Short answer
When a furnace won't heat, check the thermostat (heat mode, set above room temperature, fresh batteries), the furnace power switch and breaker, and the filter. On gas furnaces the usual failures are a dirty flame sensor, a failed igniter, or a pressure switch tripped by a blocked vent or drain. If you smell gas, get out of the house and call the gas company or 911 from outside.
Upstate homes heat three ways: a heat pump, a gas furnace, or electric resistance heat in an air handler or electric furnace. Plenty of houses have two of those working together. Alpine Cool works on all of them.
What's usually wrong with a gas furnace
Dirty flame sensor
The burners light, run for a few seconds, then shut off, over and over. A coated flame sensor can't prove there's a flame, so the furnace shuts the gas off for safety. It's one of the most common winter calls.
Failed igniter
Hot surface igniters crack with age. No glow, no flame, no heat.
Pressure switch or venting
High-efficiency furnaces prove the vent is clear before they light. A blocked vent pipe, a sagging vent, a clogged condensate drain or a failing inducer motor trips the pressure switch and stops the sequence.
Blower motor or capacitor
The burners run but little or no air comes out of the vents, or the furnace overheats and shuts itself down on the limit switch.
What's usually wrong with electric heat
Electric furnaces and the heat strips in heat pump air handlers use heating elements, sequencers or relays, and limit switches. A burned element or a failed sequencer takes part or all of the heat with it. On a heat pump system, the strips are also the reason a January power bill can jump: if the heat pump isn't pulling its weight, the strips do the work, and resistance heat is expensive to run.
What we check on a service call
- Thermostat mode, settings, wiring, and whether it's calling for heat.
- Power: breaker, service switch, door safety switch and control board status code.
- On gas: gas supply, igniter, flame sensor, burners, inducer and pressure switch, and the vent and condensate drain.
- On electric: heating elements, sequencers or relays, and limit switches.
- Blower, filter and airflow, since restricted airflow causes overheating.
- Safety checks, including the heat exchanger and flue on gas equipment where accessible.
- What failed and what the fix is, explained plainly.
What it costs
Ask what the service call costs when you call. For what drives the price of a repair or a new system, see what HVAC work costs.
When repair stops making sense
Most furnace repairs are small parts, and they're worth doing. The line is a cracked heat exchanger on a gas furnace, which is a safety problem and usually means replacement, or a major failure on an old unit that's already needed several repairs. If your furnace is paired with an old AC, replacing both together is often the better value. See installation and replacement.
Safety first
- Gas smell (rotten eggs): don't touch switches. Leave the house and call your gas company or 911 from outside.
- Burning or electrical smell: turn the system off at the thermostat and the breaker, then call.
- Carbon monoxide alarm: get everyone out and call 911.
A little dusty smell the first time the heat runs each fall is normal and should clear within the first hour or so.
Local detail
January lows at GSP average 32.1°F (National Weather Service, 1991–2020 normals). That's cold enough for heat pumps to lean on their heat strips on the coldest mornings, and cold enough that a furnace that limps through November will often quit in January. A fall heating check finds the weak flame sensor or igniter before the cold snap does.
Common questions
Why won't my gas furnace ignite?
Common causes are a dirty flame sensor that shuts the burners off seconds after lighting, a failed igniter, a tripped pressure switch from a blocked vent or drain, the gas valve or supply turned off, or a thermostat or power problem. Check the thermostat, the furnace switch and breaker, and the filter. If you smell gas, leave the house and call your gas company or 911 from outside.
What does it mean when my furnace blinks a light?
Most modern furnaces have a control board with a status light that flashes a code. The code chart is usually on the inside of the furnace door. Write down the pattern before you turn the power off; it tells the technician where to start.
Why is my electric heat so expensive?
Electric resistance heat, whether it is an electric furnace or the heat strips in a heat pump air handler, uses a lot of electricity. If a heat pump is failing and the strips are carrying the load, the bill climbs fast.