Can Flex Duct Be Used for a Range Hood? (What to use instead)

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Ensuring that your range hood maintains its functionality well into the future is incredibly important, and the type of duct used has a massive influence on its efficiency. But, there are also rules and codes to adhere to concerning ducts, and users will not have complete freedom when determining the type of duct used. The rules are in place to protect the residents.

According to the International Residential Code (2015) Section M1503, using a flex duct for a range hood is not permitted for a variety of safety reasons. Some exceptions allow the use of schedule 40 PVC pipe for downdraft range hoods, provided that ducts are installed according to specific requirements. 

There are plenty of reasons why using a flex duct is not permitted, but there is also an abundance of reasons why it’s not the best idea regardless of rules and codes. Join us as we discuss the rules surrounding using a flex duct for a range hood and what ducts you should be using for the best functionality over time. 

Can you use PVC pipe for a range hood?

PVC pipe is not allowed to be used for a range hood unless when the case falls within the exceptions stipulated by the International Residential Code, which permits the use of schedule 40 PVC pipe. Even in these cases, there are many rules and regulations to adhere to regarding the duct’s installation and location. 

Ducts for domestic kitchen cooking appliances equipped with downdraft exhaust systems shall be permitted to be constructed of schedule 40 PVC pipe and fittings provided that the installation complies with all of the following:

  1. The duct is installed under a concrete slab poured on grade.
  2. The underfloor trench in which the duct is installed is completely backfilled with sand or gravel.
  3. The PVC duct extends not more than 1 inch (25 mm) above the indoor concrete floor surface.
  4. The PVC duct extends not more than 1 inch (25 mm) above grade outside of the building.
  5. The PVC ducts are solvent cemented.

Although the code communicates such stipulations and these regulations are followed by most states and areas, some AHJ (Authority Having Jurisdiction) does permit the use of heat-rated PVC. These details can be confirmed by contacting your local building department or City Code Department. Users will need to ensure that they meet the rules involved before going ahead with any PVC pipe installations.

Why Flexi Pipe and PVC Pipe Can’t Be Used to Vent a Range Hood

The type of duct being used will influence its longevity and functionality and pose various complications for users if it’s not up to standard.  All of these motives should be considered before investing in pipes and ducts for range hoods. Users should be mindful of their material compositions and properties concerning the care and maintenance needed over time. 

Rules and Codes

According to the International Residential Code (2015) Section M1503, ducts serving the range hood need to be airtight, equipped with a backdraft damper, and should have smooth inner surfaces. These ducts need to be free from all other exhaust systems in the surrounding area and should never occupy a crawl space or attic inside the building.

Here is an helpful guide if you would like to learn more about ventilation dampers.

Duct placement in such manners will lead to a buildup of chemicals, dirt, and grime in these areas, demanding that the range hood is vented to the outside of the building.

The code demands that ducts are comprised of rigid stainless steel, galvanized steel, or copper with double walls, and it should be airtight with smooth inner walls. This rules out all Flexi ducts since they do not have smooth inner walls and are usually made out of aluminum.

Ducts for range hoods should never pose the risk of fires or combustibles, which are enormously dangerous for those occupying the building. Ducted range hoods and downdraft exhausts require fan sizes according to Section M1507.4 of the International Residential Code, in addition to many other necessities for the placement and workings of the ducts. 

Local codes can differ when it comes to whether or not range hoods are required (you can read my articles for North Carolina, California, Ohio, Ontario, Colorado, and Florida). However, they all stipulate that if you have a range hood, it has to have smooth ducts.

Related article: Are range hoods necessary?

Risks, Functionality, and Usability

Even if using flex pipe or PVC pipe to vent range hoods was allowed hypothetically, it still wouldn’t be an ideal approach.

Over time a lot of grease will collect in the duct and this could potentially ignite and cause a fire.

Aluminium which most Flexi ducts are made out of is not a durable material and is susceptible to cracks and tears. This means it does not retain its connections and functionality very well over time.

In addition, the use of these materials in ducts makes them incredibly complicated to maintain safely. Flex ducts are extremely challenging to maintain over time, particularly concerning the hygiene and safety aspects of ducts and the processes which need to be followed to ventilate the range hood to the outdoors successfully. These materials result in a much higher chance of the duct experiencing grease buildup over time, which may be nearly impossible to clean effectively. 

This potential for a more significant and more problematic grease buildup is not only awkward concerning cleanliness. Excessive fat and grease buildups pose the risk of duct fires and even cooking/kitchen-related fires, which are far more common than one may think. These dire possibilities can become far more complicated to resolve as well, as most insurance companies will not pay out if your ducts violate codes and safety requirements in your area. 

If users do happen to have a flex duct for the range hood, the duct should be checked for grease buildup. If the duct is clean without any grease buildup, then it may be used until it is replaced with a code-compliant duct.

When Can You Use a Flex Duct or PVC?

Although the use of PVC pipe is generally discouraged as it violates the code in most areas, there are some users who still use PVC or flex ducts for their range hoods. What’s even more concerning is that these ducts are encouraged or installed by professionals in many cases, whether or not they meet the code of the area. Users should always consult the relative departments in their area in order to identify whether or not these options would be suitable for their circumstances. 

Flex duct should never be used for a range hood. Even when the duct is advertised as “range hood vent kit”(amazon link) it does not mean that it is OK to use it. You will fail home inspection when it comes time to sell. Insurance might reject your claim in case of a fire if the fire was started by the range hood.

However, there are a few exceptions that permit the use of PVC pipe for ducts serving the range hood on an international scale, many of which demand that the aspects meet specific requirements when installed according to the manufacturer’s instructions. The only exception to the code’s stipulations is when the duct is being used for kitchen cooking appliances that have been equipped with downdraft exhaust systems

Where Can Flexi Duct Be Used (Helpful Table)

Within the confines of these exceptions to the code, ducts for range hoods are allowed to be made of schedule 40 PVC pipe and fittings, as long as the installation process involves the following: 

  • The duct being installed under a concrete slab poured on grade
  • The underfloor trench beneath the duct is fully filled with gravel or sand
  • The duct cannot extend more than 1 inch above the concrete floor surface indoors
  • The duct cannot extend more than 1 inch above grade when outdoors
  • The ducts are required to be solvent cemented

The use of flex ducts and PVC pipes will be permitted if its installation and functionality meet these requirements.

What Type of Duct Is Best for Venting a Range Hood?

The best type of ducts for venting a range hood is solid and durable and meets the necessities stipulated by the codes involved. Semi-rigid ducts comprised of materials such as aluminum are relatively flimsy in comparison to rigid ducts, making rigid ducts the best option for venting range hoods and most effective when meeting the relative codes as well. Rigid ducts are made of materials that are much higher in quality, making them far more durable and functional in the long run. 

Rigid ducts, which are comprised of sturdy stainless steel, demand a once-off installation and only necessitate that users clean them as needed. Other than these involvements, the maintenance required by rigid ducts is reasonably minimal. The use of rigid ducts for range hoods affords users with much lower risks involved, and they generally last more than twenty years or more with proper care and maintenance. 

Choosing the proper ductwork according to the International Residential Code, risks for safety, as well and usability and functionality long term can be quite perplexing, especially with so many manufacturers and service providers providing false information regarding the appropriateness of various products. Always ensure that your duct installations adhere to the rules and codes involved, support your daily activities, and ensure your safety over time. 

Related article: 8 Range Hoods Made In USA

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