Each elbow adds some resistance to your duct system, which makes it harder for air to make it to the outside. A couple of elbows are OK; you might need them just to navigate through your walls. But don't use more than two to keep your hood running as efficiently as possible.
A straight, short duct run with limited elbows and transitions will allow the hood or downdraft to work most efficiently.
Of course, all of this is contingent on the fact that your kitchen hood, the crawl space and other installation limitations allow the use of this duct size all the way through. Does a range hood duct need to be rigid? No, semi-rigid flexible ducting is available that makes installation a lot easier.
Ventilation to the Outdoors (IRC M1503. 1): Residential range hoods should vent to the outside to effectively expel smoke, grease, and cooking odors, maintaining the home's air quality. This requirement is crucial for preventing the recirculation of contaminants within the home.
A semi-rigid duct offers more durability than a flexible duct. However, the best ducting for your kitchen hood is rigid. It's the most durable option because higher quality materials are used to produce them.
Quieter than Flexible Ductwork
Rigid ducting is quieter than its flexible counterpart. Due to its shape, sound waves are smoothly channeled and directed by the material. On the other hand, flexible ductwork is often noisy as its walls expand and contract due to air pressure and temperature changes.
Here is a Breakdown of the Duct Size You Need Based On Your Range Hood's CFM. If you have a range hood with a CFM of 0-400, you will need a minimum duct size of 4”. If your hood is between 401 and 600 CFM, you need a minimum duct size of 6”. If your hood is between 601 and 900 CFM, you need a minimum duct size of 7”.
Semi-rigid aluminum duct is another choice for range hood venting. It is flexible enough for ease of installation, yet holds its shape without sagging. It has smaller ridges inside for better air flow than regular flexible ducting, which is NOT suitable for kitchen range hood venting.
A vented range hood, aka a ducted range hood, pulls in air and leads it outside either through the exterior wall or the roof. Going through the exterior wall of your house requires less waterproofing. If you decide to vent through the roof, make sure the vent goes all the way outside and not just into the attic.
If you have an electric cooktop, range hoods should typically be 20–24 inches above the heat source. For gas cooktops, your range hood should be around 24–30 inches above the stove. Range hoods are a great way to complement your cooktop or range and can help clear the air while you cook.
Each elbow adds some resistance to your duct system, which makes it harder for air to make it to the outside. A couple of elbows are OK; you might need them just to navigate through your walls. But don't use more than two to keep your hood running as efficiently as possible.
Yes, Flex ducts are a perfectly valid option to use a conduit for your bathroom exhaust fan.
Disadvantages of Flex Duct
Flexible ducts are more prone to air leakage around the seams, which can lead to lower HVAC system performance. Additionally, the pliable nature of flex duct tends to make them prone to tangling or crushing when they pass through tight spaces or fixtures.
Yes, you can use flexible aluminum duct for a range hood, but it's not ideal. While it's easier to install, flexible ducts can create more airflow resistance and collect grease, leading to reduced efficiency and potential fire hazards.
Proper drain venting and slope are essential for a smooth-flowing plumbing system. A poorly sloped drain can lead to clogs and backups, while inadequate venting causes gurgling noises and foul odours.
Downdraft Range Hoods
The ductwork can be installed on the floor to vent smoke and steam. Downdraft range hoods are often used in kitchens with islands or peninsulas.
A mechanical exhaust directly to the outdoors shall be provided in each kitchen. The fan shall run intermittently (on demand) or continuously. A readily accessible manual control designed to be operated as needed or an automatic control shall be provided for intermittent operations.
The Bottom Line
A range hood shouldn't be vented into your attic. There are many ductless options available that will work great if you don't want to add additional ducting to direct kitchen exhaust outdoors. However, if you want a ducted range, there are many ventilation systems to select as well.
And both are usually connected to a ducting system that removes air from the kitchen and sends it outdoors. The difference is that a vent hood is designed to draw air, while a range hood is designed to push air. A range hood uses a forced-air fan to push air out of the system.
150mm diameter semi rigid ducting will hold shape and full diameter even when bends are applied.
The exception to this section allows the use of listed ductless (recirculating) range hoods as an alternative to the ducted type. However, exhaust to the outdoors would be required if natural or mechanical ventilation was not provided for the kitchen area.
NEVER use a PVC dryer duct or any plastic duct on a range hood installation as is not fire-rated. The minimum size duct allowed on most range hoods is 125mm ranging to 200mm round. More powerful hoods require even larger duct sizes up to 250mm or 300mm round. Consult the specifications for the model you have chosen.
When it moves from a smaller to a larger duct, the velocity decreases. In both cases, the flow rate — the amount of air moving through the duct, in cubic feet per minute — stays the same.
For most home kitchens, a 400 CFM range hood suffices. But if your cooking involves high heat or your stove has a high BTU output, consider a higher CFM.
Most hood exhaust systems will work more efficiently with a round duct system, however if you are running an older system then a rectangular system may be required.