Whole House Air Purifiers, Filters and UV Lights in Oakland County

MERV ratings: which furnace filter should you use?

Furnace filters are graded on the MERV scale, which runs from 1 to 16. The higher the number, the smaller the particles the filter can catch. The EPA's advice for homes is to use MERV 13, or the highest rating your system's fan and filter slot can accommodate. The second half of that sentence matters as much as the first.

A dense filter resists airflow. Put a MERV 13 one-inch filter in front of an older blower and you can starve the furnace of air, which can lead to overheating in winter and a frozen coil in summer. That is why we check the blower before any filter upgrade. As for HEPA, a filter that dense generally will not work in the filter slot of an ordinary furnace.

• MERV 8: at least 70 percent of particles from 3 to 10 microns, the size range of much household dust and pollen
• MERV 13: at least 90 percent of that size, 85 percent of particles from 1 to 3 microns, and 50 percent from 0.3 to 1 micron
• HEPA, for comparison: 99.97 percent of particles at 0.3 microns

Why a thick media filter beats a one-inch filter

A media filter is several inches deep and folded into many pleats, so it has far more surface than a one-inch filter of the same height and width. Air spreads across all of that surface and passes through more easily. You get a high MERV rating without the airflow penalty, and the filter typically lasts much longer between changes because it has more room to hold what it catches.

The cabinet matters as much as the filter. It is sealed to the duct and closes with a gasketed door, so the air cannot sneak around the edges the way it does in a loose filter rack. For most homes with a forced-air furnace, this is the upgrade that gives the most for the money.

Electronic air cleaners and UV lights, honestly

An electronic air cleaner gives particles an electrical charge and collects them on plates that need regular washing. It can capture fine particles, but the EPA notes that some electronic cleaners and ionizers generate ozone as they work. If you already own one, we will service it. If you are buying new, a media filter does a similar job with nothing added to the air.

UV lights are sold hard, so here is what the EPA has found: typical home UV lamps have limited effectiveness against bacteria and mold, and there is no standard way to measure how well a UV cleaner works. A UV light also removes nothing from the air. Dust, dander and smoke pass straight by it. Treat a UV lamp as an optional extra after good filtration, never as a substitute for it.

Whole house or portable air purifier?

A system in the ductwork covers every room with a supply register and needs attention only when the filter is due. Its limit is that it cleans only while the blower runs. On smoky days or in pollen season you can set the thermostat fan to ON or CIRCULATE to keep air moving through the filter.

A portable unit makes sense for a single bedroom, or in a house heated by a boiler, with no ducts at all. Neither kind deals with gases and odors unless it has a carbon stage, and neither changes humidity or replaces stale air. Those are separate jobs, covered on our indoor air quality page.