Q: We have a musty odor that comes out of our vents when the a/c and heat kick on. We had a new system put in with UV lights and had some of the ductwork replaced at the same time.  We have been told that the solution is to replace all of the ductwork as the HVAC technician thinks there is mold in the ductwork.  Is there any other solution?  The HVAC business won't guarantee that this will solve the problem and we have already spent a lot of money on a new system, trying to get rid of the odor.

A:If you are sure that the musty odor only occurs when the A/C operates, then the problem may still be inside the ductwork that hasn’t been replaced. What type of ductwork is in place? Is it internally insulated with duct liner or is it externally insulated? If it is internally insulated and a relatively old system, then there may be a build up of dust and debris, and possibly mold, in the ductwork and, so, the ductwork needs to be  either cleaned or replaced. If the ductwork is externally insulated, then there would be less dust, debris and possible mold inside and cleaning could be relatively effective.

UV is generally effective at cleaning and killing mold on surfaces where  a direct UV light is applied. So, if the cooling coil of the A/C unit has UV lighting directly shining on it, the coil would be kept relatively clean. UV lighting has not been as effective on cleaning the rest of the system where there is no direct light, such as air flowing through a duct. UV light won’t necessarily kill the mold spores passing through it.

Another key area of the A/C system to review is the return air shaft, ductwork or grille. Review the entire pathway from the return air grille to the A/C fan unit. There may be possible sources of the musty odor in the pathway causing the odor to permeate throughout the system.

Response provided by: Rick Anderson, P.E. of EFI Global, Inc.

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