IAQ – Allergies and asthma – Ductwork

Mother_kidsIndoor Air Quality

In the previous article on Indoor Air Quality, the indoor machinery of your air conditioning system was covered. This installment covers the remainder of the air conditioning system, the ductwork.

I was going to include a photo of a dirty section of ductwork but decided against it. There is much misplaced concern about mold in ductwork and I don’t want to add to it.  Most of the dirt and mold to be found has been covered in the two previous articles. Your ductwork’s cleanliness depends on, more than anything else, changing your air filter. Filter changes should be done on a regular basis, starting as soon as your air conditioning system is installed. If you have recently purchased your home and do not know the history of the air conditioning system maintenance, I’ll explain later in this article how to have your ducts checked by a professional or to self-assess your duct system. During most visits requesting that we check the duct system, we find a minimal amount of foreign matter in the ductwork. Those that are dirty can be cleaned by equipment operated by trained professionals. (Some ducts should not be cleaned. I’ll expand on this later.)

You can have your ducts checked by an AC technician or check them yourself. To check the ductwork yourself, get a ladder and remove a register. If you aren’t steady on a ladder, leave this to a technician. Take a white cloth and rub the inside of the duct surface. What you see on the cloth is a good representation of the condition of the rest of the ductwork. If there is anything dark on the cloth, put the cloth in the kitchen sink and pour a few drops of chlorine bleach on the dark debris. If it disappears, its mold; if not its dirt.

This self-assessment will tell you about everything except the first few feet of duct after the cooling coil, near the blower section. Air coming off the cooling coil in summer is at 100% relative humidity. It is saturated with water and this is an area for mold growth inside the duct. This area requires cutting and patching of the ductwork and should be left to a professional. He can cut a hole, photograph the interior for your viewing, and patch the hole. If there is mold present, it can be spot treated very effectively without a complete duct cleaning. Actually, duct cleaning will not correct mold problems in this area.

You want to know two things prior to deciding on duct cleaning:

  1. how dirty your ducts are
  2. the material used in constructing your ductwork.

The most common methods of duct fabrication use the following materials:

  1. Fibrous glass board
  2. Sheet metal internally lined
  3. Sheet metal externally lined
  4. Flexible ducts

1. Fibrous glass board – The most common material used in new construction in the past thirty years is fibrous glass ductboard. It is made of glass fibers formed into a sheet that is fabricated into rectangular ducts. On its inner surface is an adhesive binder that is porous. It deadens sound and prevents the glass fibers from being entrained in the airstream during duct operation.

2. Sheet metal duct internally lined – Rectangular metal ducts internally lined with a fiberglass material similar to ductboard and a fabric inner face.

3. Sheet metal duct externally lined – Round or rectangular metal ducts wrapped with fiberglass insulation and a vapor barrier of foil or kraft paper.

4. Flexible duct – Round plastic spiral wire reinforced liner wrapped by fiberglass insulation and foil coated mylar plastic outer cover.

There are two types of equipment in use by companies that specialize in duct cleaning:

1. Rotary Brush/vacuum – This is a powerful vacuum cleaner with a concentric shaft driven rotary brush. The brush stirs up the dirt and the vacuum sucks up the loosened debris.

Pros – Relatively inexpensive method of cleaning Sheet metal externally lined ductwork. Rotary brush vacuums are the most common machine used to clean ducts.

Cons – Internally lined duct and fibrous glass board duct can have its inner adhesive binder surface permanently abraded by rotary brush cleaning. This leaves loose fiberglass to blow into your home for as long as you have the ductwork. Also, internally lined ducts are porous and if mold is present and a problem, brushing the surface is ineffective. Flexible ducts can be damaged by the brush without the awareness of the person cleaning the ducts.

2. High volume HEPA blowers/compressed air – This equipment uses high-velocity/high-volume blowers to vacuum the duct system while a high pressure air wand blows debris backwards toward the built-in HEPA filtration system.

Pros – Very effective at removing loose debris without contacting the surface of the ductwork. Works on all types of ductwork except flexible ducts.

Cons – Costs more to purchase and operate this equipment. I don’t know of any companies in this area currently using this method.

If you have ductwork that is indeed very dirty, I recommend replacement. This is an expensive process; it takes more than three times the manhours to replace ductwork in an existing home than during new construction. Dirty flexible ductwork can be replaced at about the cost of cleaning. New flexible ducts have better insulation and can be sealed better for an improved system. If internally lined ductwork is so moldy that it must be replaced, wintertime is the best season to have this work performed.

So what now? Your options depend on your goals. In order of decreasing costs, they are:

  1. Replace the ductwork
  2. Clean the ductwork
  3. Install ozone or peroxide generators
  4. Install UV lights
  5. Nothing

Replacing the ductwork – Price range about $2.50 or more per square foot of heated and cooled living space when performed during winter. Summertime duct replacement is quite a bit more.

Cleaning the ductwork – Price range about $.50 per square foot of living space (for rotary brush method).

Ozone Generator/UV light – Price range $200 unshielded/$450 shielded per AC system. NB – Unshielded UV light can destroy fibreglass ductboard, metal duct liner, plastic condensate drain pans, motors and wiring. We commonly see these in areas that they should not be installed. Unshielded UV lights are bare bulb devices designed for use around metal cooling coils or unlined ducts. UV lights generate ozone which is an oxidizer. Oxidizers such as ozone kill mold and suppress its propagation. Shielded UV lights are bulbs within an enclosure designed to be placed directly in the airstream of lined ducts.

Peroxide Generators – $750.00


For allergy sufferers, the word ‘mold’ itself brings a reaction. Mold is a type of fungi that is present everywhere in South Alabama. It’s propagation is carried out by spores. The moldy odor is a signal to us that a wetted surface is present. The odor from the blower and cooling coil is the most common source of complaint in a home. That is not to say that ductwork can’t have mold.

If the conditions for mold growth – humidity, darkness, and bacteria – are not present, mold will not grow and you will have no odor. Extant spores in the duct will remain dormant causing no symptoms for the occupants.

In Summary

For every endeavor, budget and cost will restrict the available options. Of primary importance in a project to control mold, mold odors, and dust is the air filter. Bigger is always better when it comes to air filters. Next is the cooling coil in the air handler or furnace. If this is dirty, there is no substitute for having it cleaned. As regarding ductwork, if it is ‘clean enough’, peroxide generators will kill mold and odors. Peroxide generators seem expensive at $1,000 per device but when they allow you to achieve a symptom-free home environment, precluding the expense of ductwork replacement, they can be a bargain.

This article concludes the discussion of control of mold and allergens by your air conditioning system. If your home is of sound construction, you can achieve a clean, healthful environment inside your home. Your air conditioner can help you keep it that way.


About Swinson AC Co

Residential Air Conditioning and Heating Contractor in Central Baldwin County Alabama and the Eastern Shore 964-9009 990-0998
This entry was posted in Air Conditioning, Allergies, Asthma, Ductwork, Mold, Odors and tagged , , , , . Bookmark the permalink.

1 Response to IAQ – Allergies and asthma – Ductwork

  1. Johanna says:

    The air conditioning maintenance London service providers always suggest keeping the system
    neat and clean. These modern units keep the house warm without all the work of
    cutting and splitting wood. Among them, Pescod decrease of 8%, 10%
    drop Hisense, Chunlan drop 12%.

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