How Ductwork Moves the Air in Our HomesBefore getting into the HVAC industry, the idea of ductwork had me imagining a movie where someone’s crawling through a duct to escape an alien or a monster. It’s a cool scene in a movie, but it’s clearly not the primary function of ductwork. Outside of Hollywood, the purpose is to move heated or cooled air from your furnace or air conditioner to all the rooms in your house. The concept is simple enough, but there’s actually a lot more to designing ductwork so it’s the right size, and length, for your home. HVAC engineers design your HVAC system much like the structure of a tree. The bigger the trunk, the longer, bigger, and taller the branches will be. The same goes for your HVAC system. Ductwork is sized based on how large your unit is, and how far it is from all the spaces that need to be heated and cooled in your house. Designers then use the specifications of the unit, and the layout of your house, to design ductwork that will move a certain amount of air (measured by cubic feet per minute, or CFMs) to each room. The distance a room is from the HVAC unit plays a big factor in how large a piece of duct will be. If this ductwork isn’t sized correctly, it can cause the system to become imbalanced, meaning that each room may get different amounts of air, leaving some cool and some hot.
Unbalanced Airflow in Your DuctworkIf you could crawl up into your ceiling to see your ductwork right now, you would probably see that it comes in all different sizes. This probably means it has been specifically sized for your home—but it doesn’t necessarily mean it’s balanced. You may also notice that, like other appliances in your home, there’s some wear and tear. All these factors contribute to the unbalanced air flow in different rooms.
- Ductwork is the wrong size: Even a slightly oversized or undersized duct can affect the air balance from room to room. Surprisingly, a larger duct doesn’t mean you will get more air. This is because the force, or static pressure, in the large duct will be less than it is in a small duct. So while there might be more room for air in a large duct, there will be less delivered—the airflow will be slower due to less static pressure.
Longer duct runs have less static pressure as well, so rooms that are a large distance from the unit have to be taken into consideration when sizing ductwork. Otherwise, the rooms farthest from your unit will be the slowest to heat and cool.
- Dampers aren’t balanced: Most ducts contain a device called a damper that is used to balance the airflow in each duct run. There are many types of dampers, but a common type is called a balancing damper. True to its name, this device has blades that open and close to let more or less air move through the duct.
Think of it like your blinds—opening them lets light come in, while closing them keeps the light out. For dampers, this allows ducts of different sizes and lengths within the same house to be balanced so they all have the same static pressure.
- Ductwork is leaking: Everything may be sized perfectly, and the dampers may be balanced but, over time, the seams in your ductwork can begin to leak air. Ductwork is designed from aluminum sheets that are cut and bent to specific sizes. Each of these pieces is then installed in the ceiling of your home and attached together with industrial staples, tape, and a glue or sealant. With age and use, the glue, tape, and staples begin to deteriorate, causing ducts to slowly begin leaking air.
Fixing Unbalanced Ductwork for Even HeatingThere is nothing fun about finally getting comfortable in your bedroom, only to find that when you get up and go into the kitchen, it’s freezing cold. But luckily, if the issue is unbalanced ductwork, it should be easy to diagnose and correct. Here are some common solutions to ductwork airflow issues:
- Test and balancing: You can have a professional HVAC technician, or test and balancing expert, measure the airflow in your ductwork. If it’s a simple air flow problem, they can correct it by moving balancing dampers into different positions to even the flow to each space. They do this by determining the airflow at each duct, then adjusting damper positions manually to increase or decrease the static pressure until the same amount of air flows to each room. They can also detect leaks in ducts that are negatively affecting the pressure.
- Increasing fan speed: Another thing that your HVAC professional may be able to do is to increase the fan speed of your unit. If the ductwork has been balanced, but it’s not achieving the right airflow, the motor speed of your unit can be adjusted to improve the airflow per minute to each space. The increased speed will push more air through the ducts, increasing static pressure—and the amount of CFMs delivered to each are of the home.
- Resizing ducts: If balancing dampers and increasing fan speed doesn’t correct the air balance issue in your home, the ductwork may need to be resized. This will solve the problem indefinitely, but may be more expensive than just balancing your ducts.
This is not uncommon in homes that have had different sized HVAC units installed, or have renovated rooms or new additions added onto the home. If this is the case, an experienced HVAC professional should be consulted to conduct load calculations, and to determine the proper sizing for your ductwork—then they can determine where ductwork may need to be replaced or altered in your home.
- Sealing leaking ducts: If your ducts are losing pressure, it may be due to leaks. In fact, the average home duct system loses 20% to 30% of its air because of leaks due to aging sealant and tape. A certified HVAC professional can find the leaks and seal them, restoring the pressure in the ductwork.