A member sent us the following question:

"I have a problem with mold on three registers of a 16 register system. The indoor coil and ducting near the unit is clean as a whistle. The three affected registers are on three different floors, not near each other. Two are ceiling registers in the middle of the room with a floor above them and one is a low sidewall register on an inside wall. One room has two registers but only one of these has any significant mold growth. The rooms include two bedrooms and a spare, little used, kitchen. They say this started after their electronic air cleaner was replaced with a mechanical filter (4" paper type filter). Do you see any correlation between the two? The blower runs continually. I did find some very slight mold build up on the trough of the humidifier. I cleaned all areas noted with diluted bleach but would appreciate your insight."

ACCA's Glenn Hourahan responds:

"In general, mold growth indicates that moisture is condensing out of the air and growing on a host surface. Since you indicate that the ducts and the coil are clean ... it would appear that the mold is not propagating from the HVAC system to the affected registers. Rather, it sounds as if localized cold spots are coming in contact with humid areas (could also say that localized humid areas are coming in contact with cold spots, i.e., the duct registers).

"This causes the moisture to condense out at those particular locations. This is often seen when a register blows cold air directly onto an exterior surface. Another example is when bathrooms are improperly vented (or the vents/windows are not used for hot showers) and the moisture laden air hits a cold surface. Hence, you need to assess whether these are localized cold spots (are they getting more air? Are they closer to the coil so the air is colder?) or localized humid spots (close to baths, fountains, etc.).

"Another key statement you made is that the customer runs the fan all the time. Someone may have advised the customer that they would get better filtration if the fan was left in the 'on' position as opposed to 'auto.' This is a true statement. But, this is a no-no in most areas of the country as airflow distribution inefficiencies (and something as small as shutting interior doors can count) can over-pressurize some rooms and under-pressurize others. This allows for uncontrolled moisture flows (say from an over pressurized bathroom to a bedroom or from an under pressurized room drawing air through the outdoor envelope).

"Additionally, constant running of the fan (with the outdoor condensing unit cycling on and off based on T'stat demand) also allows the evaporator coil to get cold, then warm, then cold ... resulting during the warm stages with reduced moisture removal from the air and can even cause re-entrainment of moisture from the drain pan or the evap coil into the air stream.

"I don't see a connection with the customer changing filter type, unless doing so resulted in a marked change in airflow (and temperature?) to the affected registers.

"Finally, there is an excellent resource on mold that should be on every single contractor's bookshelf. The author crafted a resource that contractors will be able to read, understand, and apply. You can obtain it from the ACCA bookstore right here."

Copyright 2005, Air Conditioning Contractors of America. Reprinted with permission. For more information, visit www.acca.org.