Airflow and ducts

Supply vents and return grilles are the two halves of a ducted system. A furnace heats air and uses a blower motor and ducts to send warm air through the house. A central air conditioner sends cool air through the home the same way, through ducts and registers. The filter sits on the return side, in the return duct or plenum.

The grilles on your walls, floors and ceilings are where that air enters and leaves each room. Correctly sized supply and return grilles help air move well through a home. Grilles that are the wrong size, poor quality or badly placed can leave you with drafts or stagnant air, noise and other comfort problems. If ducts leak or there is not enough return air, they can pull outside air into the house and add to the heating and cooling load.

Source: U.S. Department of Energy · ASHRAE

Think of static pressure as how hard the blower has to push against the ducts, filter and coil. Airflow itself is hard to measure, because air speed varies across a duct, while static pressure is much easier to measure and more even. That is why technicians measure it: it can reveal a mismatch between the ductwork and the air handler. Each piece of equipment has a minimum acceptable airflow and a maximum allowable static pressure, which a technician finds in the maker's literature.

As static pressure increases, some blowers deliver less air. Variable speed blowers can change their speed and torque to keep airflow steady as static pressure rises. Fewer duct bends, right-sized ducts and removing obstructions lower the pressure the fan works against. Airflow should be checked both with a new filter and with one that is due for changing.

Getting airflow right matters for more than comfort. Furnaces and air conditioners are designed to run within certain airflow ranges for proper and safe performance. Air conditioners are typically designed for 350 to 400 cfm of air across the indoor coil for each ton of cooling. Too little airflow means too little heat exchange at the indoor coil, and improper airflow can also cause that coil to ice up. Too much airflow in cooling can leave the air too damp.

Source: U.S. Department of Energy · ASHRAE

Ducts often run through basements, attics and crawlspaces you never see. Leaky ducts waste energy dollars and can reduce airflow, making the home less comfortable. How much a duct leaks depends on the size of the leak and on how hard the fan pushes air through that duct. Poorly sealed return ducts in unheated spaces can draw in contaminated air and spread it through the house. Keeping ducts in good repair can prevent heat loss of up to 60% at the registers.

Sealing is the usual fix. Sealing duct air leaks can help save energy on heating and cooling bills. Ducts in spaces that are not heated or cooled, like attics and crawl spaces, should be properly sealed and insulated, which also helps keep condensation out of the system. Sealing a very leaky duct system can raise pressure and so reduce total airflow. Some rooms that are always too hot or too cold can point to duct problems, poor equipment operation or thin insulation.

Source: ENERGY STAR · U.S. Department of Energy · U.S. EPA

Weak airflow at a register has a handful of common causes. Airflow is shaped by the furnace or air handler, its motor speed, the ducts, the air filter and the condition of the whole air system. An undersized or dirty filter, or a coil clogged with dirt, can restrict it. Disconnected or crushed ducts can keep warm or cool air from ever reaching the room it was meant for. Furniture that blocks supply or return registers can affect the air too, so place it with airflow in mind.

Poor airflow is worth fixing. It can lower efficiency, raise utility bills, leave the air too damp, reduce comfort and shorten your equipment's life. Start with what you can see: the filter, and anything sitting on or in front of a grille. If those look fine, tell us which rooms are weak and when.

Source: U.S. Department of Energy · U.S. EPA · ENERGY STAR

Before paying for duct cleaning, it helps to know EPA's view. EPA does not recommend cleaning air ducts routinely, only as needed, though it finds no evidence that proper cleaning does harm. Duct cleaning is not a necessary part of yearly upkeep, which consists of cleaning drain pans and coils, changing filters and inspecting heating equipment each year. Duct cleaning has never been shown to actually prevent health problems. Little evidence suggests that simply cleaning the ducts will make your system more efficient.

There are times to consider it. EPA names ducts clogged with excessive dust and debris, or particles actually released into the home from your supply registers. If no one in your home has allergies or unexplained symptoms, and a look inside the ducts shows no large deposits of dust or mold, cleaning is probably unnecessary. If a problem does exist, its cause must be fixed first, or it will likely come back. Mold in ducts is covered on the mold page.

If you hire someone, EPA offers some guidance. Do not hire duct cleaners who recommend cleaning as routine maintenance or make sweeping claims about health benefits. Ask them to show you the contamination that would justify cleaning. Talk to at least three providers and get written estimates first. EPA does not certify or endorse duct cleaning companies. Before cleaning, the system should be checked for asbestos-containing materials, such as insulation or register boots.

Source: U.S. EPA

Some duct cleaners offer to spray chemicals inside the ducts. EPA says not to allow chemical biocides or treatments unless you fully understand the pros and cons. Even EPA registered biocides may cause eye, nose and skin irritation. No products are registered by EPA as biocides for fiberglass duct board or fiberglass lined ducts. Spraying these materials into a running duct system may carry much of them through the system and into other parts of your home.

Sealants sprayed inside ducts have their own limits. Lab tests show that sealant sprayed into a running duct system tends not to coat the duct surface completely. It may also change the noise and fire resistance of fiberglass lined ducts and may void the maker's warranty. Coating the inside of a duct is a different practice from sealing duct air leaks.

The lasting answer is keeping ducts dry and clean in the first place. Controlling moisture is the most effective way to prevent biological growth in all types of air ducts. Condensation or high humidity is an important sign that mold could grow in a duct. Whether or not you clean, keeping water and dirt out of the system is the most effective way to prevent contamination.

Source: U.S. EPA · National Institutes of Health