A gas furnace that keeps the blower running after the flame goes out is usually doing its job. The control board holds the fan on for a set period so the heat left in the heat exchanger moves into the house instead of going up the flue. The trouble starts when that period seems too long or too short and nobody has written down what normal looks like on this particular unit.
What the off delay controls
When the thermostat calls for heat, the board starts the inducer, proves airflow, lights the burner, and then brings the blower on after a short delay. When the call ends, the burner shuts off first, and the blower keeps running for a fixed time before it stops. The thermostat does not set that interval. The furnace control does, and depending on the model it may be fixed at the factory, adjustable through dip switches, or set through a small menu on the board. The installation manual, usually tucked inside the blower compartment door or available from the manufacturer's site, lists the options for your model.
Nothing about the delay is mysterious. Heat stored in the exchanger has to go somewhere, and a fan that stops the instant the flame dies leaves that heat trapped. A fan that runs long enough to pull that heat into the duct is working as designed. Your job is to find out which pattern your furnace follows.
Why the number matters on a service call
Most cycling complaints come down to one of three things. The burner lights and shuts off too soon. The blower never seems to stop. Or the house swings between warm and cool faster than it should. Each of those descriptions is vague until you have timing data. "The fan runs after the heat stops" is not a fault report. "The burner ran 6 minutes, then the blower ran about 90 seconds after it went off, three cycles in a row" is something a technician can use.
The diagnostic tools now entering the trade are built to read equipment data from the outside. ACHR News reported that Resideo unveiled Pro-IQ Predict, an AI tool for HVAC diagnostics and maintenance, described as an expansion of Resideo's Pro-IT services platform and Resideo's new Pro-IQ Predict diagnostic tool is described as "informed by millions of interactions with connected devices." The report does not say what that tool measures inside a given furnace, and you should not assume it captures fan-off timing. Treat any platform readout as one more opinion, and keep your own log so you can compare it against what the tech measures on site.
The same week, ACHR News also ran a photo gallery of the inaugural HVAC Contractor Forum, which took place Nov. 2 to 4. The gallery notes that the inaugural HVAC Contractor Forum drew "More than 200 HVAC professionals attended." The event tells you the trade is paying attention to its own work, which is a reason for you to pay attention to yours.
The ten-minute log
Pick a normal heating call on a day when the house is not swinging wildly, and keep a note in the furnace binder or on your phone.
Start a stopwatch the moment you hear the burner light, or when you see the flame through the sight window if your model has one. Stop it when the burner shuts off and note that time. Then note when the blower stops, using a second timer or simply the clock. Repeat for three cycles. Write down the thermostat setting, the outdoor temperature, and whether the house reached setpoint.
If you own a multimeter and are comfortable with low-voltage work, check for about 24 VAC between the R and C terminals on the control board, with the thermostat connected. A reading far from that range points to a power issue worth describing to the tech. Do not work on line-voltage parts, and do not touch the gas valve, the igniter, or any safety switch to see what happens.
When to stop and call
Your log is most useful when it shows a change. Note the delay before any repair, and ask the technician to record the setting before and after any board work. If the blower never stops after many minutes, if the burner lights and quits within a minute or two repeatedly, or if the cabinet stays hot with the blower off, stop timing and call for service. Those are patterns, and they need a trained person with the right instruments.
A furnace that runs its fan after the flame is out is normally just finishing the job. The useful question is not whether the fan runs after the burner stops, but how long, how consistently, and whether that timing has changed since the last visit.