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AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FLOW MEASUREMENT CFM
APPLIANCE EFFICIENCY RATINGS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM & DUCT DEFECTS
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
Air duct leaks, condensation, flooding or wetting = contamination: this article 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. 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.
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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.
Readers should also see WATER & ICE IN DUCT WORK and see FROST BUILD-UP on AIR CONDITIONER COILS where we describe build-up of ice on the cooling coil in air conditioning air handler units.Also see TRANSITE PIPE AIR DUCTS where we describe flooding of HVAC ductwork in slabs (SLAB DUCTWORK) .
Excessive indoor humidity and its related mold, dust mite, or bacterial hazards may be traced to either a cause or an effect of high moisture inside of an air conditioning system or even heating system duct work or air handler. Here are some common examples:
Excessive indoor humidity traced to wet air ducts caused by high velocity air conditioning system coil condensation blow-off
Mike, gold star to you for good detective work. Like other areas in the Southern U.S., Dallas/Fort Worth experiences seasonal periods of high humidity that can result in an extraction rate of water from building air faster than the design of the air handler and condensate drain system can handle it.
More about dehumidification problems traced to central air conditioning systems can be read at DEHUMIDIFICATION PROBLEMS.
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.
Our flooded air duct photo (above left) shows ductwork with a layer of mud - this home was flooded to a level that included ceiling-mounted basement air ducts. The ducts needed to be cleaned and sanitized.
Soil particles themselves may not be a serious health hazard but area flooding often brings pathogens (such as bacteria or viruses found in sewage) into buildings and can form a serious health hazard. Details are at FLOOD DAMAGE in DUCT WORK.
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.
Dust & normal air duct debris: 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.
Watch out: 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 these factors:
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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Frequently Asked Questions (FAQs)
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Question: considering buying a house with very corroded in-floor rusty ductwork - can we just spray-seal the duct interiors?
I had a duct inspection which revealed severely corroded and rusted ducts with some holes through the ducting.
I have heard of a product by the Duct Seal Technologies in which the inside of the ducts are sprayed with a coating of kevlar fibers and some bonding agent. Do you know anything about this process? Thanks, Doug Spence - Spence 6/19/2011
Reply: in-slab HVAC ducts are asking for trouble from flooding, vermin, water, mold, etc.
About buried or in-floor HVAC ducts: as we suggest in the article above, even if you could reliably reline your in-ground or under-slab air ducts (and how without a detailed inspection would we know that the re-coating or relining was complete, and that it remained intact?) there remains a concern with radon gas or water leaks into the duct system.
And we are doubtful about the reliability of this approach: spray-on coatings don't bond well to dirty, rusted, corroded surfaces and may fail to seal large penetrations or holes; And even a well-sealed spray-coated in-slab HVAC duct, if subjected to future flooding from ground water or plumbing leaks, or invasion by rodents or other pests, is simply going to have new contamination in its interior.
A better, but usually much more costly and disruptive solution is to eliminate the in-slab ductwork by rerouting it through the building.
If you go ahead with the process I'd recommend first an inspection for evidence of a history of duct flooding, and an annual inspection of the duct condition until you are confident of its dryness and cleanliness.
Also take a look at TRANSITE PIPE AIR DUCTS for more examples of problems with heating or air conditioning ducts found in slabs.
Question: does use of bleach or scented candles corrode our air ducts?
I was told products such as clorox, scented candles and sprays cause corrosion to air and heating lines. Is this true? - June Walker 10/2/11
June, it's true that in areas where high levels of chlorine gas are present, such as around indoor swimming pools, corrosion of metal HVAC ducts and even evaporator coils can be a serious problem.
Scented candles? I am very doubtful that in normal use you could put enough corrosive material into the air to affect the ductwork. But indeed use of scented candles can be an indoor air quality problem source for some people such as asthmatics, and the soot deposition on indoor surfaces can be a cosmetic problem as well. (See THERMAL TRACKING & HEAT LOSS).
Question: Mold on ceiling traced to condensation in the ductwork?
I have a condo that the garage is below the living space. My tenant noticed there is mold on the garage ceiling. One of the neighbors said they had the same problem and she had it fixed.
he was told it was condensation in the ductwork. Water was damaging the drywall and leaking downward. Of course, mine is worse. What is causing this? Could this be a problem in the construction? The units may be about 10 years old. - Carol 6/20/12
Carol from the very limited information your note alone I can't be confident in a "guess" at what the problem is in your home and can only answer in general. An A/C duct that is not insulated is more likely to produce condensation inside the ductwork than otherwise, especially in humid weather.
Such condensate, if it collects in the ductwork where it does not belong, instead of in the air handler where it is drained away, can wet duct insulation (if there is some), and can leak onto or into building surfaces.
Moldy drywall is best removed and replaced, allowing you also to check to be sure the wall cavity was not also wet (and moldy).
Ask your HVAC service company to inspect the ductwork, fix the cause of condensation, help you decide if the interior of the ductwork needs cleaning AND IF it can be cleaned at all (fiberglass duct interiors don't tolerate cleaning). WHen that problem is cured, then address the rest of the building.
Question: is it ok for a window A/C unit to run continuously for 30 minutes or longer?
Is it ok for a window unit to stay on cooling for 30+ minutes or will this break the unit, see its above 96 degrees all the time outside so its hot hot and since I cleaned the evaporator coils & moved thermostat accidentally I swear it doesn't cut off & on like before but I just don't want damage to unit and also coils were dirty before so maybe that's why it cycled quicker then anyway please help - Christie 7/25/12
Under normal conditions (that is for example no abnormal electrical voltage levels, proper wiring and installation) a window air conditioner could run continuously, 24-hours a day, without damage to the equipment itself. Of course if the air filter becomes debris-clogged and is not cleaned the system efficiency and effectiveness will decrease, and of course your electric bill will reflect that continued operation.
See WINDOW / WALL AIR CONDITIONERS for details.
Questions & answers or comments about the cause, detection, effects-of and cures for wet, flooded, or water contaminated ductwork.
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Technical Reviewers & References
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