Cover the vents.

You have air coming in somewhere pushing the air through the vents. I would search for the source and seal it because it will actually save you more money.

Hi - In terms of "air coming in somewhere pushing the air through the events", I was thinking the only possibilities would be the Vents and return duct from the inside of the house. But is there another possibility?

Thanks,
Barry

The duct work in the attic may have leakage which would allow cold air to enter. You need to do a visual inspection and seal any leaks.

It is also possible that warm air may rise up through the return or supply vent and push cold air down to other vents. Closing all the vents would help stop this. I would look into covering the return also. You want to stop your warm air heated by your boiler from leaving the insulated part of the house.

This has the potential to be one or both of two messes. If the ducts are not well-insulated, warm, moist air from the living space will enter the ducts and water will condense and might drip into the house. The best solution will be to better insulate the ducts because it will also cost less to cool the house in the summer.

If the ducts actually leak air and that is where the cold drafts are coming from, the air leaks should be sealed for the Summer economy as well.