What a normal gas furnace startup looks like
Most modern gas furnaces run through a fixed sequence before you feel any air. In a 2012 Department of Energy furnace analysis, the average timings it used were:
- About 15 seconds of the draft inducer (the small exhaust fan) running before ignition.
- About 37 seconds of the igniter heating up before gas flows to the burners.
- About 30 seconds of burner firing before the main blower starts pushing air through the house.
Those are analysis averages, not a spec for your unit, but they explain two things homeowners notice. The first sound you hear is the inducer, not airflow, so a minute or more of quiet after the thermostat clicks is normal. And once the blower does start, the furnace is already hot. Cool air in the first moments is coming from the ducts, not the furnace.
Tell duct cooling apart from other causes
Pay attention to the pattern rather than the exact number of seconds:
- Air is moving between heating cycles and never gets warm: check the thermostat's fan setting. With the fan set to ON, the blower runs whether or not the furnace is heating, so that air will feel cool. AUTO lets the furnace decide when the blower runs.
- Registers near the furnace warm up fast but a far bedroom stays cool much longer: that points to a long or poorly insulated duct run. In Philadelphia rowhomes, ducts often run along the joists of an unheated basement, and second-floor runs in many suburban twins and singles pass through the attic.
- Every register stays cool and the house doesn't warm up: that isn't duct cooling. Call for service.
- The furnace starts, runs briefly, shuts off and repeats: that's short cycling, a different problem covered in why a furnace turns on and off every few minutes.
What you can check yourself
Keep it to the outside of the equipment:
- Confirm the thermostat is on HEAT and set above the current room temperature, and note whether the fan is on AUTO or ON.
- Make sure furniture, rugs and curtains aren't covering supply registers or the return grille. The DOE Energy Saver guide lists blocked registers and stuck dampers among the common, easy-to-fix causes of uneven heat.
- Check the filter. The same guide says to clean or replace furnace filters monthly or as the manufacturer recommends; how often to change a furnace filter has the details for common filter types.
- Time one heating cycle: when the air starts, when it feels warm, and whether the room temperature rises. Note whether it's one register or the whole house.
Don't open the burner compartment, clean ignition parts, adjust the gas valve, or keep resetting the furnace to force it to run.
If the ducts are the culprit
Uninsulated ducts in unconditioned space lose heat through the metal. The Building America Solution Center calls for insulating supply ducts there to at least R-8 and return ducts to at least R-6, with seams and connections air sealed. The Energy Saver guide notes that ducts leaking into an attic or crawlspace can add hundreds of dollars a year to heating and cooling bills, and that sealing work should be tested by a qualified professional for combustion safety and airflow. That's a job for an HVAC contractor or an energy auditor who does duct testing.
When to call an HVAC technician, and what to ask
Schedule service if the startup behavior changed, warm air never arrives, or the furnace can't keep the house warm. Soot around the furnace, an unfamiliar burning smell, or a damaged vent pipe are warning signs the CPSC links to carbon monoxide risk, so don't wait on those.
Ask the technician to watch a full startup and compare the inducer, ignition and blower timing to the manufacturer's specs, check the fan control and airflow, and inspect the venting. Ask what they saw that supports any part they want to replace. If you're due for annual service anyway, what a furnace tune-up should check lists what the report should cover.
Gas smell or CO alarm: leave first
If you smell gas, get out without using light switches or phones inside, and call from outside. In Philadelphia that's PGW at 215-235-1212; in PECO gas territory in Bucks and Montgomery counties it's PECO at 1-800-841-4141. If a CO alarm sounds or people feel headachy, dizzy or nauseated, get everyone into fresh air and call 911. When a CO alarm means leave the house explains the alarm patterns.