Proper placement of cold-air returns

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Q: I recently had an air-conditioning system installed and they converted the air handler to a vertical up-flow front return. They left the filter exposed, and after reading your article about getting the proper treatment for sick buildings, my concern is that because the filter is exposed it can pick up more mold and circulate it throughout my condo. Could you please let me know if this is correct? I want to have the company change the air handler to a vertical up-flow bottom return.A: The air handler, or "fan system," found in condominiums and homes is designed to supply the living areas with both heated and cooled air, depending on the thermostat settings. The ideal delivery system of the heated air is through floor registers located on an outside wall near a window or door. The registers are generally located near the window and door openings because these are the most likely sources of air entry to the home.The conditioned air forced from the registers is then pulled through the whole house to the cold-air return located on the furnace or at individual return grills located in each room. On the other hand, the ideal delivery system for cooled air is to install overhead registers with the cold-air return system located near the floor. The reasoning is that warm air rises and cooler air stays near the floor, so by having the supply registers at one elevation and the return air at a different elevation makes the system more efficient at warming or cooling the entire home.The problem, as I see it, with overhead registers is that heated air at the ceiling will warm your upper body before warming your lower extremities, and if your feet are cold, your whole body will be uncomfortable. In my opinion, heated air should be supplied at the floor level in cooler climates, but the AC registers should be located overhead in warmer climate zones.An experienced heating-and-cooling contractor can best determine the ideal installation for your seasonal needs. I have sometimes found homes where the filter was attached directly to or near the air handler. Having the filter exposed or otherwise easily accessible can be an advantage when it's time to change the filter.The important thing is to have a filter installed on the unit at all times and to regularly maintain or replace the filter. Using a high-quality filter can reduce the chances of spreading mold spores throughout the home, but a cheap paper filter offers little to no protection other than keeping large dust particles out of the fan and ducts. I would recommend the use of a high-efficiency particulate air filter (HEPA) if one can be installed.A HEPA filter is generally thicker than the normal 1-inch-thick paper filters found in or on most furnace installations. A HEPA filter is 80 percent efficient at removing dust, dander, pollen, mold spores, fungi and other allergens. The Aprilaire filter box installed near the fan measures 12 inches by 18 inches by 31 inches, which requires a lot of space and should be installed by a trained technician. If there's no room on your ductwork for a HEPA filter, you can use one of the high-efficiency sticky filters such as 3M's Filtrete filter to replace the standard filter found on most units.(Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 286, Evansville, Ind. 47702 or e-mail him at d.Barnett(at)insightbb.com.)