Turn the power off to the Air handler unit.
Remove the thermostat from the wall, (Just the face part).
Look on the back side of the thermostat and verify that the jumpers are set to electric and not gas, otherwise the fan will not run during heat mode, they come from the factory usually
set for gas.
If this is a electric heater this will solve your problem, If it is gas then it will be more detailed. Once you put it back on keep in mind that most thermostats have a 3-5 minute delay so just give it a little time and the fan should then work once you have the jumpers in the correct position.
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