The R wire (sometimes split into Rc and Rh) carries 24VAC power from your system’s transformer to the thermostat. Every other wire on the sub-base, W, Y, G, O/B, depends on R being connected correctly; get it wrong and nothing else works, no matter how the rest of the wiring looks.
For the terminal that completes this circuit, see what the C-wire does; for the full terminal map, see the complete thermostat wiring guide.
Single R vs. split Rc/Rh
Single R (most common): One transformer, usually in the furnace or air handler, powers the whole system. A factory-installed jumper between the Rc and Rh terminals inside the thermostat lets one incoming R wire feed both the heating and cooling call circuits.
Split Rc and Rh (two transformers): Some systems, most often a furnace paired with a separately added AC condenser, or a boiler paired with a mini-split, have two independent 24V transformers. In that case Rc carries power for cooling and Rh carries power for heating, and the factory jumper must be removed. Leaving it in place can backfeed one transformer from the other, which can trip a breaker or damage a transformer.
| Setup | Wires present | Jumper | Typical cause |
|---|---|---|---|
| Single transformer | R only (or Rc+Rh bridged) | Left in place | One furnace/air handler serving heat and cool |
| Two transformers | Separate Rc and Rh | Removed | Furnace + separately added AC, or boiler + mini-split |
This circuit, like every other thermostat wire, falls under NEC’s Class 2 power-limited classification (Article 725), the reason it can run without conduit, unlike household 120V wiring.
How to tell which setup you have
- Pull the thermostat off its base and look at the labeled terminals.
- If you see a single R terminal with wires landed, or an Rc/Rh pair connected by a small removable jumper, you have a single-transformer system. Leave the jumper alone.
- If you see both Rc and Rh with separate incoming wires and no jumper, you have two transformers. Do not add a jumper here.
- When in doubt, check both transformers with a multimeter (system powered): if both read the same voltage in phase, jumping is usually safe; if unsure, treat it as a two-transformer system and leave them separate.
What goes wrong when this is set up incorrectly
- Jumper removed on a single-transformer system: heat or cool stops working, because the R wire that should serve both circuits now only serves one.
- Jumper left in on a true two-transformer system: the two transformers get tied together. Depending on their phase relationship, this can trip a breaker, buzz audibly, or damage a transformer over time. This is a case where getting the wiring “close enough” is worse than getting it wrong obviously; a system that mostly works while slowly damaging a transformer is a worse outcome than one that visibly doesn’t work at all.
- Rc and Rh swapped: in most cases this doesn’t cause damage on its own, but it can cause a smart thermostat’s setup wizard to misidentify the system type. Confirm the labels match what’s actually connected before finishing setup.
Frequently Asked Questions
Q: What happens if I remove the Rc/Rh jumper on a single-transformer system?
A: Whichever circuit lost its jumpered power stops working, typically cooling, since Rh (heat) is more often the wire actually connected to the single incoming R. Put the jumper back if you don’t have two independent transformers.
Q: Can I just use R instead of Rc and Rh?
A: Only if your system actually has one transformer. If you have two independent transformers and land both onto a single R terminal, you risk backfeeding one transformer from the other.
Q: My smart thermostat only has an R terminal, not Rc/Rh. Is that a problem?
A: No. Many thermostats label the terminal simply “R” but function identically to Rc/Rh with an internal jumper; check the installation manual for your specific model to confirm how it handles single vs. dual transformer setups.
Q: Does the R wire carry line voltage?
A: No. It’s 24VAC, the same low-voltage Class 2 circuit as the rest of the thermostat wiring, not the 120/240V of household circuits.
Related: Complete thermostat wiring guide · What is the C-wire? · What is the O/B wire? · 5-wire thermostat wiring
