Probably the most important code requirement for range hoods is that they should vent outdoors.
Another important code is on the duct length and diameter of kitchen hoods.
The in-wall range hood duct should be made out of smooth metal and with a minimal amount of turns.
As a home inspector, I have inspected many range hoods over the years, and clients frequently ask me about building codes. Even though inspectors aren’t code inspectors, we do like to familiarize with building codes and best practices.
In this HomeInspectorSecrets.com code guide, I will go over…
- Minimum duct length & diameter
- Minimum CFM or fan speed
- How range hoods vent to the outside
- Makeup air systems for high powered vent hoods
Kitchen Hood Vent Codes Are Based On The IRC And Your Local Code
Range hood vent code requirements are laid out in the IRC or the International Residential Code.
The IRC is a ‘model code’ that many U.S. states and counties base their code on. For the final say on any code matter, you will need to consult your local state or city building codes or even a quick call to your local building department.
Even though I summarize the code requirements (I just looked at some of the most important issues), this isn’t exhaustive of every single code and exception.
As a side note, older houses are almost usually ‘grandfathered in’ and not required to adhere to newer code requirements except for new installations.
Read Also >> What Are The Building Codes For Dryer Vents?
7 Building Codes For Kitchen Vent Hoods
1. Must Vent To Outside
Range hoods always need to vent to the exterior. They can’t exhaust into an attic, a crawlspace, or inside a ceiling cavity.
The main code exception is for ductless or recirculating range hoods as long as they are listed and designed for that type of installation. Recirculating range hoods always require the addition of a carbon filter which helps catch smoke and smells.
Here are a few other code requirements on how range hoods exhaust…
- Interior of the duct should be smooth (no ridges)
- Range hood duct needs to be air tight
- Hood needs to have a backdraft damper (a damper or ‘flap’ closes the vent when the hood is not in use)
- Exhaust duct needs to be independent and not connected to any other exhaust system such as a bathroom fan
Read Also >> Do Kitchen Vent Hoods Have To Be Vented Outside?
2. Ducting Material
Range hood ducts always need to be either galvanized steel, stainless steel, or copper.
The one big exception to this code is if the range hood is a downdraft type (it sucks in air downwards and the vent goes into the floor).
With a downdraft hood, it is permissible to use PVC plastic piping instead of metal under certain conditions.
3. Makeup Air System May Be Required
If you have a range hood that is rated at 400-cfm or greater, you may be required to have a makeup air system installed. Makeup air systems are automated devices with dampers that brings in fresh outdoor or to replace the air exhausted by the range hood.
This code requirement only applies to gas or propane ranges and if it isn’t already direct vented.
The makeup air unit also needs to have a damper which closes when not in use so outdoor air doesn’t leak into the home.
There are universal makeup air systems on the market that uses a sensor to open the damper if it detects the range hood turned on. These systems only need low voltage wiring and they work well with new construction or existing kitchen hoods.
4. Duct Length
The code requirements for the duct length of range hoods is based on the duct diameter, whether it is smooth walled, and it’s CFM rating.
For example, if your range hood is rated at 200-cfm, and the flex duct has a 6-inch diameter, then the maximum duct length is 18-feet. And a 5-inch diameter sized duct or below isn’t even permitted with a 200-cfm hood and 6-inch flex duct.
The exception to this section is if the range hood satisfies the requirements of the manufacturer or if you independently test the air flow of the hood with some type of measuring device.
The X marks in the table provided below means that under these conditions a duct is not allowed because of the high pressure drop.
5. Exterior Cover
Range hood vents should always be located on the outside and they need to terminate at least 3-feet away from windows, doors, and other vents such as bathroom fans.
If you have a powered air intake such as from a high efficiency furnace, then the range hood vent needs to be at least 10-feet away. The exception to the 10-foot rule is if the range hood vent is at least 3-feet above the intake vent.
6. Minimum Exhaust Rate Or CFM
The absolute minimum CFM for a kitchen range hood is 100-cfm.
CFM stands for cubic feet per minute and it is a range hood rating for how much air the hood can exhaust every 60-seconds. Most range hoods are in the 150-350 cfm range.
7. Must Use Individual Or Dedicated Branch Circuit
Range hoods need to be supplied by an individual branch circuit.
For higher powered range hoods, a dedicated circuit will be needed.
The minimum cord length will be 18-inches and the maximum cord length is 48-inches.
Makeup Air Venting Code Prevents Carbon Monoxide Poisoning
The main reason for the makeup air code requirement (code #3 above) for some range hoods is to prevent a carbon monoxide hazard.
If you have a very strong range hood, it will create a large air pressure imbalance in the home (negative air pressure) which can possibly create a backdrafting scenario.
Backdrafting is when negative air pressure actually reverses the flow of the exhaust gases into the home. If exhaust gases are going into the home rather than to the exterior, carbon monoxide poisoning can occur.
Makeup Air Should Go Into Kitchen
If you need a makeup air system by code, the ‘makeup’ or fresh air needs to be sent into the kitchen or room where the range hood is located.
The exception to this rule is that it can go into a different room as long as they are connected by a permanent opening.
The type of damper for the makeup air system should work either by gravity (opens and closed based on air flow) or it can be electronically operated. The electric dampers open automatically when the range hood is turned on.
The biggest takeaway for range hood code requirements is that they need to vent to the outside. They can’t vent into a crawlspace or an attic.
Also, there are duct requirements for safety such as the length and the type of material used. If you use too small diameter of a duct, it can cause a pressure drop, and create a hazardous situation.
Another important code requirement is for makeup air.
If your range hood is 400-cfm or greater, you may need to install a makeup air system to prevent a dangerous backdrafting situation with carbon monoxide poisoning if you have fuel-burning appliances.