Good morning I have a ICF foam block house, with 8 inches of closed cell insulation in the attic. The house is 900 square feet. The air exchanger ducts go from the basement into the attic to 2 inlet and to discharge ducts on the ceiling. They are 6 inches in DIA. I have a back draft damper in the system and verified no air is comming in from out side. But with the exchanger off I have a fair amount of cold air blowing into the house from 2 of the ducts(fresh air in duct) the duct in the attic is 6 inch flexible insulated duct and about half covered with the closed cell. I have no clue on why I have cold air coming in and I need help on what I can do to fix it. Thanks Larry

Are the rafters insulated, so that the attic is an insulated space? Or is the attic a cold space?

Where is the damper located in the system? It sounds as if you have a leak in your envelope somewhere And warm air is exiting powered by stack effect. The make-up ducting is of course doing it's job and providing cold makeup air. How are you heating the house? The heating system could be providing this pathway or the pressure difference to cause the air exchanger make-up to flow inward.

Do you have any gas appliances? Those will have a stack/vent to exhaust combustion gasses but in a more ideal installation would be provided with their combustion air directly so it is not drawn from the rest of the house which can be a performance or safety problem.

I agree with ronmar - sounds like you have something that is creating a negative pressure in that area. Best to find and stop this. Since a negative pressure (at the ceiling) implies heated air leaving the house, using a thermal camera on the outside would work (if simpler approaches fail).