AC Ducts And Your HVAC System Part 2
So basically, there are 4 common air duct designs. The first is the plenum or radial duct system. Radial systems have the HVAC unit and return grill at the center of the home with the supply ducts extending out around the plenum like the legs of a spider.
The second type is the reducing extended plenum system. As the name implies, the plenum reduces in size as it extends across with the air ducts coming out from either side.
The third is the extended plenum system. This time the plenum, which does the distribution or removal of air for the HVAC unit, does not reduce in size as it extends across.
The last is the perimeter loop system – a radial system, not as many spider legs, with a supply duct that encircles the building.
Good HVAC contractors calculate airflow, determine the types of ducts to use to meet the home’s requirements, and sketch the air distribution system for proper placement of equipment and duct sizing.
Typical Air Duct Design Flaws
Air Vent Location Not Optimal
When it comes to air vents, “location, location, location” makes a huge difference. If the air vents are in out-of-sight areas like closets, supply air cannot properly heat or cool the room. Likewise, if the vents are blocked by furniture or some other kind of obstruction, your HVAC system will have to work harder to achieve the desired temperature. This wear and tear will shorten your HVAC system’s lifespan considerably, sometimes making air conditioning repairs extremely expensive.
The Central Return Problem
In an ideal world, there would be returns for every room of the house except bathrooms, and utility rooms. But builders like to save on costs. With only one central return duct to feed the HVAC unit, closed doors will starve the furnace or AC and create a negative pressure environment. Dirty outside air then infiltrates the house through fissures, which can be bad for allergies, among other problems.
Too Many Air Ducts Coming Off The Plenum
Plenums, as we mentioned, are boxes connected to the heating or cooling unit that fill with hot or cool air and distribute or remove it, usually at a greater atmospheric pressure. What some call a “ductopus” is a poorly designed radial system with air ducts coming off the plenum in all directions, like the tentacles of an octopus. When too many duct branches are connected, air flow can be all around poor throughout the house.
Long Duct Runs
This is a common problem with the reducing plenum duct system. Not enough air reaches the room at the very end of the supply duct. The solution is usually to install a bigger duct size.
Bad Takeoff Placement
As air moves from a supply plenum or main duct into a takeoff, it changes direction. Airflow has a hard time making turns at high rates of speed, just as you might in a car going 100 mph. With moving air, we want a gradual and smooth turn since we’re moving really fast. When there is a change in air direction in a duct, it takes roughly 24 inches for airflow to restore its pattern. Takeoffs should thus be 24 inches away from any turns, transitions, or the end cap.
Undersized Returns And Undersized Supplies
When an HVAC system has undersized returns or undersized supplies it is like breathing through a straw. Air cannot circulate properly, and the unit struggles to heat and cool the building.
Improper Duct Installation Issues
If you suspect your duct work is designed or installed improperly, there can be a host of heating and cooling issues that you never asked for. Take all your problems as a sign that it is time to make adjustments to your existing duct work.
The good sheet metal workmanship of the 50s and 60s is long gone. Since the 1970s, air duct installations have been typically a lot cheaper and are generally inferior to the craftsmanship of the past. Here are the common air duct installations issues:
Bad Air Duct Seals And Connections
Remember that in a typical home, at least 20% of the air traveling through the ducts escapes through leaks, holes, and bad ducting connections. In many cases, HVAC installers are forced to use cheap materials to keep their business afloat. Using cheap seals instead of proper airtight ones leads to noise, dust, draft, humidity, and much more. This is because poorly fastened air ducts can become loose with the motion caused by flowing air.
Unbalanced Air Flow
Poorly installed ductwork leads to poorly balanced airflow, which leads to hot and cold spots, which leads to higher utility bills. HVAC best practices call for an air balance after installation to determine whether the air is flowing where it’s designed to. If it’s not, dampers can be installed on the duct trunk to adjust air flow. Unfortunately, this is rarely done.
Un-Insulated Or Under-Insulated Air Ducts
Air ducts are frequently made of thin material with little insulating capacity. Insulation keeps the air inside your ducts at the temperature it should be. In the summer, insulation prevents the cool air condensation that can lead to mold and mildew. In unconditioned spaces such as attics and crawl spaces, the recommended air duct insulation level is R-8. Unfortunately, most air ducts are only insulated at R-2.
Unnecessary Ducting Bends
Just like the airplane that experiences heavy turbulence within air pockets, air colliding with the duct becomes turbulent and moves slower. A good HVAC specialist will avoid unnecessary bends when installing the HVAC duct system and opt for smaller tree branch type ducts where possible.
Twisted, Tangled, & Kinked Flex Ducts
The bottom line is many HVAC contractors that are not Air Check Mechanical Service are too lazy to stretch flex ducts and cut them to fit into place fully extended. Instead, the overly long runs are left twisted, tangled & kinked. Remember the metal rib cage in flex ducts? Now airflow is reduced even more, costing you big money.
Unsupported Duct Runs And Butt Joints
It’s a basic fact: flex ductwork cannot be unsupported for more than 4 feet or it will start to sag. The same thing will happen when contractors take two flex duct runs, fit them together with tape, and wrap insulation to hold them together. While this may pass an inspection, neglecting to install a metal butt joint connector creates a weak connection and leads to sagging ducts and escaping air.
Air ducts, like almost anything these days, doesn’t last forever. In fact, because of the thin materials, they can be subject to serious wear.
Air Duct Wear & Tear
Moving air is a powerful force. Your air ducts experience wear and tear over time when air is ricocheting off the walls, so unfortunately, they don’t last forever. Expiration dates vary – metal ducts generally last longer than flex ducts because metal is more rigid and its surface allows air to glide, unlike the coil around flex ductwork. That said, the average air duct lifespan varies from 15 to 25 years.
As your air ducts age, problems begin arising:
Leaky, Corroded Ducts
Old seals will erode with enough wear and tear, letting air slip out (or in). If moisture gets into the air duct, it will corrode the ducts over time. You’ll see the same corrosion if your basement or crawl space is overly humid, or if water is leaking from your roof, plumbing, or humidifier. Unsupported flex duct runs are more likely to sag, and condensation will gather in the dip.
Dirty & Clogged Air Ducts
The nature of duct work is unfortunately that one duct problem leads to many. If your indoor air is dirty, the impurities can accumulate inside your ducts and, if the debris is substantial, clog them. This is made worse with holes, loose connections, and general uncleanliness in your home.
Molds In The Air Duct Are Hazardous To Your Health
Warning: do not let moisture enter your air ducts. Mold will form. And while it may not physically deteriorate your air duct, it will affect the quality of your air and by association, your lungs. Supply vents will blow mold spores to every room. And in worst case scenarios, this could lead to the decay of furniture or walls.
Be sure to contact Air Check Mechanical Service to evaluate all of your options during a ductwork repair.