Range Hood Duct Sizing Guide
Range Hood Duct sizing and the proper size of duct-work for your new Vent Hood is very important to ensure that your Range Hood or Vent Hood is efficient and quiet.
Diameter of your Range Hood Ductwork is the most important factor
The diameter of your Ductwork is the single most important consideration. Here are some guidelines based on your Range Hood or Vent Hood CFM. Now please note that we will use the most commonly available sizes of Ductwork for our recommendations and that in many remodel applications for older homes there is an existing 4” Duct which in some cases is the only option other than a recirculating hood. We always recommend venting outside over recirculating even if you must adapt to an existing smaller duct size than is recommended. If you have any questions please call us or contact your local HVAC professional.
- 0-400 CFM Minimum Duct Size of 4” Recommended Duct Size of 6” if possible
- 401-600 CFM Minimum Duct Size of 6” with a Recommended Duct Size of 8” if possible
- 601-900 CFM Minimum Duct Size of 7” with a Recommended Duct Size of 8”-10” if possible
- 901-1200 CFM Minimum Duct Size of 8” with a Recommended Duct Size of 10”-12” if possible
- 1200 CFM and above will depend on the motor type and duct run length but at least a 10” diameter duct is required and 12” is recommended.
The recommendations for Range Hood Ductwork Size are suggestions. After you consider the size of your Ductwork the next most important factor is the “Run”. How long is the the ductwork and how hard will the Range Hood Blower have to “push” the exhaust air to get it out of ductwork.
Length of Ductwork and the number of changes in direction are the second most important factor.
Next to Ductwork Size, the Length is the most important consideration. In some cases even if you are dramatically undersized you can still exhaust the cooking air efficiently if you have a very short run of ductwork. Here are the necessary things to consider in the ductwork size and design assessment.
First, Shorter is always better. Next, the straighter the better. And finally, Upward for hot air is always the best way to go. Remember, hot air rises, so if you have to choose between a 30’ run with 2 elbows out the side wall or a 40-50’ run straight up with no elbows these will be about the same in static pressure at the exhaust point. Every time you change direction you add a small amount of resistance to the air pressure and increase the pressure required to move the air along the length of the discharge run. Our standard deduction in length for each elbow or 90 degree change in direction is 10’. So a 20’ run with 3 elbows is equal to a 50’ straight run.
Here are a couple of real life examples to show how we would advise our customers. If you have any questions please call us. We are always happy to help.
Customer “Smith” living in an older apartment/condo in Boston. Has an existing 4” Duct that goes up the wall, turns a corner, crosses the ceiling about 15’ then exits the side of the building. Customer wants to know if he can hook up a 600 CFM Range Hood? Should he just switch to Recirculating? Customer cannot afford and does not want to disturb the existing ductwork.
Our suggestion is to connect the range hood to the existing 4” ductwork versus using the Range Hood in Recirculating mode. Here is why, this Range Hood was a 4 speed Vent Hood. On the lowest 2 speeds, 240 CFM and 360 CFM a 4” Duct is large enough. However on the 3rd and 4th speeds the pressure inside the ductwork would increase and the blower would have to work a little harder to push the air. Yes, if this customer chooses to run this Range Hood on the highest speed everyday then the additional air pressure will reduce the usable life of the motor. But in most real life situations, the range hood can be installed and hooked up to the existing ductwork once the customer understands the limitations and the trade offs.
Customer “Andrews” lives in Phoenix AZ, and is building a new home. He is venting over a 48” Wolf Range and plans to cook all the time. In fact, when he and his wife entertain, they enjoy cooking together and will have the cooktop on for hours. He wants to make sure his Range Hood Ductwork is sized correctly for his new home and he wants the Range Hood Blower motor on the rooftop of his house. The BTU’s for his Cooktop are Maximum of 78,000 and he plans to use the grill that is part of his cooktop.
Our Suggestion is a minimum Ductwork Size of 10” diameter and 12” if the ductwork extends more than 100’ including the 10’ deduction for each elbow. This customer will use a 1200 CFM roof top blower controlled at the range hood. The 10’ size will accommodate every speed but the at the highest speed may require the blower to work a little harder. Depending on the financial considerations we would recommend the 12” size especially if the indoor grill is to be used often. We also had some recommendations for correct Range Hood Size and Placement.
Hopefully this helps provide some answers and if you have any questions please feel free to give us a call.
Happy Cooking and enjoy your kitchen!