The duct work of a house is how the air from the heating and air conditioning system is transferred throughout the building. Since this is the air that you breathe, it's best if it's clean, which is why there are filters in place. However, if the ducts themselves get dirty with mold or some other contaminant, the filter isn't of any use. You need to clean the ducts to kill the mold.
Things You'll Need
Spin head vacuum with long hoses
Ozone output system
Cover all furniture with plastic sheeting.
Insert the hose of the vacuum into the various vent openings of the house and suck out the dust. Push the hose into each vent as far as possible. Also suck out the air return duct. In the attic, detach the duct from the system and vacuum it out as well. Reattach it when you are done.
Turn the air system off and cover all but one of the vents and air returns with plastic and tape it down. You don't want air getting into the system. Cover the outside unit with plastic as well. The one vent you leave open is for the ozone system.
Insert the hose of the ozone system into the vent. Cover the opening around it with plastic and tape it down
Run the ozone system for at least an hour. Several hours is better. If it's a large duct system, you can rig up two ozone units in different parts of the house on two different vents. Seal around the hoses so air doesn't get in.
Higher levels of ozone are best for killing mold. A 28,000 mg output machine is what professionals normally use for duct work cleaning. However it's possible to use lower outputs if you install two or more machines in separate vents of the system. When you use smaller outputs, try to get them to equal close to 28,000 mg. The cost of the 14,000 mg up to 28,000 mg machines can be in the thousands, but if you shop around, you can find them on sale for less than $1,000. You can also rent the machines from equipment rental businesses or Internet sites.
Ozone can irritate lungs, which is why it's essential that all vents are sealed when the you operate the machine, and no one should stay in the house during the ozone application. When you are finished with the process, open up all windows and doors, then remove vent seals and turn the air conditioning system on. This will disperse the ozone from the unit and allow it to make its way outside. Leave the doors and windows open for at least an hour while the system runs, and stay outside during this time.