A central air purifying system, added to existing HVAC equipment, is not the cost effective solution for household air quality. Unless you are wealthy or building a new, carefully custom designed home, this cash is best spent on other aspects of the indoor air situation.

I do not claim a whole home air filtration system is not a good idea for those with extra cash. But it is not a substitute for room air purifiers. This article will focus on the shorcomings and cost effectiveness of air purifiers for an entire home that add on to existing heating, ventilation, and air conditioning systems.

Notice that air duct ultraviolet light systems, for defense against microrganisms, are considered seperately.

Consumer Reports, in a 1992 issue, says portable air cleaners are more effective than a central system.

Since such a filtration system shares the central system's blower and ductwork, the first concern must be the wholesomeness of the HVAC (Heating, Ventilating, Air Conditioning) internals. In my opinion, dirty HVAC systems and ductwork are second only to wet, moldy basements, in the “Nightmare on Elm Street” awards.

How many of us ever remove the vent louvers and inspect our ductwork, raise your hands.

That's not very many.

So how many want to do it today?

Oh, that's better.

Who wants to go first?

Nobody?

OK, I'll do it.

In the photograph, taken in my Houston apartment, the red arrows point to accumulated fine dust which has come through the fiberglass air filter over the years. The blue arrow indicates a new section of duct wall which has less dust. Because this dust has been screened by a coarse air filter, whitening of the duct wall is the more dangerous fine particles! The lower red arrow points to dust I piled up with my finger.

Yuck! Are we done yet?

Fiberglass HVAC air filters are placed upstream of motors and fans to protect them from sandpaper-grit sized particles and lint, not as air purifiers. No expensive upstream filter or partial bypass whole house air cleaner can do anything about this accumulated mess!

HVAC Blowers

HVAC blower systems, while they may seem powerful, with maybe 1000 cubic foot per minute (cfm) airflow, do not set up a strong enough air current in most areas of the household to be meaningful air purifiers. Stand near the airflow from an AC duct, you won't get blown away. The airflow right at the duct is about as strong as a small table fan.

A quality portable air purifier creates strong internal pressures. Air-Purifier-Power is focused on how this pressure is handled in air purifiers of varying quality, and how many builders solve the problem by allowing bypassing.

Air from central system registers does not flow evenly through the cross section of the home, it seeks the path of least resistance. Cooled air makes a beeline straight to the bottom, without visiting the out of the way nooks and crannies.

Warmed air fans out along the ceiling until it finds a cool surface to start a downdraft. Where corners, steps or obstacles are encountered, eddies form and larger particles fall out. While the house as a whole may receive several air changes per hour, the net result in the lower flow areas is not strong enough.

Imagine a meandering stream, with surging bank cuts and rapids, followed by lazy drifts over sandbars; the speed is not uniform across the whole stream. This is not an effective model for air purifiers.

With a central purification system, the blower must run all the time. Residential HVAC blowers are not powerful enough to draw air through a big, tightly sealed, whole house HEPA air filter. In fact they must be protected from overloading which could occur with an ordinary dirty furnace filter.

Real HEPA filters create serious static pressure, so they cannot be installed inline with the standard residential system. Vendors have incorporated costly bypassing vent circuits and more powerful secondary filter blower mechanisms to get around this. This is nothing but a high dollar version of the bypassing I cry about every day in cheap, loosely built department store air purifiers!

Who'd pay to replace what ain't broke?

This has created a marketing opportunity for add-in ionizers, ultraviolet sanitizers, and electrostatic precipitators, which are less restrictive. Some of these create ozone.

Cleaning your air conditioning air handler system and duct work could be the best single indoor air improvement money can buy. It depends on how old the system is and when (if) it has been cleaned before.

Don't assume a new home has clean duct work. Air ducts will collect dust during construction. Site cleanup is a hidden cost on every job, and with subcontracted crews coming and going, you never know what might be in hidden areas of a new house. You can hire a professional duct cleaning service to vacuum and clean the air vents and ducts.

Add Air Purifier when Clean?

Once the system is clean, install an upgraded quality air filter in the HVAC main air return or air handler. These are cheap: 3m Filtrete are widely available. I line the edges with polyurethane foam to limit bypassing. Regular maintenance will keep your system clean in the future.