A modern furnace runs a strict startup sequence, and knowing where the sequence breaks is most of the diagnosis. The thermostat calls, the inducer motor spins up to prove draft through a pressure switch, the hot surface igniter glows, the gas valve opens, the burners light, the flame sensor confirms fire, and only then does the blower deliver heat to the house. In Hagerstown, MD, the failures cluster at predictable points: a flame sensor coated in oxide that can no longer prove the flame, a hot surface igniter cracked from years of thermal cycling, a pressure switch that will not close because the inducer is weak or the condensate trap on a high-efficiency model has clogged, or a control board with a burned relay. Each one stops the sequence cold, and each one mimics the others from the hallway.
When the AC quits in summer, see AC Repair.
Warning Signs for Furnace Repair
The sooner you notice these red flags, the easier and cheaper the fix. Watch for these telltale signs:
- Soot, rust, or moisture near the furnace cabinet or vent pipe
- The igniter glows and clicks, yet no flame ever appears
- Frequent on-off cycling that never reaches the thermostat setting
- A fan that keeps running while the registers blow cool air
- Booming or banging at light-off from delayed ignition
- Squealing or grinding from the blower compartment
- Burners light, run a few seconds, then shut down repeatedly
- A flame that burns soft and yellow rather than steady blue
Don't Wait for a Total Failure
Left unchecked, even a small fault or temperature swing often signals a system working its way toward a breakdown. If you notice any of these signs, call (240) 896-8249 for a prompt inspection.
