Unresolved smell from vents

[Svend W.]

For a few weeks, a tenant of mine in Chicago has been complaining about an intermittent sewer-like smell from the vents when the heat is running. 

I visited twice wasn't able to reproduce it and also had an HVAC technician take a look, as well, and he wasn't able to reproduce it, but my guess is that there really is a problem, as they swear up and down that it gets bad sometimes and I see no reason for them to make it up. So I'm going to have somebody take another look. 

So, it's intermittent and there's no question that I've tried to resolve it--I brought both that HVAC technician to check furnace and somebody from the gas company to check for leaks--but I'm wondering whether given Chicago's more tenant-friendly rules people think there's any chance that the tenant could withhold rent or file a complaint if takes some time for me to fix this.

Thanks.

[Allan C.]

I've chased sewer smells a number of times in the past, and often they end up being legitimate. It can also be a combination of factors, like issue always being there, but not as noticeable with colder AC air. Some things you can check:

- are there any cast iron vent lines near your air intakes? 
- does your HVAC condensate drain line connect to sewer?

If your vent/drain lines are PVC or ABS it's lower likelihood to crack but cast iron will crack over time. I also experienced an issue in personal residence where HVAC unit pulls a vacuum while in AC mode, and sucked in sewer gas from condensate line. During winter your condensate line p-trap (if there is even one) may dry up.

Your HVAC system should run under positive pressure, so I think it's more likely that former than the latter, though don't rule anything out.