'Exhausty' smell in house when boiler runs --
This is a long post, but I think the details will be needed since a professional has checked the obvious and finds no explanation.
My wife reports an 'exhausty' smell in the house when the boiler runs. We have had two long cold spells (for St. Louis) recently with temperatures not above freezing for more than a week. The smell is particularly noticeable in the morning when the boiler runs for a while bringing the temperature of the house back up after the overnight thermostat set-back. It is most noticeable on those very cold (for St. Louis) days such as we had during those two cold spells. The smell is most noticeable in the second-floor hallway. It is not noticeable in the basement. I previously made an effort to close openings between the basement and first floor, and the basement door is weatherstripped. On the other hand, I have made no effort to seal the basement, which is quite leaky to the outside (and needs to be to provide combustion air to the boiler). I am less aware of the smell. Now that it is warmer, the boiler is running less and the smell is weaker.
I have asked my wife to smell the radiators. They have the normal warm radiator smell, and that is not the exhaust smell.
I tend to trust her observations because the last two times she mentioned this kind of symptom, I investigated and found that the old water heater was spilling exhaust (candle went out) on one side of the hood. A professional from the company I call when I need a pro came out and tweaked the hood until it was no longer spilling (candle does not go out). Next time there was a smell complaint the water heater was spilling again. I could not adjust the hood to fix it, and now we have a high-efficiency water heater (uses less gas than the boiler pilot light, on average) with a fan-blown PVC exhaust.
The steam boiler is the only gas appliance that has been running in the house when the smell happens.
The house was originally built about 1905. Two stories, plus a walk-up (narrow stairs) attic that contains many fiberglass insulation batts laid on the pine floor. The boiler is a Burnham, installed in 1994, before we moved in. It exhausts through an 8" Zflex stainless steel flue liner installed after we purchased the house, about 20 years ago. When the boiler is running or is just on pilot, it was drawing well when the technician tested it. The technician also looked at the areas of the house where the chimney runs up through the walls, went up to the attic to inspect the chimney there, and then used the 1/2 height door at one end of the attic to reach the second-story roof, and inspected the top of the chimney. No problem found. At that point the technician had no other suggestions for investigation.
We have a CO detector with a digital display in the basement (where the boiler is) that has not gone off. Nor does it display a non-zero value. I moved the detector to the second-floor hall for a few days -- no display, no alarm. I tested the detector with a test kit that included a plastic bag and a little glass vile of CO. The detector showed a value in the 780 ppm range at peak, which was consistent with what was described in the test kit.
The boiler has been skimmed on a regular basis. When I do that, I always save a little cup of water from the start, and from the end. The first time I skimmed it, there was a noticeable film covering most of the surface of the start sample. Last fall, there were barely discernible scattered dots of oil in the start sample.
Ideas I have:
Get someone to make a camera inspection of the flue liner. The chimney has an odd bend in the attic -- perhaps the liner was damaged during the original install and has now failed at some point.
Make an extra effort to seal the upstairs hall from air infiltration from the chimney behind the wall on one side, and the crawl space over the first floor addition on the other.
Re-skim the boiler to see if anything is present now that was not present last fall.
That's all I've got. I would love to hear suggestions from others.