The diameter of your duct and the length of the ductwork are critical considerations to ensure that your new Range Hood will operate efficiently and quietly.
Why does the diameter of my duct matter?
The diameter of your ductwork is vital to the performance of your Range Hood or Vent Hood. When the Blower is pulling air into your Range Hood it must exhaust it through the ductwork to keep your kitchen environment fresh. The ductwork needs to be large enough to handle the amount of airflow being pulled through it without making the fan work harder than intended. The way to determine the appropriate diameter of your ductwork is by the Range Hood CFM (Cubic Feet per Minute).
Here are some guidelines to follow based on your Range Hood or Vent Hood CFM.
0 – 400 CFM: Minimum Duct size of 4” inches
Recommended Duct size of 6” inches
401 – 600 CFM: Minimum Duct size of 6” inches
Recommended Duct size of 8” inches
601 – 900 CFM: Minimum Duct size of 7” inches
Recommended Duct size of 8” – 10” inches
901- 1200 CFM: Minimum Duct size of 8” inches
Recommended Duct size of 10” – 12” inches
1200 CFM+ : It will depend on the type of motor and the length of the
duct run, but at least a 10” diameter duct is required
and 12” is recommended.
Sometimes it is not possible, due to space restrictions, to accommodate a large duct. You can always use 2 smaller diameter duct runs that will fit the space available. For example: you have a 1200 CFM blower but only have an 8” or 10” floor joist/roof joist to fit the ductwork run in. You can use two 6” duct runs and have more than enough ventilation capacity.
Many older home remodels have an existing 4” duct and no option of increasing the size without considerable cost. It is still recommended that you vent outside rather than use a recirculating application, even when you must adapt to a smaller existing ductwork. Please refer to your local HVAC professional for any questions specific to your situation.
Is the length of my ductwork or changes in direction important?
After you consider the size of your ductwork based on CFM, the next most important factor is the length or “Run” of the ductwork. How long your ducting runs from the Range Hood to the exterior determines how hard the Vent Hood Blower will have to work to “push” the exhaust air out of the ductwork. Depending of the length of the run, in some cases a smaller duct size than recommended will be sufficient if you have a very short run of ductwork.
Here are the main things to consider in your ductwork size and the design evaluation.
1) Shorter is always better. The less distance the air must be pushed through the ductwork, the more efficient and quiet your Vent Hood will operate. The actual length of ductwork will also take into consideration any bends or directional change.
2) Straighter is better. Every time you change direction you add a small amount of resistance to the air pressure, requiring the Range Hood to use more power. A very simple “rule of thumb” is that each elbow or change of direction in your ductwork is equal to an additional 10’ feet in your calculation. For example a 20’ ft run with 2 elbows is measured the same as a 40’ ft straight run.
3) Up is always the best way to vent. Hot air rises, so ducting out the roof is often more effective than ducting out the side wall depending on the ductwork design. For example a 30’ ft run to an outside wall with 2 elbows is equivalent to a 50’ ft straight run up to the roof. They require about the same static pressure at the exhaust point.
Hopefully this has provided you with some answers and things to consider when determining your ductwork size and venting design. Please refer to your local HVAC professional for any questions specific to your situation.
Have Fun and Happy Cooking!!