No, you should not use flexible duct for a downdraft or kitchen exhaust system.
It depends on several factors, but most residential kitchens require 400-600 CFM for effective ventilation. Gas ranges need 100 CFM per 10,000 BTUs, while electric cooktops typically need 100 CFM per linear foot for wall-mounted hoods.
Cons of Flexible Ducts
Since they aren't as smooth on the inside as rigid ducts, they may not be as energy efficient. The biggest drawback to flexible ductwork is that they don't last nearly as long as rigid ducts. Their flexibility can make them more prone to snags and tears.
When installing an exhaust fan in a location that does not have attic assess, using flex ducting is an almost must. It allows you to make the connection to the fan's duct collar without needing to cut a larger opening (and then repairing) in the ceiling.
Dryer venting tips and practices
Flexible transition hose between the dryer and the wall outlet should be either the foil type or the aluminum flexible duct (most preferred). Do not use the plastic or vinyl.
The safest and most efficient dryer duct is a rigid metal duct. For the short, flexible connection between the machine and the wall, DryerFlex is widely considered the best transition duct. Never use vinyl or thin foil hoses, as they are a major fire hazard.
M1502.
The maximum developed length of a clothes dryer exhaust duct shall not exceed 35 feet from the dryer location to the wall or roof termination. The maximum length of the duct shall be reduced 2.5 feet for each 45-degree (0.8 rad) bend, and 5 feet for each 90-degree (1.6 rad) bend.
While flexible duct can technically be used for vent hoods in some cases, it is strongly advised against, as rigid metal ducting is the standard for proper performance and safety. Flexible duct creates air turbulence, reduces airflow efficiency, is harder to clean, and poses a higher fire risk due to grease buildup.
A 12×12 inch (rectangular) duct typically handles between 680 and 800 CFM (Cubic Feet per Minute), depending on the airflow velocity and static pressure.
Therefore, the duct for a bathroom exhaust fan cannot be shared with a dryer ventilation system. Section M1502.
In HVAC duct design, the two-foot rule is a guideline stating that branch ducts (takeoffs) must be placed at least 24 inches away from any air direction change, transition, or end cap. This spacing allows turbulent air to re-pressurize and stabilize, ensuring consistent airflow to every room.
While it is true that the ridges inside a flex duct can lead to higher friction and therefore, slightly lower airflow, this impact can be minimized with proper installation. Keeping the duct as straight and short as possible, and avoiding tight bends, can significantly mitigate any potential reduction in airflow.
Flexible HVAC ductwork typically lasts 10 to 15 years. While it can occasionally endure up to 20 or 25 years in ideal conditions, flex duct degrades faster than rigid metal due to its vulnerability to tearing, sagging, mold, and pest damage.
They usually require about 30 inches between the top of the cooktop and bottom of the cabinet, depending on the cooktop dimensions. Downdraft hoods are installed behind the cooktop and designed for use in a cabinet at least 24 inches deep, though some models require more depth.
Reduced System Efficiency: Too much CFM can actually cause the system's efficiency to decrease and in-turn cause it to work harder than necessary. Potential Damage to Ductwork: If the ductwork is not designed to handle the increased airflow, it could lead to leaks, vibrations, or even damage over time.
The answer: neither is ideal. Properly sized ductwork helps your system move air efficiently, keeps your home comfortable, and prevents unnecessary strain on your equipment.
One of the most surprising places they can hide? Your ductwork. That's right: your heating and ventilation system may be more than just a pathway for warm or cool air; it could also be a cozy travel route (and nesting spot) for mice.
The higher the cubic feet per minute, the larger the duct you'll need. At 600 CFM, the minimum duct size is 6". At 900 CFM, the minimum duct size is also 6". Finally, at 1200 CFM and above, consider a 8" or 10" diameter duct.
For high-temperature air, fumes, and exhaust gas applications, flexible ducting made of fiberglass is one of the best options. It's often used with an elastomeric compound like silicone or neoprene to coat the fiberglass body and make it more resistant to high temperatures.
The duct serving the range hood should be made of rigid metal such as galvanized steel or stainless steel and should have a smooth interior surface. Metal or plastic flex ducts are prohibited. This is a common code violation related to kitchen exhaust systems. PVC ducts may be used for downdraft exhaust systems.
General sizing recommendations:
600-800 CFM hoods with elbows: Minimum 7-inch duct. 800-1000 CFM hoods with elbows: Minimum 8-inch duct. 1000+ CFM hoods: Always use 8-inch or larger, even without elbows.
Dryer Connection – There are various styles of flexible hoses made for connecting dryers. Flexible hoses must not exceed 8 feet in length. If the run is longer than 8 feet, rigid pipe must be used. Best – Rigid pipe, as it doesn't trap lint.
6-Inch Duct: Suitable for range hoods with CFM ratings up to 600 CFM, typically used in smaller kitchens or with lower-powered range hoods. 8-Inch Duct: Suitable for range hoods with CFM ratings between 600 and 1200 CFM, commonly used in medium-sized kitchens or with moderately powerful range hoods.
It's not a concern. Dryer exhaust might hit 200 degrees, but most have thermal fuses far below that. You'd need twice that and then some to start a fire. Heck, many dryers are vented through cheap vinyl hose.