SAFETY: Turn off power at the breaker before touching any thermostat wiring. Confirm the thermostat display is dark before pulling it off the wall.
Emerson thermostats, including the White-Rodgers-badged 1F78 family and the newer Wi-Fi Sensi line, use the same universal terminal letters as every other 24V thermostat: R (power), C (common), W (heat), Y (cooling), G (fan), O/B (heat-pump reversing valve). There’s no separate “Emerson color code.” What’s actually Emerson-specific is the 1F78’s heat-pump jumper trick, its Rc/Rh auto-jumper, and, on the Sensi side, a C-wire rule that’s stricter than most competitors. This page covers all three, plus the terminal-by-terminal reference. For the universal color chart, see thermostat wiring color code; for the full terminal system, the thermostat wiring guide.
TL;DR
| Your situation | What to do |
|---|---|
| 3 wires (R, W, G), heat only | Match letters, no jumper needed unless old stat had one |
| 3 wires (Rc, Y, G), cool only | Match letters |
| 4 wires, heat + cool, no C | Match letters; C wire optional on basic models, required on all Sensi models |
| 5 wires, dual transformer (Rc + Rh) | Land both; do not jumper them if they arrive as two separate wires |
| Heat pump on a 1F78 | No W wire; jumper Y to W instead. O/B switch defaults to O |
| Installing a Sensi (any model) | C wire required, full stop. No power-stealing option |
Old thermostat to Emerson: terminal and color reference
Before wiring, translate what the old thermostat used to what the new Emerson expects. This mapping is specific to the White-Rodgers/Emerson terminal-letter conventions documented for the 1F78 family:
| Wire color (typical) | Terminal | What it does |
|---|---|---|
| Red | R (or Rc / Rh) | 24 V AC power from the transformer. Rc = cooling power, Rh = heating power; one system uses a jumper between them. |
| White | W (W1, W2) | Heat call. W2 is second-stage or auxiliary heat. |
| Yellow | Y (Y1, Y2) | Cooling call — energizes the compressor contactor. On heat pumps, Y runs the compressor for heating too. |
| Green | G | Indoor fan (blower) relay. |
| Blue | C | Common — completes the 24 V circuit and gives smart thermostats constant power. Sometimes black. |
| Orange | O (or O/B) | Heat-pump reversing valve, energized in cooling (most brands). |
| Dark blue | B | Heat-pump reversing valve, energized in heating (Rheem/Ruud). Occasionally used as C — verify. |
| Brown | E | Emergency heat — manual backup that locks out the compressor. |
| Varies | AUX | Auxiliary heat — automatic electric backup on heat pumps. Often shares a wire with W2. |
Two terminal-letter quirks worth knowing before you touch a wire:
- B wire, old thermostat: if the old thermostat had a wire in its O terminal, label the B wire C on the new one. If the old thermostat had nothing in its O terminal, label B as O/B instead. This is the one Emerson-documented case where a terminal’s old function determines what a same-lettered wire becomes on the new unit. Worth double-checking rather than assuming.
- Y2 (light blue): on 1F78-family charts this is documented as a reversing-valve wire on some heat-pump variants, not a second-stage-cooling wire as Y2 usually means elsewhere. If you have a light blue Y2 wire, confirm at the old thermostat which job it was doing before assuming “second stage compressor.”
Wiring by system, 1F78 model family
The 1F78 (White-Rodgers/Emerson’s long-running non-programmable and programmable single-stage line) covers four base configurations:
Heat only (3 wire): R, W, G
Furnace or boiler with no air conditioner. R is power, W is the heat call, G runs the fan.
Cool only (3 wire): Rc, Y, G
First-stage air conditioner with no furnace heat call. Rc is cooling power, Y starts the compressor, G runs the fan.
Heat and cool (4 wire): R, W, Y, G
Standard furnace-plus-AC setup. If only one R wire comes out of the wall, jumper Rc to Rh (see below); if you have separate Rc and Rh wires, leave them unjumpered.
Dual transformer (5 wire): Rc, Rh, W, Y, G
Separate heating and cooling transformers, each with its own R-family wire. Land Rc and Rh on their own terminals; never bridge them when both are live, that risks shorting two out-of-phase 24V feeds.
Heat pump wiring on the 1F78: the Y-W jumper
Heat pump wiring on the 1F78 works differently from a standard heat/cool system in one specific way: there’s no separate wire for the W terminal. Instead, a jumper connects the Y and W terminals on the thermostat itself, and the compressor call (Y) does double duty for both heating and cooling. Direction is set by the reversing valve on O/B, not by a separate heat call. In summary, a 1F78 heat-pump install lands:
- Y, jumpered to W at the thermostat
- O/B, the reversing valve wire
- Rc and Rh, jumpered together if only one R wire arrives from the wall (the 1F78 connects Rc and Rh internally by default; if your install has two separate R wires, clip that internal jumper to isolate them)
- G, fan relay
- C, common, if present
The O/B switch on the 1F78 ships factory-set to O (reversing valve energized in cooling), which covers most heat-pump equipment. If your system is one of the minority that energizes the valve in heating instead (Rheem/Ruud-style B logic), flip the switch. Don’t rewire the terminal.
