“iComfort” isn’t one wiring scheme. It’s a product line with two completely different electrical designs under the same name, and getting the model wrong means either wiring it incorrectly or buying a thermostat that can’t talk to your equipment at all.
The short version: the S30 and S40 are communicating thermostats that use a 4-wire proprietary bus (R, C, i+, i-) to Lennox-compatible equipment, not standard R/Y/G/W terminals. The E30 and M30 are conventional 24V thermostats that wire exactly like a Honeywell or Nest (R, C, G, W, Y, O/B). If you have the older, now-discontinued 10F81 (“iComfort Wi-Fi”), it’s also communicating, and its cataloged replacement is the S40.
Which iComfort do you have?
Check the model name printed on the thermostat face or in its menu (Settings > About). If you can only see a part number stamped on the back or a sticker, match it here:
| What’s on the unit | Model | Wiring type |
|---|---|---|
| “S30” or “iComfort S30” | S30 (catalog 19V30) | Communicating |
| “S40” or “iComfort S40” | S40 (catalog 22V24) | Communicating |
| “E30” or “iComfort E30” | E30 | Conventional |
| “M30” or “iComfort M30” | M30 (catalog 15Z69) | Conventional |
| “10F81” or “iComfort Wi-Fi” (no letter/number suffix, older black-and-silver touchscreen) | 10F81 (discontinued) | Communicating |
If you’re not sure and the thermostat has a small round antenna sticking out of a separate box behind the wall plate (the “smart hub”), that’s the S30/S40 architecture. A single flat unit with no separate hub is E30 or M30.
S30 / S40: communicating wiring (4-wire RS-Bus)
The S30 and S40 don’t use R/Y/G/W terminals at all. They connect to a compatible Lennox indoor unit (S- or E-series furnace, air handler) over four wires carrying a proprietary digital bus (Lennox calls it the RS-Bus), not simple on/off signals.
Smart hub terminal designations (per Lennox’s own S30 installation guide):
| Terminal | Function | Wire spec |
|---|---|---|
| R | 24VAC input | 18 AWG unshielded |
| C | 24VAC return | 18 AWG unshielded |
| i+ | RS-Bus i+ | 18–22 AWG shielded (recommended) |
| i- | RS-Bus i- | 18–22 AWG shielded (recommended) |
| ACC1 / ACC2 | Accessory (ventilator, fresh-air damper) | 18 AWG unshielded |
| + / – | 12VDC to HD Display | 18 AWG unshielded |
| A / B | Communications bus to HD Display | 18–22 AWG shielded (recommended) |
Four wires run between the smart hub and the indoor unit (R, C, i+, i-), and four more run between the smart hub and the HD touchscreen display (+, -, A, B). Total run length across the whole system is capped at 1,500 feet, with no more than 300 feet between any two components.
What this means for compatibility: the S30/S40 is built to run a genuine two-way conversation with matched Lennox equipment, commanding exact blower speed and compressor stage, and reading back fault codes. It is not a drop-in replacement for a generic thermostat, and a generic thermostat isn’t a drop-in replacement for it either, without giving up that communication (see Replacing a Communicating Thermostat for what you lose making that swap).
E30 / M30: conventional 24V wiring
This is the part that trips people up: despite sharing the iComfort name and touchscreen look, the E30 and M30 are ordinary 24V thermostats. Lennox’s own E30 manual states it outright: “The iComfort E30 smart thermostat CAN NOT be connected as a communicating device to indoor or outdoor units. Only conventional 24VAC wiring to indoor and outdoor units is supported.”
E30 conventional terminals:
| Terminal | Function |
|---|---|
| R | 24VAC power |
| C | 24VAC common |
| W1 | 1st-stage heat |
| W2 | 2nd-stage heat |
| W3 | 3rd-stage heat (air handler electric heat) |
| G | Fan / blower |
| Y1 | 1st-stage cooling |
| Y2 | 2nd-stage cooling |
| O / B | Heat pump reversing valve (O = energized in cooling, B = energized in heating) |
| H | Humidifier signal |
| DS | Dehumidification signal |
M30 conventional terminals (functionally the same core set, described as a universal thermostat for “residential units and approved commercial split-system matches,” not restricted to Lennox equipment):
| Terminal | Function |
|---|---|
| R | 24VAC power |
| C | Common 24VAC |
| G | Fan relay |
| W1 | 1st-stage heat (or emergency heat) |
| W2 | 2nd-stage heat (or 4th stage, heat pump) |
| Y1 | 1st-stage cooling |
| Y2 | 2nd-stage cooling |
| O/B | Heat pump reversing valve |
| D/H | Dehumidifier or humidifier |
| Tc / To | Optional outdoor temperature sensor |
| ACC1 / ACC2 / R2 | Accessory (ventilator) power |
Both require a C-wire to operate; there’s no battery-only mode. If your old thermostat only had R/W/Y/G with no C, see thermostat C-wire options before you buy.
