Heater not working?

What this usually is

• The burners are not lighting, so the blower moves air the furnace never heated. On most modern furnaces that points to the igniter or the flame sensor.
• A safety control has shut the burners off on purpose, often because the furnace is overheating or its exhaust is not venting properly.
• The thermostat fan is set to ON rather than AUTO, so air keeps moving between heating cycles and feels cool.

When to stop and callIf the thermostat is on HEAT and AUTO and the air is still cold, stop there. What is left is ignition, gas, or safety-control work, and none of it is safe to open up yourself.

What this usually is

• A tripped breaker, or the furnace service switch near the unit has been turned off. It often looks like an ordinary light switch.
• The thermostat has lost power or its batteries, or it is still set to COOL from the last warm day.
• On a high-efficiency furnace, a condensate safety switch has stopped the system because its drain is blocked.

When to stop and callIf the breaker trips again after one reset, stop. A breaker that keeps tripping is reporting a fault, not causing one.

What this usually is

• A dirty flame sensor cannot confirm the flame, so the control board shuts the gas off seconds after the burners light.
• A clogged filter or closed vents restrict airflow until the high-limit switch cuts the burners to protect the heat exchanger.
• The furnace is oversized for the house and satisfies the thermostat before the rooms have warmed evenly.

When to stop and callIf it still cycles with a clean filter and open vents, stop. Repeated overheating stresses the heat exchanger, the part that keeps combustion gases out of the air you breathe.

What this usually is

• Dust that settled on the burners and heat exchanger during the warm months is burning off. It is common on the first cold days and fades with use.
• The blower motor is running hot, or wiring insulation is overheating. That smell is sharper, closer to hot plastic than to dust.
• The filter is so loaded that the blower strains to push air through it.

When to stop and callA dusty smell that fades is normal. A smell like hot plastic or burning wires is not: turn the system off at the thermostat and call.

What this usually is

• A bang as the burners light usually means gas gathers for a moment before it ignites, which is known as delayed ignition.
• A squeal or a grinding sound points to the blower motor bearings, or to the belt on older units.
• A rattle can be a loose access panel, but it can also be a cracked or broken part moving inside the cabinet.

When to stop and callA bang on start-up is a reason to stop and call on its own. Delayed ignition strains the heat exchanger every time it happens.

What this usually is

• Gas is escaping at a fitting, at the gas valve, or along the supply line to the furnace.
• A burner failed to light while gas kept flowing into the combustion chamber.

When to stop and callDo not look for the source, and do not switch lights or appliances on or off. Leave the house first, then call 911 or your gas company from outside. Call a technician once the gas company has made the house safe.