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• Cause, Effects, & Prevention of Corrosion in HVAC Ducts
• Health Risks from Wet Air Conditioning or Heating Ducts
• Duct installation defects, safety hazards

This chapter of "How to Inspect the Central Air Conditioning or Cooling System" describes Wet or Corroded Heating & Cooling Ducts: Cause, Health Concerns,& Repairs to correct this condition. Rusty metal ducts or corroded HVAC ducts are an indicator of improper and possibly unhealthy conditions in the building as they are almost always associated with unwanted moisture in the duct system. Unwanted moisture in cooling ducts in particular, is an invitation to mold, bacterial, or other pathogens in the duct system and thus in the building air. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

This chapter continues with a discussion of corroded metal ducts and registers and associated health concerns. We point out and include photographs of locations where you can spot unwanted HVAC or cooling system moisture and rust, cite some related health concerns, and discuss ways to avoid duct corrosion or rust. We also offer advice on what to do about rusty ductwork. The master document, of which this is a chapter, describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects.

CORRODED DUCT WORK - Corroded Air Conditioning Duct Work

The photo at page top shows a very rusty air conditioning ductwork register, and a careful look into that system of metal duct work, found additional heavy rust in the duct system along with lots of debris. The photographs shown just above are of a less obviously rusty duct system and the rust stains on the vinyl floor suggest that the water in this duct system came from the floor level.

Are Rusty, Dirty Air Conditioning Ducts a Health Concern?

Not necessarily: To avoid inappropriate anxiety on the part of any readers we state up front that it is normal for some dust and debris to accumulate on the inside of heating or air conditioning ducts, and this material is not necessarily a hazard to building occupants. But when ducts have also been wet, or when ducts are constructed of materials like fiberglass insulation that have been damaged or can't be cleaned, some potential health or respiratory issues may be present, as we discuss further here.

Rust flakes from rusty heating or air conditioning ducts themselves are unlikely to be much of a health hazard - these particles are pretty big, not easily airborne, and probably won't be found at high levels in indoor air except in unusual circumstances. But rust in ducts is a problem indicator, showing quite clearly that the duct system has been wet.

The chief components of house dust, which will certainly collect within a duct system include fabric fibers and skin cells, often also including starch fragments and other organic debris. The combination of organic debris within a duct system and water (indicated by rusty ducts or duct registers) indicates a possible risk of mold or bacterial hazards within the air conditioning or heating system. Since blowing air through the system can pick up and distribute these hazards to occupants of the building, wet or previously-wet duct work is a potential health hazard to building occupants.

The actual health hazard level from wet or previously wet air conditioning or heating ducts in a building depends on:

• The total area of duct or other surfaces that have been wet, including both duct system interior and the interior of the air handler or fan/blower unit and evaporator coil surface
• The level of organic debris within the duct system
• The materials used to insulate the duct interior: fiberglass and similar materials form a "sticky" surface that tends to hold debris and also to act as a sponge holding moisture, possibly increasing the risk of mold or bacterial hazards, and not easily cleaned.
• The frequency of wet conditions: recurrent wetting obviously increases the risk of pathogens in the duct work, though we've seen significant contamination as a result of a single building flooding event which wet the ducts
• The health vulnerability of individual occupants in the building: elderly, immune-imparied, asthmatics, infants, people with allergies, people with COPD or other respiratory illness, are more likely to be bothered sooner by duct contaminants that are produced by wet conditions in the air conditioning or heating system.

What To Do About Rusty HVAC Ducts, Wet or Previously Wet Fiberglass Ducts or Flex-Duct

Metal ducts that have been wet but are not severely damaged can and should be professionally cleaned. Be sure that the interior of the air handler is also inspected and cleaned.

Where you observe metal duct system components which are quite corroded, badly corroded materials should be replaced. This should not involve significant expense if only limited sections of readily-accessible duct work are involved.

Leaks into ducts routed through inaccessible building areas or cavities, and leaks into duct work which has insulation, particularly fiberglass insulation, in its interior, (typically fiberglass board), may harbor a mold colony, leading to both significant cleanup costs and potential health concerns.

Fiberglass-lined ducts or flex-ducts which have been wet should be inspected thoroughly, and sections which were wet and/or are particularly soiled should be replaced.

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