Two-stage 95% furnace: low/high heat, staging and blower delays

What happens, in order

1. Single-stage thermostat (TT switch off) runs low heat for the first 12 minutes of a cycle then moves to high heat
2. Two-stage thermostat (SW1-1 TT switch ON): R-W1 gives low heat, R-W1 plus W2 gives high heat; W2 always forces high heat
3. Low heat blower comes on at low-heat speed 45 s after the gas valve opens
4. High heat blower comes on at high-heat speed 25 s after the gas valve opens; high-heat pressure switch must close and the high gas valve opens
5. If the high-heat pressure switch does not close, the furnace stays at low heat for up to 2 minutes after ignition; if it still has not closed the gas valve shuts, the heat-off delay runs, and a new cycle starts if heat is still called
6. Low to high inducer goes to high speed, the high pressure switch closes, the high gas valve opens and the blower moves to high-heat speed 5 s later
7. High to low (two-stage thermostat only) high gas valve closes, inducer drops to low, blower moves to low-heat speed 5 s after W2 opens; with a single-stage thermostat it never steps down during the cycle
8. Thermostat satisfied gas valve closes, inducer post-purges, then blower runs the heat-off delay

Timings and thresholds

Models

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