Help with Kitchen Hood ducting problem

9 years ago

We are ducting our kitchen hood to an 1500CFM external blower up on the 2nd story roof. We have a 48" range, so high CFM is needed for the BTU output). The hood and blower calls for a 10" round duct, but the duct will need elbow through two 8"x12" opening (I know, bad planning on my part..). So I am thinking of doing the following duct run from the hood:
1. 10" round
2. 10" to rectangular transition
3. 90 degree elbow to the side (rectangular)
4. 90 degree elbow up (rect.)
5. 7-8 foot vertical run (rect.)
6. 90 degree rect. elbow (heating unit located in attic right above).
7. transition to 10" round
8. 2 ft horizontal run
9. 90 degree elbow

5ft vertical run to the blower

We don't want soffits as we are doing double stack cabinetry with glass door on the upper cabinets and we want to minimize noise, hence the external blower, (although it seems the long duct run and elbows will reduce the efficiency AND add noise.)

Anyway, my question should I use transition to a 8"x12" (10.9" equiv) or a 8"x10" (9.76" equiv)?

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Comments (6)

ajc71
9 years ago
Have you done the calculations to see what CFM you are losing with the transitions/elbows etc.....does not look all that promising on paper
User
9 years ago
What about the makeup air that is required? You'll need 2x the ducting and a heating unit that can handle that airflow. Where are you ducting that to?
michellemarie
9 years ago
My husband installs fireplaces for a living and is familiar with installing venting, but for our vent hood we hired an hvac company. If you are unsure of the venting configuration call and get an estimate and have it done right.
CharissaMK
Original Author
9 years ago
ajc71: I haven't done that calculation, but the equivalent length would be less than the max 75ft specified. it would be around 65' if I use 10 ft per elbow, or around 45' if I use 5ft per elbow. Do I also need to to the CFM calc or is equivalent length enough?

hollysprings: I did an online calculation (broan website) for the make-up air, and I don't think a dedicated make up air is required by code for our set-up? We're in So Cal. I also asked the inspector specifically about the make -up air, and he said not required? We plan do connect a passive damper as recommended by broan.

michellemarie: We asked our heating and A/C contractor, but he's not very knowledgable about kitchen exhaust. What should I look up for? A commercial kitchen exhaust designer? HVAC ME? Would a duct cleaning company have this expertise (that's all I seem to find when I google kitchen exhaust duct design)? How much should I expect for a design consultation? We are on a pretty tight budget for a whole house addition/remodel.

Thank you all!
PRO
Joseph Corlett, LLC
9 years ago
" I did an online calculation (broan website) for the make-up air, and I don't think a dedicated make up air is required by code for our set-up? We're in So Cal. I also asked the inspector specifically about the make -up air, and he said not required? We plan do connect a passive damper as recommended by broan."

CharissaMK:

The building code is what the inspector says it is on the day he says it, regardless of whatever has been drawn, written, and signed previously.

You need to make a compelling case as to why whatever you're doing meets or exceeds industry standards, just in case.
jackfre
9 years ago
This is, as you know not ideal, but...it is what it is. I would have a 10" pipe off the top of the hood connect to the 8x12 duct right above the hood. You would need the first horiz 8x12 piece to tie into the 8x12 riser. At the top you tie your 10" round back into the 8x12 rectangular riser. You just cut a 10" hole into the 12 face of the riser. It has to fit tightly. The tricky part on this is the first joist bay where the 8x12 transitions to vert. However you do this make sure you SEAL ALL THE JOINTS with real duct sealant.

I piped my new hood with round pipe last year. The roof cap was mostly flat on top. When it rained it was like a drumhead. I put a piece of ice and water shield on the cap and it is now all good.