Hvac issue, water heater or boiler co detectors

[Isaac Zimmerman]

Hey guys, longtime lurker, just here for some real estate advice from the hive mind....

I’ve got an over/under duplex that shares a boiler and waterheater, split electric.

The furnace and water heater are both gas, located in the furnace room downstairs. The washer dryer for downstairs is also located in this same furnace room.

I lived in the lower unit before moving out, top and bottom rented out for 2+ years now.

About a month ago, co detectors started going off downstairs, then upstairs. We thought it was the perfect combination of attic fan kicking on, boiler running, and dryer running with the door to the furnace room closed, all at the same time which was causing a deficiency in combustion air, leading to unburnt gas. Again, I never ran into this living there, and it’s been rented for a couple years since me and this has never happened until now.

So we cut a vent to the exterior to take care of combustion air. Intermittently ran into the same problem. Then we turned the attic fan off, intermittent same problem.

We changed the co detectors out and gave the tenants mobiles ones as well, to locate areas and levels if it were to go off. Still went off, seems to be generally coming from the furnace room and not somewhere downstream like the attic etc

We’re thinking about an inline exhaust fan, maybe drafting issues? The water heater is newer and has been looked at several times, nothing found. The boiler is over forty years old, has just been professionally cleaned, and 4 separate hvac guys including guys from the utility company have looked at it and all thinking it’s functioning extremely well, good blue flame and can’t find an issue.

What are we missing? We looked at the soffits, a little dirty but that shouldn’t be causing this, should it? Any ideas what could be the culprit? We’re running out of ideas, I’d rather not replace the boiler just to have the same issue with a brand new boiler, especially if the current one is functioning great...

Any ideas?? The duplex layout is extremely simple, just an over/under rectangle, most boring roof and attic that I can think of....and we’ve never ran into this co thing before.....