you're going to spiral or otherwise cram 14' of flexible vent around the inside of that 'box'? That sounds a bit restrictive, to say the least.

I'm not even sure it's legal to use flexible hosing with a vent hood - too much risk of grease build up and fire. I think Broan gave you excellent advice - get an integrated hood and vent directly out the back. Most of the noise is NOT from the motor but from air noise - in fact my limited understanding is that shorter runs give you less noise and more effective venting. I think you're actually lucky to be able to vent directly out the back.

Weissman, the flexible aluminum duct is rated at over 200 degrees internal and 500 degrees external, broan requires 140 degree internal, weedmeister, do you mean restrictive from the standpoint of airflow through the pipe, or from the standpoint of cleanly looping the pipe inside the box?
thanks, TMY23

I'm not talking about the rating per se - I'm talking about building/fire code - I really think you need to check about flexible ducting for a range hood. And even if you keep your external blower, I think you should vent directly out the back to it - why do you think looping a longer duct will decrease the noise?!?

Weissman, I got that from the tech at Broan who said that the longer the duct, the less blower noise makes it back into the kitchen through the insert

The longer the duct the less blower noise is because the blower is farther away from your kitchen. Winding all that duct up inside a box is going to create wind resistance which will cancel out the point of going with an external blower.

Can you mount the external blower on the outside of your house and just run the short piece of ducting?

I seem to remember a rule that every 90deg bend is equivalent to 5' of straight pipe. Assuming you have a 'coil' of 14' with lets say 7 coils, thats 7 x 4 x 5 + 14 or about the equivalent of 155' of pipe. I doubt your blower could successfully pull anything through that 'length' of pipe.

If the blower is mounted on the outside wall, and the box is sufficiently large, I would think about using insulation on the inside of the box.

Make the ducting as short as possible, not longer for reasoning regarding noise.
Use galvanized smooth duct, not flex.
Use two 45-degree turns with a short length of straight duct in between them if you have the room.

Yes, I can go right out the back wall of the "box" through the outside wall.

Flex duct is not allowed under most building codes for venting a range hood.

It will not contain the fire that can start from the grease build up.
"rated at over 200 degrees internal and 500 degrees external"
is not going to come close to containing a grease fir.

The corrugations required to make the duct flex will also trap grease, and the convoluted run will allow the air speed to drop low enough the grease WILL settle inside the duct.

Noise is the price you have to pay for moving a lot of air.
Get a variable speed blower and only turn it up enough to remove the smoke.

Ok, got it. I assume standard 26 or 30 guage galvanized round ductwork is sufficient to meet code?
thanks

tmy
I echo what others have said - not using flex, going straight out, additionally - you have only 600 CFM rated hood for a power house of a range - by the time you get done with your configuration you may have cut the CFM's in 1/2.
Broan did give you excellent advice - If you have not already purchased one, see if you can get a higher rated CFM one.

The noise is indeed relevant - People go out of their way finding the quietest hoods & dishwashers, but knock down walls creating a very noisey new kitchen.

Good luck

As always, thanks to all those more experienced than I, I appreciate the advice. I will go to rigid metal duct, straight outside.

Now another question about noise and how to mitigate it. What is (are) the major sources of noise?

- Is the noise primarily from the motor right down the duct through the insert into the room? In which case I can't think of anything other than something like a fantech silencer which will fit, but forces me into 2 more elbows than I'd have with a direct out. Maybe two additional elbows with rigid duct is a worthwhile tradeoff to incorporate a silencer?

- Is the noise from the motor through the wall of the house into the plywood box and then into the room, in which case should I consider filling all gaps in the outside and inside wall with foam or some sound absorbtive material, pack insulation around the duct where it comes in through the outside wall?

- Is the noise due to vibration? Would you consider vibration isolation between the external blower and the sheething on the outside of the house? Would you consider sound absorbtive material inside the plywood box to absorb sound there?

Thanks...