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Your Prescription for Better Air
Duct systems are constructed differently depending on the part of the country you're in. Houses built on basements typically have 'trunk and branch' systems, whereas homes built on slabs (think desert southwest) will have an attic air-handler and a 'satellite' duct system.
*Gas-powered air compressors are necessary to get traditional 'trunk and branch' systems cleaned. Electric compressors simply aren't powerful enough to be effective.
'Source Removal' is the only NADCA (National Air Duct Cleaner's Association) approved method of duct cleaning. This involves putting the entire duct system under negative air pressure (suction) at once. This can be accomplished using electric or gas powered vacuums. Both have their uses.
It is best to use gas powered vacuums on homes. This exhausts all of the dirty air to the outdoors, and prevents any cross-contamination. They also have larger filter bags and catch basins for containing the dirt, debris, and other dry filth being dislodged from the ductwork. Thus resulting in less loss of suction as the cleaning process progresses.
Electric duct vacuums are necessary on large or multi-story buildings where it is impractical to run pipe to the HVAC system from a vacuum outside. However, electric duct vacs can quickly become plugged and lose suction. This increases the risk of letting dirt out into the building. There is also a risk of cross-contamination from disassembling the component parts of the vacuums for transporting them. While being disassembled, the filter compartment is separated from the motor compartment and the dirty filters are exposed to the open air.
You could equip an unskilled technician with a gigantic gopher-sucking vacuum (like the one I started with)....and they would still find a way to blow dirt and filth all over inside the building. *It is the skill level of the duct cleaning technician that matters more than the size of their vacuum.The technician's 'house count' is more indicative of the quality of service than any measure of CFMs (air volume) or Static Lift (air velocity) of the machine they're using. So when they start bragging about their vacuum...ask them how many houses they've cleaned the ducts in (the technician working at your house, not the company).
Dirt sticks to the inside of ductwork, so a simple blast of air won't dislodge it. It takes a variety of brushes and pneumatic agitators (whips and skippers) to actually get the ducts clean. Whip tentacles tend to swirl around inside of round pipes, so round brushes are needed to get these branches clean. Conversely, round brushes won't get rectangular trunks clean, so whips are needed.
There's no such thing as a duct vacuum so large and powerful that the dirt just magically flies loose from the duct walls when it's started. That's not how it works. The duct vacuum is only there to keep dirt and debris from flying out into the house while the ducts are actually being cleaned by other means. The type of duct vacuum required depends on the accessibility to the ductwork and the skill level of the technician performing the work.