I have a c/a system. My furnace is heated by oil. My thermostats are set to heat. When the furnace kicks on hot air comes up. Once it gets to the temp furnace kicks off but I get cool air flowing. I have two zone heat and when I turn up the heat upstairs I get that smell as if it's the first time my heat has been on?? I just don't undertand why cool air is coming up threw the vents? Help!

Is the fan continuing to run & if so, for how long?

I think that's what I'm hearing from the furnace??

It seems like its on all the time unless the heat kicks in

Is the fan switch on the thermostat set to "on"?