Had a look at a property today, it was generally in need of some TLC but one thing that I noticed that I'm unsure of the cause is that there were brown patches around the DVS vents. My initial thought would be that it was from staining from smoke, but there is no fireplace and no evidence that the current tenants smoke.

My understanding of these systems are that they are pushing air into the house from the ceiling cavity, therefore the marks would be caused by particles in the air coming in - which is meant to be filtered.

Is it possible that this would be caused by a bad / clogged filter? Any idea what would cause this. The area of the stain / mark is probably about 10-20cm around the ducts, and it seemed to be around every duct in the house.

Thanks

Ben