Courtesy of Touchstone Energy Cooperatives
Q: We are replacing our old heat pump and evaluating different filter options. Our kids have allergies, so we want the cleanest possible air in our home. What MERV rating should we look for when selecting filters?
A: MERV is short for minimum efficiency reporting value, and it measures how well a filter or air-cleaner appliance traps particulates, including dust, smoke, pollen and mold spores.
The rating ranges from 1 to 16. An inexpensive, 1-inch-thick fiberglass filter is about MERV 2 at best. A filter used in hospitals for surgery may be MERV 16. My geothermal heat pump uses a MERV 11 filter.
Consult your doctor for a precise diagnosis of allergens, and choose filters accordingly. Many common allergens (pollen, pet dander, dust mites) can be removed by a MERV 4 filter. Mold spores vary in size, but a MERV 7 filter can remove most of them. Removing tobacco and other smoke requires a minimum of MERV 13.
Installing air filters with the highest MERV is not always the best choice for a home. These filters can decrease airflow through ducts to the point where the heat pump’s efficiency drops, and with lower airflow, fewer allergens make it to the filter to be removed.
Most filters rated MERV 7 or higher use some type of pleated media. This filter material is folded back and forth like an accordion inside a stiff paper frame, often with reinforcing wire mesh. High-MERV filters are typically about 2 inches thick. Thicker filters (up to 6 inches) do not necessarily filter better, but they can hold more particles before needing to be replaced.
Keep in mind, a central filter only removes allergens that reach it. Many of the larger allergen particles never get drawn into the return air ducts. They puff up when someone sits on a sofa and settle back down again. Small, room-air cleaners—stand‑alone appliances with built-in fans and filters—are another important tool when trying to control airborne allergens.
For families without allergies, installing a basic, pleated-media filter is adequate. Its main purpose is to keep dust and dirt from getting into the heat-exchange coils inside the heat pump. Dirty coils can inhibit heat transfer and reduce system efficiency. Experts recommend changing air filters every three months to keep HVAC systems in peak working order.
Washable electrostatic air cleaners can be an effective choice. The air flowing over the filter media creates a natural static charge, causing particles to stick to it. Rinsing the filter in the bathtub neutralizes the charge and washes the trapped particles away. Washable filters cost more than disposable filters, but if properly maintained, they can provide years of service.
Whole-home air cleaners, which use a combination of thick pleated media and electrostatic grids, can be built into a new HVAC system in place of traditional air filters. Most manufacturers offer these combination systems as an option that is programmed to work with the heat pump’s computerized thermostats.
Other manufacturers, such as Aprilaire, sell their own whole-home air cleaners that can be installed to work with existing heat pumps and other HVAC systems. These cleaners have their own control panels, which allow you to program the timing of the filter to fit your family’s at-home schedule. They also offer apps to control the system from a smartphone.
As with any filter, the pleated material in a whole-home air cleaner will, over time, become clogged with the particles it removes and require cleaning and maintenance. Follow the manufacturer’s recommendations and guidelines to breathe easier and ensure peak operating efficiency.
Send questions to Energy Q&A, South Carolina Living, 808 Knox Abbott Drive, Cayce, SC 29033, email energyqa@scliving.coop or fax (803) 739-3041.