Abnormal conditions and protection

What is going on

Summary of how the furnace responds to abnormal conditions.

Likely causes

• Excessive combustion pressure from wind over 40 mph or vent/flue blockage
• Loss of flame during heating
• Power failure during heating
• Gas supply failure
• Induced draft blower failure
• Condensate drain blockage

Check in this order

1. Excessive pressure pressure switch opens and shuts off gas until pressure is acceptable.
2. Loss of flame control closes gas valve, retries ignition, then locks out if ignition fails.
3. Power failure ignition restarts automatically when power returns if thermostat still calls for heat.
4. Draft blower failure unit will not start if pressure is not sensed.
5. Condensate blockage pressure switch stays open until the drain is cleared and condensate flows freely.
6. Lockout reset manually as described in the reset tip.

More furnace guides

• Adjust gas valve manifold pressure
• Air for combustion and ventilation in non-direct vent installs
• Airflow adjustment check
• Airflow and temperature rise check
• Bottom return filter rack installation (upflow)
• Carbon monoxide detectors and signage for Massachusetts side-wall venting