Ten Questions about Duct Cleaning

This Engineering Control Column provides information on duct and HVAC system cleaning.

Abbreviations: IAQ = indoor air quality; HVAC = heating, ventilating, and air conditioning; NADCA = National Air Duct Cleaners Association

What is "duct cleaning"?
Ductwork sometimes can become both the source and the pathway for dirt, dust, and biological contaminants to spread through the building. In this case, duct cleaning usually means the removal of dirt, slime, mold, debris, and other materials found in ductwork and other HVAC components (e.g., cooling coil, drain pan).

When should duct cleaning be performed?
Supplier schedules.
HVAC equipment suppliers and manufacturers sometimes provide instructions on when and how "cleaning" should be performed. Cleaning schedules typically depend on operating schedules, climate, filtration used, air contaminants, costs, and building occupant expectations.

New ductwork. When new systems are being installed or when older systems are being updated, suppliers should provide access for cleaning system components that are cleanable or replaceable when they become contaminated. New equipment should be kept sealed until installed. If not, new construction or updating of older equipment should always include duct cleaning. New ductwork often contains oil, tools, construction debris, and dirt. These must be cleaned from the ductwork before connection to the air handling system.

Older ductwork. As time passes, ductwork can become contaminated with dust, dirt, debris, mold, slimes, and other contaminants. The presence of these materials does not necessarily mean IAQ problems or complaints will occur. Most ducts have small amounts of dry dust collected on their surfaces--a common occurrence that occasionally requires duct cleaning.

You definitely would want to provide cleaning (or duct replacement) if:

  • there is permanent water damage
  • there is slime growth
  • there is debris that restricts airflow
  • dust is actually seen emitting from air supply registers
  • offensive odors originate in the ductwork or HVAC system.

A duct cleaner says we need cleaning, but how can I know this is the case?
Remember, duct cleaning is a competitive business. You have probably seen their advertisements. Don't be taken in by extreme claims (e.g., "EPA says ducts should be cleaned every year" or "Our customers have reported no allergy symptoms following cleaning," and so forth.)

Before you actually contract for duct cleaning, you should be able to answer yes to these questions:

  • Are there observable or known contaminants in the ductwork?
  • Has testing or observation confirmed their type and quantity?
  • Can or do they (or their odors or byproducts) leave the duct and enter the occupied space?
  • Do you have a good idea of the source of these contaminants? Can the source be controlled? (If not, cleaning is only a temporary measure.)
  • Will the proposed duct cleaning effectively remove (neutralize, inactivate) the contaminants?
  • Is duct cleaning the only (or, the most cost-effective) solution?
  • Have you identified a qualified and reputable duct cleaning firm?
  • Have you checked the firm's references?
  • Has the firm a sensible, sound approach? Do they have the right kind of equipment? Do they belong to and follow NADCA recommendations? (See the next question.)
  • Will the cleaning process protect your HVAC equipment and the occupants of the space during cleaning?
  • Will they give you a guarantee the duct will be clean after completion?

Any "no" answer should delay duct cleaning until adequate answers are obtained.

Are there standards for duct cleaning and duct cleaners?
Currently, official standards and guidelines are not yet available to determine when duct cleaning is necessary. Common sense and the ground rules provided here can help you decide when cleaning is necessary.

NADCA supports a standard of good cleaning practices for its members titled ACR 2005. A NADCA-sponsored certification program has also "certified" hundreds of duct cleaning companies. Check with NADCA (www.nadca.com) for a list of approved members in your area, a list of current publications, and the status of its standard ACR 2005.

Are duct cleaning firms licensed to perform cleaning?
Not at this time. It is a good idea to use duct cleaning firms that have met the certification criteria established by NADCA and that follow NADCA standards.

What actually happens during "duct cleaning"?
Duct "cleaning" typically consists of contaminant removal (e.g., through brushing/vibration plus vacuum cleaning).

If duct cleaning is performed by contractor personnel, ask them about the following typical good practices:

  • Will they keep the ducts being cleaned under negative pressure during the cleaning operation? (This minimizes the discharge of dirt and dust into the occupied space.)
  • Will they protect the duct system? (Avoiding unnecessarily cutting holes in the duct or duct liner, for example.)
  • Will they schedule the cleaning when the building is not occupied?
  • Do they provide at least 10 air changes in the building after duct cleaning and before occupants are re-admitted to the building?
  • Are vacuum cleaning and collection equipment located outside the building? Where vacuum collection equipment is inside, is HEPA filtration provided for vacuum discharge?
  • Is vacuum cleaning coupled with gentle brushing to lift settled materials? (Vacuum cleaning alone is usually not very effective.)
  • If biocides are used, will they select only EPA-registered products and follow label instructions completely?
  • If sanitizers, deodorizers, or chemicals are used, are they sure to rinse and completely remove chemicals before occupants return to the building?
  • Will they avoid using sealants to cover interior-contaminated ductwork? (Sealants have not been shown to be effective as a barrier to microbiological growth, their long-term health effects are unknown, and they may void fire safety ratings.)
  • Are they removing (rather than cleaning) water-damaged or bio-contaminated porous materials?
  • Will they help you develop a contamination prevention program so ducts are not cleaned again?

A duct cleaning firm says it wants to use sealants, encapsulants, disinfectants, biocides, or other chemicals in the ductwork. Is this okay?
Not normally. You should check with your local industrial hygienist to discuss this issue.

What about cleaning fibrous insulation in the ductwork?
Ductwork is often insulated for thermal efficiency and noise control. Insulation generally consists of fibrous material mounted on the interior or exterior of the ductwork. IAQ concerns include:

  • Fibers and fiber coatings flaking off and contaminating the ductwork and the occupied space
  • Microbiological growths within the insulation.
  • Current thinking suggests the following prudent approaches:
  • Remove wet or contaminated insulation.
  • Use only dry vacuuming on dry insulation during cleaning.
  • Don't disturb the insulation during cleaning.

These practices will help minimize the need for cleaning insulation:

  • Keep existing fiber insulation dry and clean.
  • Keep system filters properly maintained and replace them on schedule.
  • Install thermal insulation on the outside of the duct when possible.
  • Avoid interior open fiber insulation in the first 10-15" of the duct as it leaves the AHU. (Substitute noise control approaches may be required.)

What do we do about people in the space served by the ductwork?
During duct cleaning, be sure it is done in a way that protects building occupants (e.g., it is almost always done during non-occupied hours).

What can we do to avoid the need for duct and system cleaning?

Dirt, debris, and microbiological growths can be minimized by:

  • well-maintained filter systems
  • using filters rated at MERV 6 or greater
  • regular HVAC maintenance
  • providing good housekeeping in the occupied space
  • locating air intakes in non-contaminated locations
  • keeping all HVAC system components clean and dry (or water drained at the coils).

This column appears in the May 2006 issue of Occupational Health & Safety.

This article originally appeared in the May 2006 issue of Occupational Health & Safety.

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