Our company recently surveyed all of our foremen so that we could better serve our clients. We wanted their experienced opinions of why HVAC systems get dirty in the first place. What are the contributing factors?

The foremen are all seasoned veterans averaging 12 years each as duct cleaners. (Our company has been around more than 30 years.)

Here’s what surveyed out as the top ten reasons:

  1. No filters.
  2. Gapped filters or poorly-fitted filters.
  3. Poor filters.
  4. Filters not changed or cleaned frequently.
  5. Neglect of the units: not inspecting them occasionally to spot dirty build-up or
    a. problems (such as filters not in place).
  6. Dirty environment such as factory, new construction, or highway nearby.
  7. Duct leakage.
  8. Poor or no condensation drainage.
  9. Deteriorated fiberglass insulation.
  10. Leaks in air handlers, such as worn seals around doors or holes in cabinets.


By far, the number one way to contaminate a duct system is to simply omit filters. This isn’t a common occurrence, but when it happens, the duct cleaner has a real mess on his hands. Dirt stacks up rapidly in the air handler and ductwork and is soon pouring into the building.

I recall one disaster where this occurred in a fiberglass-lined duet system. The lining looked like caked mud. A thorough cleaning was factually impossible.

When cleaning such a system, ensure you include with your service a heart-to-heart talk with the building owner or maintenance people. All HVAC systems must be filtered or they will be filthy within a year.


A miniature version of “no filters” is gaps in the filter rack. This can happen in many ways, such as:

  1. Missing filters.
  2. Tears in the filters.
  3. Wrong-sized filters.
  4. Filters warped or bent by air suction.
  5. Worn gaskets in filter frame.

The bottom line is: air takes the path of least resistance. If any open spaces exist among the filters, unfiltered air will happily take that route. The result: rapid dirt build-up.

As a standard part of every cleaning, ensure the filter rack is not a Swiss cheese of gaps and holes. If you spot errors, fix them or call the problem to the client’s attention.


Discussing filter preferences is almost like talking religion or politics. Almost everyone you speak with has his own opinion – particularly filter salesmen.

A former president of the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) stated, “One of the major problems facing the filter specifier and user is the wide range of confusing, exaggerated and/or misleading claims made for filter performance.

This is especially true for the range of filters applied in the residential and commercial market.”
But ASHRAE has established some basic recommendations for filters. One of the ways filters are rated is the “dust-spot” method. This measures how efficiently a filter captures particles in the 0.3-6 micron range. The rate of efficiency is measured in percent, such as a “30 percent dust-spot efficiency.”

For office buildings, ASHRAE recommends filters have a dust-spot rating of 35-60 percent or better. For residential areas such as apartments, dormitories, and motels the recommended rating is 20-30 percent or better. Hospitals should have at least a 90 percent rating.

If a client wants to increase filtration beyond the usual (as above), be sure his HVAC system can handle the extra burden. Higher efficiency filters slow down the air flow and causes the fans to work harder because it’s tougher for air to go through them. Unless you are familiar with the mechanics of HVAC design, consult an HVAC contractor or engineer before dramatically upgrading filters beyond the normal.

Further information on filters is available from ASHRAE (404) 636-8400 and the National Air Filtration Association (202) 628-5382.


Once filters become saturated with particles, a number of problems follow. The filter becomes more a barrier to the air flow. The blower motors strain. The HVAC system puts out less air.

Clogged filters are more likely to tear from the air pressure or warp or get sucked into the air handler. Additionally, dirty filters act as a breeding ground for microbes which can then get into the air stream.

A general rule of thumb is that filters should be changed when dirt becomes visible on the downstream side. Flat panel filters are commonly changed every one to two months, whereas pleated filters are usually on a 3-6 month schedule.


Most homeowners and a great many commercial building maintenance people rarely look inside their air handlers. If they did, they’d see what the duct cleaner commonly sees: clogged filters, torn fiberglass insulation, standing water, frayed fan belts, and maybe a few bird nests.

A simple inspection of the unit every time the filters are changed could improve air quality, reduce the need for cleaning, and extend the life of the air handler.


Sometimes this is obvious and sometimes it isn’t. Two buildings close to a highway may have very different dirt build-up in the ducts due to such things as prevailing winds in the area or which side of the building the air intake is on.

But “dirty environment” is a very clear possibility if you have a client with an inordinately dirty system or who has to call for
duct cleaning more often than most.

Under this same heading is the air intake which is located close to a source of contaminated air. We once inspected a huge air handler for one of the largest hotels in California. It was loaded with lint! Why? The laundry dryer ducts exhausted full blast about five feet below the air handler’s fresh air intake!

Solutions are available, such as extra filtration or rerouting of ductwork. Sometimes several things may need remedying.


This is classic – and rather common. Duct leakage simply means a hole or gap in the ductwork. This normally occurs at the joints where duct pieces are fit together. As a result, dirt from outside the duct can get sucked into the air stream. It soils the ductwork faster and can even cause particulate to come out.

Our company once ran into this on a baffling job at a college. They called us because dirt was blowing out. As a quick fix, we were asked to clean the unit and the supply side of the system, which we did from stem to stern. A few days later, more dirt was coming out.

After several inspections we found the culprit. A dirty return duct was routed right through the air handler above the fans. One of the return duct seams had a big, hidden gap in it. Every time the unit was switched on, the fans sucked a big gulp of dirt from the hole in the return duct and spit it into the building.

Either fix leaky ducts or tell the client about them. They are a real dirt source and energy waster.


What’s worse than a dirty duct system? A dirty, infected duct system.

But microbiological growth requires water. And water is usually only found in the air handler (except in humid climates). When that water begins to stand in stagnant pools, you have a recipe for trouble.

Mold, algae, even bacterial growth accelerates under these conditions. As a result, the air stream can get contaminated and health problems may occur in building occupants.

How bad can this get? We were once called in to handle a school that had standing water in the air handler. But by the time we arrived, the air smelled like fish. And outside the school marched angry, picketing, gas-mask-wearing parents claiming their kids were getting sick – with plenty of TV cameras capturing it all for the six o’clock news.

A firm rule: report or fix poor condensate drainage.


Interior fiberglass insulation can last for a long time, but occasionally it begins to break down in an air handler or, worse, throughout a duct system. This can be caused by a number of factors, such as water damage or improper installation.

Whatever the reason, once it begins, it can lead to a steady stream of particulate coming from the registers.

Some of the fiberglass sealants on the market (such as Foster’s) may solve the problem. In cases of extensive damage, the liner or even the ductwork may need to be removed.


Just as ducts can leak air, so can air handlers, check for whistling or roaring sounds around doors. Look for streaks on the inside walls (from holes or gaps).

New gaskets can cure leaky door frames. Patch any holes found. The upshot of all this is that a good duct cleaner needs to do more than just clean his client’s system. He needs to ask how it got so dirty in the first place. And he needs to fix what he can and report what he can’t remedy.

Oh sure, that means less business from that client – he won’t need your services as often. But customers will love your professionalism. And that will pay off in the end. I know. My company has enjoyed more than 30 years of steady growth from a clientele who swear by our work.

Give it a try.