Nope, she's a real leaker. And a window in the basement open. Not much for insulation, really bad single pane windows. In general, they missed quite a few codes for a house built in 1980. The chimney is on a single floor family room and the house is a two story garrison. I'd say the chimney is about 6' shorter than the main house but at least 16' from it. . The chimney also is in the garage so it is exterior job as well. So now we know, it's short so it doesn't have a great draft to begin with. OK lousy chimney, the stove pipe has 2 90* bends maybe 6-7' total in length.
Now this ONLY happens if it is cold outside, right now it's 62*, I have a draft a little over .01 idleing and it will never do it. But if it's cold out, say <35* and the boiler is .04 or better it may...... or may not backdraft at the oil side. And it doesn't matter about the combustion blower either. I'm not sure why it happens, and that is a pretty good jump, at least 1 1/2'. I've talked to a bunch about it and several people said it was common but couldn't explain why. It may be opening the damper that sets it off, I don't know. Like I say, anything done at the top of the chimney will cure it. Say a cap or an extension to either one of the offending liners cure the problem. I even tried a baffle made of aluminum across the chimney, and that worked fine.
My point was stuff happens, there are a zillion varibles going on with this stuff and most of us here are not "licensed & trained profesionals" so be careful, make changes one at a time if you can, and watch it when you do. The last thing we need around here is a R.I.P. thread (I think I'm feeling a little mortal in my old age