Hot Air Blast!

When I begin driving my 2005 Mini Cooper after it has been driven before (and especially when it is hot outside), within a minute or two the heater blasts hot air for 15 seconds to a minute. I was out of town and didn’t drive the car for 10 days and, at least for the first day, there wasn’t a blast. The Mini dealer can’t figure this out. What do you think it could be?



Thanks



John