I still need to get outside air into the boiler room. Right now it is wide open to the basement, but I would like to close it off from the basement...................................
Can you seal off the basement door from the first floor ?? That way you can provide make up air in the basement for the boiler only. Sealing the door would prevent the draw extending into the basement. Or at least severely limit it.
Our old twin farmhouse has 2 doors open to the basement from inside each side of the upstairs and also 2 bilco doors on either end of the basement. They do not seal real tight either. We do run a large about 18" diameter fan in the 2nd floor window at the top of the stairs. We rarely see a backflow on the flue.
I have mentioned on here several times before that at my old shop whenever I would turn on the spraybooth fan, it would backdraft both the coal and oil burner flues. Only solution was to fully open the far shop window. And I was using a 24" fan attached to a 1/2 hp motor. Yeah it did draw some air !!!!!