Air ducts are often hidden in residential and commercial buildings, and performance issues can go unnoticed. Deficiencies in air duct systems reduce the effectiveness of heating and cooling equipment while increasing energy costs. Air duct problems also have a negative effect on indoor air quality.
HVAC performance can be improved through the use of high-efficiency heating and cooling equipment, complemented with automatic controls to increase energy savings. However, heating and cooling can only be provided effectively if a building's air duct system is in good condition.
Some common problems that affect air ducts are poor insulation, noise, vibration, dust buildup, mold, and air leaks. However, an inspection by professional engineers is highly recommended before carrying out any work on your air ducts: if they are not handled properly, existing problems could worsen rather than be corrected.
Improve the performance of your air ducts and save energy.
How air leaks into ducts wasting energy
Space heating and air conditioning use a lot of energy, and air ducts connect HVAC equipment to indoor spaces. As a consequence, all the air that leaks after being heated or cooled represents a waste of energy. The opposite also applies: Unconditioned air can leak into ducts, damaging heating and cooling equipment while also introducing pollutants. An HVAC system can only function optimally if the air ducts are isolated from unconditioned spaces.
Even if air ducts are properly sealed, adequate insulation is also important. A poorly insulated air duct also wastes heating and cooling capacity, even if air leaks are minimized. An effective strategy is to place air ducts within conditioned spaces, as this minimizes the temperature difference across the duct walls. However, visible ducts have a negative effect on the appearance of some interior spaces and this approach is not always viable.
Insulation and air sealing provide a significant increase in the energy performance of air ducts. Air tightness also improves indoor air quality since unconditioned air from crawl spaces can no longer leak into the duct system. However, there are additional issues that can affect HVAC performance and air quality if left unattended.
Keeping air ducts clean of dust and mold
Because air ducts connect multiple interior spaces, they can spread contaminants that build up on interior walls. There are also cases where the duct system is clean, but the respective air handler is not.
Mold is known for its ability to grow hidden from view and can affect the interior surfaces of air ducts. Because mold requires moisture to grow, mold problems inside a building indicate that moisture in the air is not being removed properly. However, do not assume that any dark substance on the interior walls of air ducts is mold – have a professional inspection and appropriate laboratory testing done.
Sheet metal air ducts can be cleaned if they are affected by mold. However, air ducts and fiberglass insulation are porous and any sections affected by mold must be replaced completely. Although mold disappears when air humidity decreases, it cannot be effectively removed from porous materials. Additionally, dead spores and mold particles can also cause irritation and allergic reactions.
Importance of adequate ventilation design
The performance of air ducts is also influenced by the air handlers connected to them. Even if you have clean, airtight, insulated ductwork, you may experience noise and vibration issues due to oversized or poorly supported air handlers.
After ensuring your air handlers have the correct capacity and equipment support, performance can be further improved by adding the following measures:
- Demand-controlled ventilation (DCV), which consists of adjusting the air flow according to occupancy. Significant energy savings are achieved as the fans only run at full power when necessary.
- Energy recovery ventilation (ERV), which consists of heat exchange between supply air and exhaust air from the ventilation system. This can achieve cooling and heating savings.
Design Standards for Air Ducts
The industry authority for air duct projects in the U.S. is the Sheet Metal and Air Conditioning Contractors National Association, or SMACNA. They have published standards for metal and fiberglass air ducts:
- SMACNA/ANSI-2005 HVAC Duct Construction Standards – Metallic and Flexible
- SMACNA-03 Fibrous Glass Duct Construction Standards
When dealing with air duct requirements, local building codes typically integrate these national standards. The best recommendation to ensure code compliance is to work with a qualified MEP engineering firm.
