FA oil furnace blowing cold air when not running
WE have a forced air oil fired furnace (and water heater if that matters) and while it works well most of the time, sometimes on a windy day we'll get cool air blowing through the registers when the furnace isn't even running. What could be the cause of that? The furnace is vented out through a chimney from the basement to the roof, there are no gaps that we can see. It's been serviced regulary and no problems, but still this cooler air coming in.
Thanks.