When HVAC systems need cleaning

Out of sight, out of mind is not a good excuse for avoiding the task.

By Dan Stradford

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The heating, ventilation and air-conditioning (HVAC) system is the respiratory system — the lungs — of a building. Return ducts take in air from the occupied space and send it through an air handler, where it is warmed or cooled and returned to the occupied space through a network of supply ducts. A well-maintained system removes the stale gases and provides a steady stream of oxygen-rich, comfortable, odorless air for the occupants. But where there’s air, there’s particulate.

Why be particular about particulate?

According to the Air Quality Management District near Los Angeles, an average cubic inch of local urban air contains about 250,000 particles. This reduces to 25,000 to 50,000 particles near the coast and increases to a million or more near highways. Even a cubic inch of the purest air at mountain peaks and over the center of the ocean holds thousands of particles.

With the rise of nanotechnology, the primary focus in the medical world has been on ultrafine particles known as PM2.5 — particulate matter with a size of 2.5 microns and smaller. These aren’t good for one’s health. High exposure to PM2.5 and larger particles has been linked to an astonishing array of physical ailments, including:

  • Strokes (Annals of Neurology, May 2008)
  • Blood clots in leg veins (Archives of Internal Medicine, May 2008)
  • Wheezing in infants (Thorax, August 2008)
  • Alzheimer’s and Parkinson’s (Environmental Factor, May 2008)
  • Clogged arteries (Genome Biology, July 2007)
  • Heart risks in young adults (American Journal of Respiratory and Critical Care Medicine, August 2007)
  • Premature birth (American Journal of Epidemiology online, November 2007)
  • Premature death (Thorax online, July 2007)

As air is drawn through an HVAC system, particulates gradually begin to collect on the interior of the ductwork, fan, fan housing, coils and other parts in contact with the air stream. As time goes on, the particles can coat the surfaces to a thickness of three inches or more. Although contaminant buildup usually isn’t more than 0.5 inches deep, sometimes it can be several inches. A two-inch buildup on the bottom of a 12 inch x 12 inch duct makes it a 10 inch x 12 inch duct. This narrowing of the flow area produces more resistance to air going through, adding to the energy bill. A Japanese study in the 1980s, however, found that when the contaminant coating thickness reac hes 0.03 inches, the particulates start to peel off and reenter the air stream.

HVAC systems get dirty over time, but at variable rates. Contaminant accumulation rates and peel-off rates are determined by a variety of factors, including duct size, air speed, air volume, particulate load in the air and particulate size. Some systems accumulate large amounts of debris and some reach a point of near equilibrium, where the rate of accumulation is only slightly higher than the rate of peel-off.

Energy costs

Before and after.

One of the first things to be affected by particulate buildup is energy cost, particularly from the heating and cooling coils. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) found that simply cleaning the coils after a year of use produced considerable energy savings by increasing the cfm by 14% and by improving heat-transfer ability by 10-25% (ASHRAE Journal, November 2006). But many plants allow their HVAC systems to go years without a good coil cleaning, so it seems reasonable to conclude that industry is missing out on greater energy savings. As time goes on, accumulation on other parts of the system adds to the energy loss.

Some fan blades are slightly cupped to make them aerodynamic. Accumulated dirt alters the fan-blade profile to render them less efficient and they pull fewer cfm. Also, dirt adds weight to the fan, so more energy is required to turn it. A clean fan is cheaper to run. Turning vanes at duct elbows collect particulate matter and all manner of debris over time. I’ve seen cardboard, pieces of insulation and dead birds clogging the elbows.

Clogged registers also introduce inefficiencies. Besides particulate buildup, after five, 10 or 20 years, a lot of foreign objects can find their way into an HVAC system and get caught behind the registers. Because air flow can be restricted in so many ways by particulate buildup, a cleaning can cause a dramatic increase in cfm, particularly in older systems that have never been cleaned.

Preventive measures

You have control over the rate of dirt accumulation in your HVAC system. The following items are the primary contributors to system contamination and efficiency loss:

  • Missing filters — not common, but it makes a system dirtier faster
  • Filters poorly fitted or with gaps between them
  • Improper filters — use those recommended by the OEM
  • Dirty filters and failure to clean the filter rack when changing filters
  • Neglecting the air handler — not inspecting periodically to spot functional problems
  • Dirty operating environment
  • Duct leakage — gaps at duct joints, where unfiltered air can be drawn in
  • Poor or no condensate drainage in the air handler
  • Deteriorated fiberglass liner — friable, small or large pieces get into the ductwork
  • Leaks in air handlers — worn door seals or holes in the cabinets

Monitor the symptoms

Unless you’re in a clean environment, such as a hospital, where HVAC cleaning is part of routine maintenance, you might wonder how to make an educated decision on when to clean a system. Here are some basic guidelines.

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