Thermostat Rc Rh Jumper: Which Wiring Situation You Have and What to Do

SAFETY: Turn off power at the breaker or furnace disconnect switch before touching any thermostat wiring. Confirm the display is dark before removing the thermostat from the wall.

If heat stopped working the moment you installed a new thermostat, or if you just blew a fuse on the furnace board, the Rc/Rh jumper situation is the first thing to check. This is the most common wiring mistake on thermostat swaps and it plays out in two opposite directions: some homeowners fail to jumper when they should, others jumper when they shouldn’t.

The fix depends entirely on which of three wiring situations you have. Read the decision tree below before touching anything. For every other terminal and wire count, see the full thermostat wiring guide.

[FIGURE: rc-rh-dual-transformer-wiring.svg | alt: Rc and Rh dual-transformer thermostat wiring diagram showing separate heating and cooling transformer feeds landing on their own terminals]

TL;DR

What Rc and Rh actually are

Rc is the 24V AC power feed from the cooling transformer. Rh is the 24V AC power feed from the heating transformer. On a thermostat that has both terminals, R stands for “relay power”: it’s what closes the heating or cooling circuit when a call for heat or cool is made.

The reason two terminals exist: some older homes have a separate 24V transformer in the furnace and another in the AC condensing unit. Keeping them separate on the thermostat board prevents two live power sources from being shorted together.

On most modern systems, a single 24V transformer in the air handler or furnace feeds both heating and cooling through a single R wire. In that case Rc and Rh are the same electrical potential, and the two terminals must be connected (either by a jumper wire or by the thermostat’s internal switch).

The three situations

Situation 1: Single transformer (most homes built after ~1985)

Your old thermostat had one R wire, possibly with a short factory-installed jumper clipped from R to Rc or from R to Rh. The furnace or air handler contains one 24V transformer that powers both heating and cooling calls.

What to do: Rc and Rh must be connected. Most modern smart thermostats handle this automatically.

• Nest Learning Thermostat (4th gen): uses an internal electronic jumper that activates automatically. Remove any physical jumper from the old thermostat before installing. Do not carry the jumper over. Insert your single R wire into either the Rc or Rh terminal; the Nest uses whichever you connected plus its internal jumper.
• ecobee Premium: same behavior. The ecobee has an internal automatic Rc-to-Rh connection for single-transformer systems. Do not add a physical jumper wire to the ecobee terminals. If your old thermostat had a wire jumpered between Rc and Rh, leave it behind. Only label and move the wires that run back into the wall.
• Honeywell T10/T10+ Pro: uses a physical R Slider Tab on the back of the unit. Set the slider to the UP (1 wire) position. This physically joins Rc and R internally and hides the Rc terminal from the wiring diagram. Put your R wire into the R terminal.

The failure mode if you skip this: the thermostat powers up (the display runs off Rc) but the heating circuit never closes because Rh is floating — no path to the heating transformer. You get cooling but not heat, or neither.

Situation 2: Dual transformer (older homes, or systems with separate furnace and AC units)

Your old thermostat had two separate R wires coming out of the wall — one from the furnace transformer (Rh) and one from the AC condenser transformer (Rc). You may see two wires both labeled R, or one labeled Rh and one Rc, or one red and one red-with-stripe.

What to do: Land each wire on its own terminal. Do NOT jumper them together.

Why it matters: the two transformers are running on 120V AC primary feeds, and unless they are perfectly in phase, shorting their secondaries together creates a voltage differential between Rc and Rh. Measure voltage between the two terminals with a multimeter before installing the thermostat. If you read more than ~12V AC between Rc and Rh, the transformers are out of phase. Jumping them will not add up to a clean 0V short. Instead you get up to 48V AC across the short, which trips the fuse on the furnace control board or damages the transformer windings.

Even if the transformers happen to be in phase (reading close to 0V between Rc and Rh), their commons must stay separate. Connecting the commons together on a two-transformer system can cause ground loops and intermittent faults.

• Nest (4th gen): Insert the Rh wire into the Rh connector, Rc wire into the Rc connector. The internal jumper stays inactive when both connectors are populated. Do not physically join the wires.
• ecobee Premium: Insert each wire into its labeled terminal. The ecobee detects that both are present and does not activate the internal jumper.
• Honeywell T10/T10+ Pro: Set the R Slider Tab to the DOWN (2 wires) position before inserting any wires. This separates the Rc and R paths internally. Then land the Rh wire on R and the Rc wire on Rc.

The failure mode if you jumper them: a blown 3A fuse on the furnace control board, or a damaged transformer. The AC may stop working entirely. You will need to replace the fuse (see label on control board, typically 3A slow-blow) and correct the wiring before power-cycling the system.

Situation 3: Zoning panel

If your home has a multi-zone control panel (Honeywell TrueZONE, Ecojay SmartZone, or similar), the panel acts as the intermediary between the HVAC equipment and each zone thermostat. The panel typically has its own 24V AC transformer or reuses the equipment transformer and provides a switched R output to each thermostat zone terminal block.

