is the one most likely to blow a fuse if it slips. Connect it last and turn the power on only when every wire is in place.
Other labels you may see for R
Older and non-US thermostats sometimes print V, VR, 4 or 24V where modern ones say R. If there are two power terminals and each has its own wire from the wall, you are looking at a dual-transformer system: see Rh and Rc before connecting anything. On a new smart thermostat, a single R wire goes on Rc on ecobee, on R on the Honeywell T9 and Amazon Smart Thermostat, and on either Rh or Rc on Nest.