IAQ – Allergies and asthma – Return Air Duct

SneezeIndoor Air Quality

Allergies and asthma are triggered by the body’s reaction to substances in the environment. Sometimes the cause is:

  • known from experience
  • told to you by your physician
  • unknown

I am starting a series of short articles on these problems and how they relate to air conditioning. This guide will allow you to investigate it yourself or to look over the shoulder of a technician. There are many deceptive practices in the field of mold remediation; some of it is just overkill on a problem; some is dishonest; some is inattention to the problem. As in all issues of your health, it is up to you to keep plugging away and asking questions until you find your answer.Heater Closet - 1

Our interest here is to ascertain how well the system is sealed from the outside air and that air’s entrained moisture and contaminants. The first place to look is the largest air grille in the system, the return air grille. Starting at the return grille to the supply grille where the conditioned air enters the home, all surfaces inside the air conditioning system are shielded from view and cannot be cleaned by the homeowner. The purpose of these articles is to explain what goes on in these spaces.

At this location you should see an air filter. Many homes have the filter in the furnace or air handler and not here at the return air grille. Having a filter at locations other than the return air grille is universally sub-standard. Not having a filter at the starting point of the system causes heavy accumulation of household dust under the furnace and in the return air plenum or return air duct.

Open this grille and use a flashlight to look around the space.

1. If the furnace is in the closet above you –
The only holes should be the ones for the filter and at the bottom of the furnace; the rest of the space should be ‘bubble-tight’. This is very important. When the filter is in place, the plenum is at a negative pressure and will pull air in through every crevice. As the filter loads with dirt, this negative pressure increases. This intruding air is unfiltered, humid and from the attic.

If there are holes, the repair for this does not require high skills; it is pretty tough getting in and out of the hole amongst the piping nonetheless.

The preferred material is 1/2″ thick sheetrock but that is during construction. After the fact, we use 1/2″ styrofoam exterior building sheathing and expanding foam. We are attempting to build a smooth, sealed, cleanable enclosure between the return air grille and the furnace.

2. If the furnace is under the house or in the attic –
Most homeowners are out of their depth at this point. This space behind the return grille is an extended duct to the furnace. Reliable repairs to duct material require developed skills and only the handiest of homeowners should make these repairs. As with the closet mounted furnace, this section is at a negative pressure and pulls in outside air if defective. Here is a good tip for anyone with a furnace in the attic. If you are spry enough to walk the ceiling joists of an attic or it is decked, look at the point where the return duct rises through the ceiling to the attic. There will be insulation covering this location. If the insulation is dirty in a ring 4-6 inches away from the duct, you have found an important duct leak. The negative pressure from the duct is pulling air through the insulation. The insulation filters dirt from the air and becomes darkened with age.

Modern air conditioning works best when it doesn’t have to process new air. It recycles the same air through the system removing moisture and heat and contaminants. In a normal house, clothes dryers, range hoods, bath fans and door traffic supply plenty of fresh air.

If you are going to win the battle with mold you have to deny it the things it needs to live:

  1. moisture
  2. darkness
  3. food

If you live near me, your summer consists of 8 months of 90% or higher relative humidity. Mold cannot grow in an environment of 55% relative humidity or less. When we replace indoor air conditioning equipment, we always improve the seal; very often we find huge holes that have been pulling attic air into the house, sometimes for decades.

If you will follow the things I post on here in the coming weeks you can educate yourself about mold allergies related to duct systems. This treatment of the return duct is just the start of the process but I will take others in turn. Some houses are easier to correct than others but the causes of mold allergies are known causes. There is no reason for you to suffer in your home from allergies caused by your home.


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.

One Response to IAQ – Allergies and asthma – Return Air Duct

  1. Pingback: IAQ – Allergies and asthma – Indoor AC Equipment | SwinsonAC's Blog

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