Venturi Effect in Duct Work

There is a Terrible Smell Coming from Your Air Vents.  Will a Professional Duct Cleaning Rectify This Situation?

From time to time we hear our technicians talk about the venturi effect in duct work.  Many New Jersey homeowners  are not familiar with this term nor do they know how it affects them.  I would like to explain this phenomenon now and see if I can clear the air.

Duct work generally starts with large trunk lines originating near the furnace. The trunk line(s) are the largest and have the biggest volume of air.  As the ducts get farther and farther from the blower of the furnace they generally get narrower.  This reduction in size also results in a venturi effect; when you narrow something, the velocity of the liquid or gas passing through it will naturally increase.

Duct work in a NJ home or commercial structure is not airtight.  Even if the heating contractor is extremely conscientious during the installation, there are going to be some spots through which air can pass.  Seams, bends and other connections will not be completely sealed.   The venturi effect can also apply here, in a different way.  As air passes quickly through ducts (as mentioned above) the velocity increases as it passes through a narrower and narrower opening.  If there are any openings or gaps along the sides, air from these gaps is going to be sucked in to the passing,  moving air.  Logic would have you assume the opposite (that the air would be pushed back away from this moving force, but instead it is sucked in to the flow by the “venturi effect”).   Based on this principle, if you have any kind of odor or smoke outside of your ductwork, and if there are any openings in the duct (big or small), that odor will be drawn into the flow of air and thus distributed throughout your home.  This is why it is really difficult to isolate where a smell or musty odor is coming from.  If you duct work passes through a crawl space, basement or attic, odors from these areas can be drawn into the air and then disbursed throughout your home.  The only way to truly clear up the odor is to eliminate the source completely. If it is not eliminated, and if you clean the ducts, the smell will just continue to be drawn into your system.

Viruses, bacteria, mold are all airborne contaminants and these too can be picked up by the venturi effect. A genuine concern for homeowners is the transmission of the avian flu.   Birds can nest in areas in which your ducts pass through, such as an attic or crawl space.  Avian influenza is rare in humans but if it is contracted, it is often deadly. More than half the people who catch the avian flu will die from it.  Another rare airborne virus but of concern is the Hantavirus.  Hantavirus is a respiratory illness that humans can get from ingesting dust contaminated with the feces of rodents such as mice or rats.  Like the avian flu, your HVAC system can pick up this virus from dust in your attic and crawl space and draw it into air where it can make you ill.

Again, the only true solution to thoroughly remove these and other contaminants is to first thoroughly eliminate the source and to clean out the ducts

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