We purchased a 50 year old home 4 months back and it has a HVAC system that is about 20 years old. The AC over the summer was blowing nice and cold, but the actual fan blew very weak. Now that the colder temps are here and we have the heat on, it works great and blows strong. Fresh filter when we moved in and everything looks and sounds OK. Now I'm no home HVAC specialist, but this has me confused? Any thoughts are much appreciated.

Same fan blows through same filter in same ductwork.

My Guess is you have a fan capable of more than one speed and the controller is not telling it to run fast? Do you have a warranty when you bought the house? If so might be worth calling an HVAC guy to come out and diagnose it before it expires.

Typically, fan speeds are higher for AC over heat because the cooler air is denser. You may have the fan leads reversed. If you can, look at the motor. Many have diagrams on them that show what for hookup for AC or heat.

You have me confused. You just said that everything works well and you want to know why. "If it ain't broke, don't fix it". A quote from Bob Jones.

When you heat comes on do you heat any changes in the air volume/speed? Can you tell us the brand and model of your furnace and the source of the energy?