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CoconutPete 01-25-2012 09:09 AM

Booster fan on exhaust hood in kitchen?
Well ... one thing about tearing into old houses - there's always surprises :thumbup:

Long story short... because of the unique way the pipes are, I'm not going to be able to vent my hood to the outside the way I originally wanted. This is going to be a combination microwave / exhaust fan by the way.

The duct will need to come up from the microwave and turn right, run 3' to the right and then make a more or less 90 degree turn. The duct will then need to run just short of 12' to the outside wall.

Because of the 12' run I'm thinking I might need a booster fan. Is there a rule of thumb that I can use to find out if I actually need one? Number of 90 degree turns? Feet length of duct line?

jklingel 01-26-2012 12:12 AM

Coco: See what the instructions or manufacturer says. That always depends on the horsepower of the motor and size of pipe. You may already have a "booster" motor, in effect. BTW: When will the burgers be done?

CoconutPete 02-02-2012 01:05 PM

Burgers! We've been living off Microwaved frozen stuff my wife cooked before we tore the kitchen out haha.

Started working on hanging the microwave last night but decided to suspend those plans in favor of getting the sink working so we actually have a barebones kitchen again.

Still didn't see anything in the documentation about length of ductline, just a LOT about taking the blower out of the microwave and facing it a certain way depending on how you are venting it.

astersia 02-05-2012 02:52 PM

Why not disable the motor in the microwave and use an inline or exterior mounted fan with higher cfm's to account for the longer run?
I guess this is a "booster" fan, in a sense, but just replaces the inside motor completely. Outside mounts are much quieter, too.
Maximize the diameter of the duct.

Perhaps this is what you're thinking of:

I've not used this, but do have an exterior mounted Broan unit, many years old.

abracaboom 02-05-2012 05:13 PM

I think you're over-thinking this, Coconut Pete.

CoconutPete 02-06-2012 01:57 PM

I like the idea of the exterior mounted unit, takes some variables out such as accessing the hidden unit etc. I will look into that one - thanks.

abracaboom - why do you think I'm overthinking it? Do you think my run is not long enough to worry about a secondary fan?

beenthere 02-06-2012 03:24 PM

Doubt you'll need a booster fan. Is there a rating such as X CFM @ X static pressure?

CoconutPete 02-07-2012 08:12 AM

Not sure. Will check paperwork tonight.

hairy 12-13-2016 02:51 PM

Re: Booster fan on exhaust hood in kitchen?
I've got the same question as CoconutPete..

I am redoing the kitchen. The layout of the house requires a 12' run of the 4" duct from the over the range microwave/exhaust fan.

The fan in the microwave is rated at 400 cfm (in boost mode).
Because of the long run, I am considering putting in a booster fan rated at 135 cfm (A number I believe more than the microwaves fan spec).

The fan would be located about midway of the 12' run.

The booster would have a separate 3 position switch (off/low/hi).

I don't know:
1. If this is really necessary or of the microwave fan can push the air 12' sufficiently.

2. If the presence of the booster will actually help or hurt the exhaust capability of the (according to spec) higher powered fan from the microwave.

3. Can the 2 fans be used separately, or if, when using one, I should always use the other.

Pete, Are you still around? How did you make out?

Bret86844 12-13-2016 04:08 PM

Re: Booster fan on exhaust hood in kitchen?
12' is not very far to go. Your elbows will probably have more pressure loss than your 12' straight run. If you are worried, use smooth, radius elbows for the duct(no flex crap).

Hairy, do NOT put a 135 cfm booster in a duct carrying 400 cfm. Even if your microwave is pushing less than 400, it's still probably pushing more than 135 and this fan will reduce the airflow instead of speed it up. The motor is only made to spin so fast.

user_12345a 12-13-2016 05:01 PM

Re: Booster fan on exhaust hood in kitchen?
You pretty much have no chance in hell of moving 400 cfm or even 200 cfm through a 4" pipe.

If you can get 100 cfm through a short run of 4", you're doing well. over a long run i would expect less.

4" is too small for range hood fans. it's a common mistake. it's fine for bathroom fans which move 50 to 100 cfm.

Use 6" and forget the booster.

If you use hard pipe the pressure drop won't be too bad.

You won't get 400 cfm though, it's totally unrealistic unless you resort to 8" round duct or something.

RRH 12-13-2016 07:46 PM

Re: Booster fan on exhaust hood in kitchen?
Just some considerations.

Don't worry about the 12ft.

400 cfm is fine occansionally on a 4" line see link below.

I will mention the panasonic inline fans are quit and good quality.

My gripe with kitchen exhaust fans is how they let in so much outside air when not in use. A real energy waste.
You can find a damper that seal good due to having some use at low speed. Matter of fact only know of a couple dryer vents that seal pretty good.
But these can be used for a dryer since exhaust is one high speed only.

Next project I may consider only a kitchen exhaust with one speed. High only. In order to use one of these dryer vents.

user_12345a 12-13-2016 08:03 PM

Re: Booster fan on exhaust hood in kitchen?

400 cfm is fine occansionally on a 4" line see link below.
That page was written by someone who doesn't know anything.

um, 400 cfm through a 4" pipe would be 4583 feet per minute.

You would need a friction rate of 15" wc per 100 ft. If the op had 20 feet equivalent with elbows, etc that would still be 3" at the outlet of the fan.

That's absurd. Even 1" of static is very high and most fans will suffer diminished performance.

In an heating systems you wouldn't get 400 cfm through even an 8". even 100 cfm would use at least a 6" pipe - granted the available static is different vs what an exhaust fan produces.

In reality there's no chance in hell a run of the mill exhaust fan will move anything near 400 cfm even through a 6".

The manufacturers often test them with no ducts connected to get a bigger cfm number for marketing purposes.

The proper way to do things is to get a fan performance curve, get the equivalent length and size the duct for the cfm you want.

No one doing residential would go through that though for an exhaust fan.

it's the only proper way if you need a minimum flow rate.

Use the right size, don't cut corners based on flawed rules of thumb.

user_12345a 12-13-2016 08:39 PM

Re: Booster fan on exhaust hood in kitchen?
Here's a duct calculator:

If you google "hood catalog pdf" and click the first link, you'll get a spec sheets of commercial grade hoods which are probably infinitely better than the consumer ones.

example - model B300 is speced to move 300 cfm at 0.1"

For a 12' run like the op's with 3 elbows, it says just over a 8" duct.

Use a much smaller duct, like a 4" and the performance would be a fraction of the rating.

Use 6" and it will move lots of air, but not 300 cfm.

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