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Negative Chimney draft issues with window AC/ fan units


We are constantly having issues with CO in our boiler room. Our bedroom is on the same level, so that's a pretty big concern. I have 4 CO detectors between that room and our bedroom. 

Only issue is during the summer when we turn on our free standing AC unit in our back porch for the dogs or use box fans and windows ac units that blow air out. 

Current setup:

Central MN 1960's four level split with tar paper for house wrap and cedar siding. Hot water baseboard heating system with glo-heat hot water heater.

We added a fresh air intake next to the boiler because it did not have one at all. And we lined the clay tile chimney and put a cap on it that came with the liner kit. This has helped but not totally. We also keep a window cracked in the back porch were the AC is and had an additional window open on the main floor when we turn a box fan on in the front window blowing out. 

Future plans: 

I would really like to add a whole house fan and central air unit to our attic space above the main floor so we can have some AC in the really humid days but I am really worried about what that would do to the Chimney situation with negative draft from just a window fan. 

What do we do?

Can we add an inline duct fan on the chimney for the boiler that turns on when the dampner opens or is there a better solution?

Thanks, Dan 

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