DIAGNOSIS The thermostat's "Heat Anticipator" or adaptive recovery feature is off. The fan starts before the furnace burners are lit.
FIX Go to Installer Setup (press and hold the Menu button for 10 seconds). Navigate to System Setup > Fan Control in Heat Mode. Change the setting from "ON" or "CONTINUOUS" to "AUTO." This makes the fan wait for the furnace's heat call.
IF THAT DIDN'T WORK Your furnace's fan control board may have a built-in delay. This is normal. If the delay is excessively long (over 90 seconds), the furnace limit switch or flame sensor may be faulty.
HEADS UP In "AUTO" mode, you may hear the burners ignite before the fan starts. That's correct operation. Do not set the fan to "ON" in winter.