One more heat-pump-specific control on the 1F78: the GAS/ELECTRIC switch. On electric-heat or heat-pump systems where you want the fan to start immediately on a call for heat (rather than waiting, as a gas furnace’s fan control would), set this switch to ELECTRIC. Leave it on GAS for a fossil-fuel furnace. For the O/B convention in more general terms (which brands use O vs. B, and why) see heat pump thermostat wiring.
Sensi: same letters, stricter C-wire rule
Emerson’s Wi-Fi line, Sensi, follows the same universal terminal letters (R, C, W, Y, G, O/B) as the mechanical 1F78. The Sensi-specific wrinkle is the C wire: every current Sensi model requires one, for every system type. Copeland’s own Sensi support documentation is explicit: the base Sensi and Sensi Lite need a C wire for heat-only, cool-only, and heat-pump systems, and Sensi Touch / Sensi Touch 2 require a C wire across the board. That’s a stricter rule than some competing smart thermostats, which can power-steal on a basic 4-wire heat/cool system without one.
If you’re installing a Sensi and don’t have a C wire, Sensi’s documented fallback order is:
- Look for an unused wire already in the wall (a spare conductor from a 5- or 6-conductor cable that was never landed).
- Use the G (fan) wire as a common wire, only if you’re not relying on independent fan control, since this repurposes the fan terminal.
- Install a Common Wire Maker Kit, the fallback when neither of the above applies.
For the fuller decision tree on no-C-wire installs generally (not Sensi-specific), see how to install a thermostat without a C wire.
Who makes Emerson, Sensi, and White-Rodgers now
The brand names on the box have shifted in the last few years, and it matters for finding the right manual. Sensi smart thermostats and White-Rodgers controls were both formerly sold under the Emerson brand. In May 2023, Blackstone completed its acquisition of a 60% majority stake in Emerson’s Climate Technologies business (Emerson retained the other 40%), and the newly standalone company took the name Copeland. Sensi and White-Rodgers kept their product names but now sit under Copeland’s site and support structure, so if a link to emerson.com support for your Sensi or White-Rodgers thermostat is dead, check sensi.copeland.com or copeland.com instead; that’s not a broken product, just a moved one.
How to replace an Emerson thermostat: 6 steps
- Turn off power at the circuit breaker. Confirm the display is dark.
- Photograph the old wiring, then label each wire by terminal letter, not color.
- Note any jumpers on the old thermostat (commonly Rc to Rh) and what they connected.
- Mount the new base, using the built-in level if your model has one.
- Land each wire on its matching letter. On a heat pump, jumper Y to W and check the O/B switch position. On a Sensi, confirm you have a C wire before starting, or line up your fallback.
- Restore power and test every mode: heat, cool, fan, and, on a heat pump, that cooling doesn’t produce heat or vice versa.
FAQ
Are Emerson thermostat wires color coded?
By convention only, the same as every other brand: red to R, white to W, yellow to Y, green to G, blue or black to C. Always confirm against the terminal letter on the old thermostat, not the color.
How do I wire an Emerson thermostat for a heat pump?
There’s no separate W wire on a 1F78 heat-pump install. Jumper Y to W at the thermostat, land the reversing valve on O/B (factory-set to O), and set the GAS/ELECTRIC switch to ELECTRIC if you want the fan to start immediately on a heat call.
Does a Sensi thermostat need a C wire?
Yes, every current Sensi model, including the base Sensi and Sensi Lite, requires a C wire for heat-only, cool-only, and heat-pump systems. If you don’t have one, Sensi’s documented fallbacks are an unused spare wire, the G wire (with a fan-control tradeoff), or a Common Wire Maker Kit.
Is Emerson the same company as Sensi and White-Rodgers?
Not anymore. Sensi and White-Rodgers were formerly sold under the Emerson brand; since Blackstone’s 2023 majority-stake acquisition of Emerson’s Climate Technologies business, they’re part of the standalone company Copeland. The product names didn’t change, but support and manuals have moved to Copeland’s sites.
What does it mean if my old thermostat’s B wire needs to become C or O/B?
On 1F78-documented conversions: if the old thermostat had a wire in its O terminal, label the B wire as C on the new one. If the old O terminal was empty, label B as O/B instead. Check the old unit before assuming either way.
⚠️ Know when to stop
Thermostat wiring is 24 V AC low voltage — safe to handle once you cut power at the breaker (do it anyway; a short can kill the transformer or blow the board fuse). But stop and call an HVAC pro or electrician if you hit any of these:
- Line voltage (120/240 V): thick wires with wire nuts, often on baseboard or wall-heater thermostats. Miswiring these can injure you or start a fire.
- Gas components: anything past the furnace control board — gas valves, ignitors, pilot assemblies.
- Refrigerant lines or sealed-system parts on heat pumps and AC units.
- No idea which system you have: if the wiring doesn’t match any diagram here, photograph it and get a pro’s eyes on it before guessing.
Last reviewed: July 2026
