I have a forced hot water (baseboard) heating system in my home and added central air conditioning about 10 years ago. The cooling system's air handler is in my attic, and duct work was added for the 1st and second floors for cooling. There are 3 returns on the 2nd floor, and about 10 supply vents throughout both floors.
My question/issue is that two of the returns and 3 of the supplies have pretty bad (cold) drafts throughout the winter. I measure the temperature at around 50 degrees F on them. All the others are at/near room temperature.
I'm considering blocking them for the winter months by purchasing covers, or likely a DIY project like http://www.instructables.com/id/Winter-HVAC-Hack.
My main question is that should there be any drafts at all. Are the drafts a sign of duct leaks that should be found and repaired by the HVAC company?