Extended Fan Timer Thermostat?

[rockncountry101]

When my central heat turns off it doesn't have a cool down cycle. This causes my ducts to just go crazy with expansion/contraction. I did an experiment where I would turn the fan on after the heater kicked off and this wasn't a problem at all. Can I buy a thermostat with a fan timer that will keep it on for a certain period (60 or 90 seconds) after the heat/cool kicks off? Thanks!

[Greg]

Most units have high & low fan switches in them, it could be that the low switch is bad and shutting off the fan too soon.

Greg