Modulating 95% furnace: adaptive heating, staging and blower delays

What happens, in order

1. Starts in intermediate or maximum heat; after 45 s at intermediate heat it moves to minimum heat or to the calculated modulating rate
2. First thermostat cycle after power-up runs at minimum heat; later cycles use a rate the control calculates from past heating run times
3. If the calculated rate is 40 to 99 percent: 45 s at intermediate heat, then the calculated rate for up to 19 minutes, then maximum heat while heat is still called; if 100 percent it runs maximum heat only
4. After a power interruption the stored rate is lost: 45 s intermediate heat, 19 minutes minimum heat, then maximum heat
5. Switches SW1-2 (minimum heat only) and SW4-2 (intermediate heat only) force two-stage operation; if both are on it defaults to intermediate
6. W2 always forces maximum heat; with a two-stage thermostat, opening W2 steps the inducer down gradually and the blower airflow changes after a 5 s delay once inducer speed is within 15 percent of the target
7. Blower-on delay 45 s after gas opens at intermediate heat, 25 s at maximum heat; humidifier and air cleaner terminals stay on through the cycle
8. Thermostat satisfied gas valve closes, inducer post-purges 15 s, blower runs the blower-off delay at minimum-heat airflow
9. With a communicating control the furnace modulates over its full range (or within set min and max CFM), with the same start-up and shutdown steps

Timings and thresholds

Models

More KeepRite furnace sequences

• Modulating 95% furnace: ignition sequence and retries · Ignition
• Single- and two-stage 95% furnace: ignition sequence and retries · Ignition
• Single-stage 95% furnace: heating cycle and blower delays · Heating
• Two-stage 95% furnace: low/high heat, staging and blower delays · Heating
• Furnace blower with outdoor A/C: cooling, dehumidify and continuous fan · Cooling
• Furnace used with a heat pump (dual fuel blower timing) · Other