How mold ends up in an HVAC system
Forced-air systems move a lot of air past many surfaces. Growth inside the system usually needs moisture inside the system: a clogged condensate drain on the air conditioner coil, a drain pan that overflows, ducts in a damp crawlspace, supply boots that sweat in summer, or a return duct pulling humid air from a wet basement. Older Hilltop houses that added central air to a mid-century furnace sometimes have coil and drain arrangements that were fitted into tight spaces and are hard to service.
Dust from a remediation job is another route. If a basement wall full of growth is torn out with the furnace running and the return grille nearby, spores and debris can be pulled into the return side, settle in the duct and reach the filter and blower.
On the other hand, a single moldy wall in a room with a supply register does not mean the ductwork is contaminated. Visible dust on a register cover is common and is not proof of growth.
Do the ducts need attention?
- IfThe coil drain pan holds standing water or the condensate line is cloggedHave an HVAC technician clear and service it; inspect the coil and nearby duct for growth.
- IfDucts run through a crawlspace or basement area that has had waterInspect those runs, especially insulated or lined duct, which can hold moisture and cannot be cleaned as effectively as bare metal.
- IfDemolition happened with the system running and no containmentAsk for an inspection of the return side and blower, plus a filter change, before assuming the whole system is affected.
- IfThere is visible growth on the inside of duct surfacesCleaning by a qualified contractor may be appropriate; growth on internal duct liner often means replacing that section.
- IfOnly the register covers look dusty and there is no moisture historyClean the registers and replace the filter; full duct cleaning is unlikely to change much.
Questions to ask a duct cleaning or HVAC contractor
- What did you see inside the system that indicates mold, and can you show me photos?
- Is the moisture source inside the system identified and fixed?
- Will you use negative-pressure HEPA collection, and how will you protect the house during cleaning?
- Are you planning to apply any chemical treatment inside the ducts, and is that product labeled for use in HVAC systems?
- Which sections, if any, should be replaced rather than cleaned, such as lined or flexible duct?
Common questions
Do UV lights or coil treatments prevent mold?
Some systems use them on the coil, but they do not replace a working condensate drain and a dry system. Ask what problem they are meant to solve in your specific house.
General information for Hilltop, not a quote or professional advice. In an emergency, call 911.