This is the second time I’ve posted this question because I did not feel comfortable with the one answer I got. Many homes older that 5-10 years have slight staining around the collar of the water heater vent pipe as it goes through the ceiling in the garage in California. The answer I got was a simple “condensation is always a problem”. I know that pipes that do not have a good vertical direction tend to have more condensation but some vertical vents also develop condensation. Question: What’s a good response to add to this issue to make sure I cover it in my report without getting a callback?
This oughta cover you.
Inspector observed a stain on the ceiling around the water heater flue pipe located in the garage. This stain was dry when tested with a moisture meter. You should have the current owners disclose in writing their knowledge and repair history of this stain. Inspector was unable to determine if a leak was active in this area due to finished coverings. Further investigation should be made to this area by a qualified contractor to determine if leaks or hidden damage exists. At a minimum, this area should be prepped, painted and monitored for future leakage. A qualified person should repair or replace as needed
Thanks John, I already use that response, mine is just a little bit more concise. I was looking for a more technical response regarding causation and a suggestion on how to prevent it.
Depending on the flue material, The fix is likely a gap between drywall and the flue separated by a thimble or fire blocking.
I was your first post one-answer bandit, I believe 
I do apologize in advance for it not helping you out.
Anyway, I don’t believe you mentioned CA as your location in the first post so I doubt it’s a condensation issue. I would venture to guess that the only other culprit would be the storm collar not being sealed properly at the rooftop. If it’s well-sealed during your roof inspection, then it’s safe to say the stain is probably a thing of the past.