We have a 2 story approx 2000 sq ft house with an unfinished basement in Michigan that was built in 2005 with a high efficiency furnace. I sealed up most of the duct work that was leaking like a sieve, we has an ERV unit installed to bring in fresh air (definitely had some stale air issues), and have been working to close most other air leaks in the house to make the house more comfortable.

At our furnace we have two 4" outlets for combustion air - one above the furnace and one that is piped down by the ground. Even when the furnace is not running, cold MI air is not just leaking, but almost flowing out through these 4" pipes. I have not actually put a thermostat down there, but it feels much colder than the typical 55 degrees of an unheated basement.

Is there anything that we can do such as a damper, etc that would block off air from flowing in through these pipes when the unit is not running, or is this a bad idea for some reason? This area serves as a play area for two kids under 4 and I am looking at ways to reduce this drafting and make it slightly warmer down there.

Thank you!