HVAC
How HVAC systems spread or prevent mold
In plain English
Heating and cooling systems control the conditions that cause condensation, and they also move air through the whole house. R520 says this makes them both a cause and a carrier of mold problems. A poorly working system can let humidity rise and condensation form widely, and mold in the system can be blown into every room. Condensation forms wherever a surface is colder than the dew point of the air around it, in any climate, though it is most familiar in hot, humid regions.
R520 notes that horizontal units in attics and down-flow units feeding crawlspace ducts are hard to service and can collect moisture unseen. Bare sheet-metal duct can be cleaned aggressively. Duct board, internally lined duct and flex duct with actual growth cannot be cleaned successfully and should be replaced. Commercial buildings add more variety, such as multi-zone and variable air volume systems, which are larger and harder to inspect. R520 also observes that mold sometimes appears with no obvious leak at all, and in those cases the interaction between the building and its HVAC system is a likely place to look.
Remediation includes returning the HVAC system serving the affected area to Condition 1. Cleaning should follow the NADCA ACR standard, and it is best scheduled after the building work so dust from demolition does not re-enter the ducts. When a system cannot hold temperature and humidity within ASHRAE 62.1 or 62.2 ranges, R520 recommends an engineering review, since the design may be undersized, altered or worn. Upgraded filtration is worth considering after a mold problem.
Key points
- HVAC problems can cause mold and spread it throughout the house.
- Lined duct, duct board or flex duct with growth is replaced.
- Clean HVAC to NADCA ACR after the building work is done.
- Seal the system off from the work area during remediation.
- If the system cannot control humidity, get an engineering review.
Official text
This topic is covered in our own words because the standard behind it is copyrighted; we cite it below rather than reproduce it.
Citations
- IICRC S520 Reference Guide, IICRC S520 Standard and Reference Guide for Professional Mold Remediation — Reference Guide — Ch. 12, pp. 127–130. Best practice