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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?

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The air in your duct work is cold because it is running through the attic which is not heated. There will be some air flow through the ducts because they are not a "sealed container" they are designed to move the air in your home.

As for the DIY project if your covers are metal I would suggest using a magnetic tape or adhesive sheets to attach them. Or they make covers:

For the ceiling registers you will want to make sure the magnets are strong enough to hold material tightly.

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What I find confusing is that there seems to be a random set of drops/returns that have a lot of cold air coming out of them and the rest have none (as verified by my laser temp reader). All the drops have slats that I can shift to close most of the air passage. Should blocking all the returns (that don't have movable slats) be enough? –  tmcallaghan Jan 3 at 20:06
    
With out knowing your specific duct design it would be hard to know why some do and some do not. Blocking the returns that are moving cold air should do the job. –  BD72 Jan 8 at 21:00
    
Thanks. I've blocked them all with home made covers that I secured with zip-ties. –  tmcallaghan Jan 9 at 14:45
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