When does the effective length become a problem? If I go out through a gable end, I am looking at about 35-40' of actual duct length, but could fit two 45's if better. If I go straight through the roof, I am looking at about 15-20' of actual duct length. My number one concern is noise. If the bends/length require higher cfm, I can size up the blower, but I want to do whatever will be quietest.

Can you fit a muffler (sound suppressor) such as Fantech sells? That is the best approach trade in minimizing pressure loss while maximizing sound reduction. Otherwise, we need to consider that the factors that cause pressure loss (and hence reduced cfm for a given fan design are:

• Lack of adequate MUA
• Baffles
• Abrupt transitions in the hood
• Duct length (actual or effective)

where the last two may swap places depending on the duct length one has and the hood design.

I think that there are calculators on the www that will provide a pressure loss with duct length for a given diameter and cfm or air velocity.

Bottom line, longer may have a slight or significant effect in reducing achieved cfm depending on a bunch of things.

Sorry for no exact answer, but the problem is not fully defined.

kas

P.S. The shorter straight up path will reduce potential grease deposition, although, again, the devil is in the details as to whether the difference is significant.

Go out the gable end.

Better to not have penetrations in your roof if you can avoid it, and you wont have to have a ramp made and flashed to support the motor.

1000 CFM at 40" is still plenty of power unless your running a deep fry operation 10 hrs a day.

Just use smooth wall 10" dia. pipe and you'll be fine.

Thank you for the responses. The various components going into hood design do tend to make it quite complicated. Basically, I have it sorted down to two options:

a) Through the roof: ~18 foot straight run up to the roof mounted blower.

b) Through the gable end: ~40 foot (actual) run, including one 90 degree bend to a wall mounted blower.

For both options, I will have 10" smooth walled ducts, an inline Fantech silencer, and either a 900 or 1200 cfm blower.

My main concern here is noise (because I believe either setup will be sufficient to eliminate gasses and odors). It seems almost as if it is impossible to say which option will be quieter without trying them both.

From a purely noise standpoint, does anybody have any thoughts about which of the above two options would be quieter (in the kitchen)?

Thanks!

The silencer will remove most high frequency blade tip noise, as well as duct turbulence noise from the portion of the duct between the silencer and the exterior blower. I expect this to be independent of duct length. Low frequency vibration from imperfect blade assembly balancing carried down the ducting will not be suppressed by duct length or the silencer, but can be ameliorated by wrapping the duct with automotive dampening materials that are (or seem to me to be) lead filled vinyl with an adhesive on one side. If used, support them with long ty-wraps or metal straps.

Baffle turbulence noise will be the same for each case (ignoring the slightly lower flow rate of the longer duct system). This fairly white noise is unavoidable in any affordable baffle system, but should not interfere with normal conversation at the cooktop or across it.

kas

Thanks Kas,

It sounds like if I am using a silencer, I should be ok with or without a 90 degree bend, and the silencer will compensate. Do you have a link to the automotive dampening materials that you referred to? How much area would have to be covered? The whole run?

I used Evercoat Q-Pads covering about six to eight feet (working from memory here) of straight duct leading up to the silencer. I recall someone here several years ago raised an alternative material that he used, but I don't recall its name.

You may not need much or any depending on whether duct dampening improves sound suppression, which depends on the blower design (maybe on its execution).

Here is an example link, but any automotive parts store that supports the body work trade would likely carry them.

http://www.oreillyauto.com/site/c/detail/EVCR/117/N1668.oap?ck=Search_N1668_-1_-1&pt=N1668&ppt=C0107

Perfect, thanks! I will look into that.