I have 95 F150 with the 5.0 8 cylinder engine. The heater used to blow hot air but now it won't blow hot air in the floor.
It will only vent through the defroster and the air is cold. I was thinking it was a vacuum leak because when running the A/C the air will switch from the vents to the defroster until you get off the gas then it will switch back...any suggestions?

 

Yes look for vaccuum leak especially where line goes thru firewall

 

+1 on the vacuum leak.

There will be a vac line running through the firewall, and once it's inside the truck you'll probably need to take the glove box and/or radio out to see where it leads. Gotta be a fitting come loose, a broken/cracked line, etc.

 

The air should still be getting hot even if it doesn't come out the right place. Better check the coolant level. But some vehicles have a vacuum operated heater valve that stops the hot coolant from entering the heater core, so that could cause the no heat issue also if you have that valve. It's operated by the same vacuum that is causing the random vent issues.