Two-stage variable speed furnace heating with two-stage thermostat

What happens, in order

1. Thermostat first-stage heat call closes; control self-checks, confirms limits closed and pressure switches open
2. Inducer starts on low speed
3. When the low pressure switch closes, a 15 second pre-purge runs on low speed
4. If the low fire pressure switch does not close, the inducer goes to high fire; after the 15 second pre-purge the high fire switch closes and the furnace starts on high fire, then drops to low fire after 10 to 20 seconds
5. 20 second ignitor warm-up follows, inducer stays on low
6. Gas valve opens on low fire and ignition occurs; the 30 second blower on-delay starts at the same moment
7. Blower starts on low heat speed; HUM contacts and the 120 V ACC terminal are energized
8. Second-stage call a 30 second recognition delay runs, then the inducer goes to high speed, the control checks the high fire pressure switch, the high fire gas valve opens and the blower moves to high heat speed
9. Second stage satisfied inducer drops to low, high fire valve closes, blower goes to low heat speed
10. First stage satisfied gas valve closes, 5 second low-speed post-purge, and the selected blower off delay starts
11. After post-purge the inducer, HUM and ACC terminals drop; the blower stops at the end of the off delay

Timings and thresholds

Codes you may see during this

E223 E224 E227 E250 E270 E271 E272 E273 E274 E275

Models

More Lennox furnace sequences

• Single-stage furnace ignition retries and watchguard · Ignition
• Two-stage furnace ignition retries and watchguard · Ignition
• Single-stage furnace gas heating cycle · Heating
• Two-stage furnace heating with single-stage thermostat · Heating
• Cooling call on single-stage furnace blower · Cooling
• Two-stage variable speed cooling call · Cooling
• Continuous fan with heat or cool calls · Standby/shutdown