In this situation:

• The thermostat may see only one wire at R (no separate Rh at all), with the panel handling zone isolation.
• Some panels provide 24V AC to both Rc and C at the thermostat, with Rh absent entirely.
• Follow the panel’s wiring diagram, not the thermostat’s generic guide. The panel manufacturer has tested the interaction; the thermostat manufacturer has not.

This is where to call a pro if you’re unsure. Zoning wiring is not standardized, and the wrong connection can put two transformers in parallel or short the panel’s output. If the panel’s wiring diagram contradicts the thermostat’s wiring guide, the panel wins.

A common mistake: treating a zone panel like a single-transformer system and jumpering Rc to Rh at the thermostat. Depending on how the panel provides power, this can short the panel’s output across a floating terminal, triggering an over-current fault in the panel.

How to identify your situation in 60 seconds

1. Turn off power at the breaker. Confirm the thermostat display is dark.
2. Remove the thermostat faceplate and photograph the wiring.
3. Count the wires connected to terminals labeled R, Rc, or Rh on the old thermostat.

– One wire, possibly a jumper clip between two R terminals: single transformer (Situation 1). – Two wires, each going into separate Rc and Rh terminals: dual transformer (Situation 2). – Wire(s) labeled by a zone panel terminal block or with labels like “ZP1”, “Z1/R”: zoning panel (Situation 3).

1. If still unsure: trace the R wire(s) back through the wall to the equipment. A single transformer at the air handler feeding both furnace and AC = Situation 1. Separate transformers at furnace and condenser unit = Situation 2.

Wiring traps by system type

Dual-fuel heat pump: these systems typically use a single transformer in the air handler. Rc and Rh must be jumpered (Situation 1 rules apply). The separate backup-heat wire goes to W2 or Aux, not to a second R terminal.

Heat pump with no emergency heat strip: single transformer, single R wire. Same as Situation 1. The O/B reversing-valve wire is often confused with a second R wire because it’s also red on some older cable runs. Check the terminal label on the old thermostat, not the wire color.

Old whole-house humidifier or EAC wired at the thermostat: some older installs tap a third transformer via an accessory terminal. This doesn’t affect Rc/Rh but can cause a ground loop if commons are shared incorrectly. Document what’s connected before removing the old thermostat.

Line voltage (120V/240V) wiring is completely separate from the 24V control side. If you see heavy-gauge black/white wiring going into a thermostat, that is a line-voltage thermostat for electric baseboard or radiant heat. Rc and Rh as described in this article do not apply. Do not work on line-voltage systems without the breaker confirmed off with a non-contact voltage tester. If you are unsure, call a licensed HVAC tech.

Verdict by situation

Single transformer (Situation 1): Nest, ecobee, and Honeywell all handle this automatically or with a simple slider — no physical jumper wire needed. If heat stopped working after your install, check that you moved only the real wires and didn’t accidentally seat two wires into one terminal.

Dual transformer (Situation 2): The Honeywell T10/T10+ Pro’s physical R Slider Tab makes this the most explicit to configure — set slider DOWN, done. Nest and ecobee handle it electrically when both terminals are populated, but there’s no visible confirmation. Before energizing, measure voltage between Rc and Rh; more than ~12V AC means the transformers are out of phase and you need an electrician, not a thermostat swap.

Zoning panel (Situation 3): Always defer to the panel’s wiring diagram. When in doubt, call the panel manufacturer’s tech line or hire an HVAC tech who has worked with that panel before.

Frequently asked questions

My old thermostat had a jumper between Rc and Rh. Do I move it to the new thermostat? No. Leave the jumper on the old thermostat. Nest and ecobee handle the connection internally and will not accept two wires in one terminal. On the Honeywell T10/T10+ Pro, set the slider to UP (single transformer) instead of using a physical jumper. The only exception: if the old thermostat used the jumper to bridge a zoning-panel situation, follow the panel’s diagram rather than carrying the jumper over.

How do I know if I have one transformer or two? Photograph the old thermostat wiring. If only one wire connects to any R/Rc/Rh terminal (plus a short jumper clip on the old thermostat board itself), you have one transformer. If two separate wires from the wall go into Rc and Rh, you likely have two transformers. Confirm by tracing the wires to the equipment.

I connected Rc and Rh on a dual-transformer system and now the furnace fuse is blown. What next? Turn off power. Remove the jumper or reconnect each wire to its own terminal. Replace the blown 3A fuse on the furnace control board (the fuse is labeled on the board; bring a spare or check the equipment manual for the rating). Restore power and test. If the fuse blows again, there’s a short elsewhere in the low-voltage wiring that needs a tech.

Does wiring Rc and Rh correctly matter for a heat pump? Yes. Heat pumps almost always use a single transformer. Rc and Rh need to be connected (Situation 1). The O/B (reversing valve) wire is what distinguishes heat-pump wiring, not a second R wire. Set the thermostat to heat-pump mode during setup so it knows to use O/B for reversing rather than treating it as a second stage of heat.

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Last reviewed: July 2026

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