The 10F81, and what actually replaces it
The 10F81 was the original “iComfort Wi-Fi” thermostat (also sold under catalog numbers 103445-01/-03/-04), and it uses the same communicating RS-Bus architecture as the S30: four wires (R, C, i+, i-) to a compatible Lennox indoor unit, not conventional R/Y/G/W terminals.
Lennox discontinued the 10F81 on November 4, 2019. Its immediate successor part number, 12U67, has since also been discontinued. The current cataloged replacement is the iComfort S40 (catalog 22V24), a communicating thermostat that will use the same wiring architecture (R, C, i+, i-) as your old 10F81, just with newer hardware and firmware. If a technician or supply house quotes you a straight S40 swap, that’s consistent with Lennox’s own parts chain, not an upsell.
Replacement and install steps
- Turn off power at the breaker or furnace switch, and confirm the display is dead before touching any wires. Communicating systems still carry 24VAC on R and C even with the thermostat removed until the equipment itself is de-powered.
- Photograph the existing wiring before disconnecting anything: both the thermostat side and, on communicating systems, the terminal block at the indoor unit. Go by terminal letter, not wire color; a 4-wire communicating run typically doesn’t use the R/W/Y/G color convention you’d expect from a conventional system.
- Confirm your indoor unit’s compatibility before buying.
– Replacing an S30 or S40 with another S30/S40: straightforward, same 4-wire bus. – Replacing a 10F81: the S40 (22V24) is the direct successor and uses the same terminals. – Installing an E30 or M30 on a conventional (non-communicating) furnace or air handler: works as long as your equipment has standard R/W/Y/G terminals. This is what these two models are designed for. – Installing a communicating S30/S40 on equipment that isn’t Lennox-communicating: not possible without a Lennox Equipment Interface Module (EIM, catalog 10T50 or the newer 22X18), which lets a conventional furnace or air handler emulate a communicating indoor unit. Without the EIM, the S30/S40 has no conventional R/Y/G/W terminals to fall back to.
- Set the unit-type and heat-stage jumpers on E30/M30 installs. Both use physical jumpers (IFC for furnace, AHC for air handler, and a heat-stage position) inside the smart hub. Get these wrong and you’ll trip an alert code rather than get heat.
- Land the wires on the new subbase per the terminal tables above, seal the wall opening, then restore power and let the thermostat complete its boot/commissioning sequence before testing a heat and cool call.
Safety box: Turn off power at the breaker or the furnace’s own service switch, and confirm the old thermostat’s display is fully dead, before disconnecting any wire, including on 24V communicating systems. Re-terminating low-voltage wiring at the wall is a reasonable DIY job. Re-terminating wires at the indoor unit’s control board, adding an Equipment Interface Module, or anything on the equipment’s 120/240V side is past that line. Call a licensed HVAC tech.
FAQ
Are all Lennox iComfort thermostats communicating? No. The S30 and S40 are communicating (4-wire RS-Bus: R, C, i+, i-). The E30 and M30 are conventional 24V thermostats (R, C, G, W, Y, O/B) despite the shared iComfort branding.
What is Lennox 10F81? It’s the model/catalog number for the original “iComfort Wi-Fi” thermostat, a communicating model discontinued November 4, 2019. Its direct replacement chain runs 10F81 → 12U67 (also discontinued) → iComfort S40 (catalog 22V24), which is the current model sold as its replacement.
Can I put an iComfort S30 or S40 on a non-Lennox furnace? Not directly — it has no conventional terminals. You’d need a Lennox Equipment Interface Module (EIM) to translate a conventional furnace or air handler into something the S30/S40’s communicating bus can talk to. If you want a Lennox touchscreen thermostat on non-Lennox conventional equipment, the E30 or M30 is the correct product, not the S30/S40.
Do I need a C-wire for an iComfort thermostat? Yes, on every current model — S30, S40, E30, and M30 all require the C terminal for continuous power.
My old iComfort has a white or blank screen. Is that a wiring problem? Not usually. A blank/white screen on a 10F81 or S30 is a documented hardware display fault, not a wiring fault. Confirm the smart hub’s status LED is lit before assuming the wiring is bad.
Where do I get the exact wiring diagram for my model? Lennox publishes installation and setup guides for each model directly: search “[your model] iComfort installation manual” on lennox.com/literature, or ask your installer for the doc number stamped on the unit (S30 is 507536-xx, E30 is 507686-xx, M30 is 507739-xx). For direct links to the current manual for every iComfort model, including the homeowner/user guides, see Lennox iComfort Thermostat Manual.
