I recently had an HVAC technician install a 900 CFM remote blower for my kitchen exhaust. The outlet on both the hood and motor is 8 inches, but he reduced it to 6 inches where it exits through the roof. I'm worried that this reduction will restrict airflow, and I want to make sure I'm not overreacting. After investing in this setup for better noise control and effectiveness, I want it to function properly. Should I ask him to redo it?
4 Answers
Definitely bring this up with your HVAC guy! Reducing the duct size from 8" to 6" is likely going to restrict airflow, which could seriously impact the system's performance. Even if the motor can compensate by spinning faster, that just means wasted electricity and increased wear on the motor. Trust me, it'll be a nagging concern knowing the ducting isn't as it should be. Better to address it now than regret it later!
You're right to be concerned. The installation manual for your blower will have the best guidance on duct sizes, and it generally states that the duct should match the blower's specified size. Reducing it can lead to negative pressure in your house and other complications. Plus, if you don't have a makeup air unit, you could run into some serious issues with airflow. I'd suggest checking out the manual and approaching your contractor about this issue, as it'll help keep your warranty intact.
By code, since your kitchen hood exceeds 400 CFM, you need a makeup air unit running simultaneously with the exhaust fan. If you don't have one, you could be creating negative pressure in your home – not ideal, especially in humid conditions! So double-check your venting arrangements, and don't hesitate to push for the right duct sizes per the installation manual.
Thanks for the heads up! I'll make sure we have the makeup air unit in the plans.
From my experience, the airflow drop isn't as drastic as it sounds. Sometimes, a small reduction in size will have minimal impact. It's similar to how a shower head can produce a decent flow despite a tiny opening. But if you're aiming for peak performance from your setup, I'd recommend sticking to the 8" throughout. If you have a makeup air setup, that should help mitigate some issues, too.
Thank you for confirming my worries!