The position of the U.S. Environmental Protection Agency (EPA) is that “If no one in your household suffers from allergies or unexplained symptoms or illnesses and if, after a visual inspection of the inside of the ducts, you see no indication that your air ducts are contaminated with large deposits of dust or mold (no musty odor or visible mold growth), having your air ducts cleaned is probably unnecessary.” A thorough duct cleaning done by a professional duct cleaner will remove dust, cobwebs, debris, pet hair, rodent hair and droppings, paper clips, calcium deposits, children’s toys, and whatever else might collect inside. Ideally, the interior surface will be shiny and bright after cleaning. Insulated fiber glass duct liner and duct board can be cleaned with special non-metallic bristles. Fabric ducting can be washed or vacuumed using typical household appliances.
Duct cleaning may be personally justifiable for this very reason: occupants may not want to have their house air circulated through a duct passage that is not as clean as the rest of the house. However, duct cleaning will not usually change the quality of the breathing air, nor will it significantly affect air flows or heating costs.
Signs and indicators
Cleaning of the duct system may be necessary if:
Sweeping and dusting the furniture needs to be done more than usual.
After cleaning, there is still left over visible dust floating around the house.
After or during sleep, occupants experience headaches, nasal congestion, or other sinus problems.
Rooms in the house have little or no air flow coming from the vents.
Occupants are constantly getting sick or are experiencing more allergies than usual.
There is a musty or stale odor when turning on the furnace or air conditioner.
Occupants are experiencing signs of sickness, e.g. fatigue, headache, sneezing, stuffy or running nose, irritability, nausea, dry or burning sensation in eyes, nose and throat.
In commercial settings, regular inspection of duct work is recommended by several standards. One standard recommends inspecting supply ducts every 1–2 years, return ducts every 1–2 years, and air handling units annually. Another recommends visual inspection of internally lined ducts annually. Duct cleaning should be based on the results of those inspections.
Inspections are typically visual, looking for water damage or biological growth. When visual inspection needs to be validated numerically, a vacuum test (VT) or deposit thickness test (DTT) can be performed. A duct with less than 0.75 mg/100m2 is considered to be clean, per the NADCA standard. A Hong Kong standard lists surface deposit limits of 1g/m2 for supply and return ducts and 6g/m2 for exhaust ducts, or a maximum deposit thickness of 60 µm in supply and return ducts, and 180 µm for exhaust ducts. Another UK standard recommends duct cleaning if measured bacterial content is more than 29 colony forming units (CFU) per 10 cm2; contamination is classified as “low” below 10 CFU/cm2, “medium” at up to 20 CFU/cm2, and “high” when measured above 20 CFU/cm2\.
Air pressure combined with air duct leakage can lead to a loss of energy in a HVAC system. Sealing leaks in air ducts reduces air leakage, optimizes energy efficiency, and controls the entry of pollutants into the building. Before sealing ducts it is imperative to ensure the total external static pressure of the duct work, and if equipment will fall within the equipment manufacturer’s specifications. If not, higher energy usage and reduced equipment performance may result.
Commonly available duct tape should not be used on air ducts (metal, fiberglass, or otherwise) that are intended for long-term use. The adhesive on so called duct tape drives and releases with time. A more common type of duct sealant is a water-based paste that is brushed or sometimes sprayed on the seams when the duct is built. Building codes and UL standards call for special fire-resistant tapes, often with foil backings and long lasting adhesives.
Signs of leaks
Signs of leaky or poorly performing air ducts include:
Utility bills in winter and summer months above average relative to rate fluctuation
Spaces or rooms that are difficult to heat or cool
Duct location in an attic, attached garage, leaky floor cavity, crawl space or unheated basement.