Where residue hides in the system
Air enters the system through return grilles, passes through the filter, crosses the blower and the furnace heat exchanger, then flows over the air conditioning coil and out through supply ducts to each room. Powdery CS tends to load the return side and the filter heaviest. Oily OC usually only reaches the system if a can discharged close to a return or the system ran while the room was fogged.
Flexible ducts, common in additions and newer infill houses, have ribbed liners that hold dust more stubbornly than smooth sheet metal. Some older Berkeley-area bungalows have ducts that were added decades after the house was built, routed through crawlspaces or attics, which can make access more difficult and inspection more important.
A sensible order for the air system
Turn the system off at the thermostat
Keep the fan setting on off rather than auto or on, so the blower does not run between cycles.
Change and bag the filter
Slide the filter into a bag at the furnace, seal it, and take it straight outside.
Clean registers and grilles
Remove covers and wash them away from clean rooms. Wipe the first reachable section of duct behind each one.
Inspect the return and plenum
A contractor can look into the return plenum and the blower compartment to see whether dust or film is present.
Decide on full duct cleaning
If the inspection finds residue beyond the first few feet, mechanical duct cleaning with negative air may be recommended.
Test run with windows open
Run the fan for a short period and note any symptoms before relying on the system again.
Is full duct cleaning likely needed?
- IfCS was deployed inside and the system ran during or after the incidentExpect duct cleaning to be part of a thorough scope; ask to see photos of the inside of the ducts.
- IfCS was used inside but the system was off and stayed offFilter replacement and register cleaning may be enough, but ask for an inspection of the returns.
- IfBear spray went off in a garage that has no ducts to the houseThe house system may not be involved at all; focus on doors and the shared wall.
- IfPepper spray discharged in one room near a return grilleAsk whether the nearest return run and the filter need attention rather than the whole system.
- IfSymptoms return every time the blower startsTreat that as evidence residue is still in the system, whatever was recommended earlier.
Questions for an HVAC or cleanup contractor
- What did you see inside the return plenum and the first runs of duct?
- Will you use negative air pressure while cleaning so dust is pulled out instead of pushed into rooms?
- How will you handle flexible duct, and would you recommend replacing any sections?
- Does the AC coil or the blower need cleaning, and how will you protect it?
- Can you provide before-and-after photos for an insurance file?
General information for Berkeley, not a quote or professional advice. In an emergency, call 911.