Horrible odor from a/c vent in wrong zone!

[Kimanjo]

This REALLY BAD ODOR problem has been driving our family batty for months! We live in a 2-story home with a downstairs a/c zone 1 and upstairs a/c zone 2. When we turn the UPSTAIRS a/c on (it operates only the upstairs) we get a terrible odor--a cross between a dead rat and sewer smell--in our DOWNSTAIRS bathroom. The smell is only in our bathroom, no other room in the house. There is a minimal air flow in the downstairs a/c vent; nothing major rushing out but the odor. Can anyone out there imagine why this might be? We live in a 2-year old house and this problem just started a few months ago. Please advise! Kim

[Jimmer]

Kim,

It sounds like you have a sewer vent pipe that is either open in a ceiling wall cavity or is open next to your exhaust from your bathroom vent. When you turn on your upstairs system you are creating a vacuum and the bathroom vent is bringing air into your home to make up for it. Check the locations of your vents and you will probably have to extend one away from the other.

Good luck, Jim

'Just a handyman trying to help'