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Old 01-18-2012, 03:58 PM   #1
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Converting existing island range hood from internal to external blower


We have an island range hood with an internal fan that is supposed to be 600 CFM. it sounds like a jet plane taking off on it's highest speed, but still fails to capture all of the smoke when we sear steaks and such (doesn't come close).

..so what I want to do is remove the internal fan and attach this 1300 CFM external blower to the roof where the ducting comes out.
http://www.ajmadison.com/cgi-bin/ajm.../VTR1330E.html

My main concern is that the external blower calls for 10" ducting and we have 6" ducting in place (it would be a major pain to change out the ducting.)

The ducting length is only about 4 feet from intake to exit (eichler home/no attic).

Can I get away with using a 10" to 6" reducing duct fitting to attach this thing?

Thanks

p.s. we have a window near by that we open for makeup air when the fan is on high speed


Last edited by ihstiv; 01-18-2012 at 05:07 PM.
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Old 01-18-2012, 07:46 PM   #2
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Converting existing island range hood from internal to external blower


no no no to small....won't handle the cfm

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Old 01-18-2012, 08:03 PM   #3
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Converting existing island range hood from internal to external blower


Noise would really get to you
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Old 01-18-2012, 08:16 PM   #4
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Converting existing island range hood from internal to external blower


It's simple math. The 10" duct is more than 30% greater capacity than the 6". It's not even close.

You said this is configured as an internal blower (i.e., "filter"). I would suggest that you'll get dramatically better performance by venting it to the outside and sticking with the 6" ducting. If that does not give you any joy, then you can explore getting a more powerful blower and changing out the ducting to meet the needs of the increased CFM capacity of the blower.

6" is borderline in terms of accomodating 600 CFM (that's trying to move 10 cubic feet of air PER SECOND through that relatively narrow duct), but if it is evacuating to the outside instead of routing through some silly charcoal filter and dumping it back in the room it might just be enough for your needs. Certainly is worth trying.

Last edited by Ironlight; 01-18-2012 at 08:18 PM.
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Old 01-18-2012, 09:01 PM   #5
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Converting existing island range hood from internal to external blower


Thanks everybody!

It is venting to the outside.. just not moving enough air and too loud.

I guess i ll have to figure out what it will take to get some larger ducting in there. There is a beam right in the way. Maybe rectangular duct...
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Old 01-19-2012, 05:11 PM   #6
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Converting existing island range hood from internal to external blower


A couple questions for your responses:

Quote:
Originally Posted by ben's plumbing View Post
no no no to small....won't handle the cfm
Makes sense that I wouldn't get the full 1300 cfm, but could I expect an improvement regardless? Would it ruin the new blower somehow?


Quote:
Originally Posted by qbert View Post
Noise would really get to you
Can you elaborate on this? would the small ducting whistle or something?
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Old 01-19-2012, 07:44 PM   #7
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Converting existing island range hood from internal to external blower


smaller duct= higher velocity= blower noise
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Old 01-19-2012, 08:35 PM   #8
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Converting existing island range hood from internal to external blower


I don't think that air is just going to "move faster" through a narrower than specified duct. Rather the blower's ability to move are at its specified rate is going to decrease due to resistance.

It's hard to say if you'd see an improvement. The blower is designed to move air through a 10" duct. It may move only half as much air through a 6" duct, and might in fact have a shortened life if it is always running under those loads at higher settings. It's $800 that may not do what you want.

Is the existing ducting smooth, rigid duct? If its not, and it's that flexible stuff, replacing it with rigid duct will give you better performance.

An Eichler home. Sweet! Those are really lovely houses.
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Old 01-19-2012, 09:45 PM   #9
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Converting existing island range hood from internal to external blower


Thanks so much guys - this is great advice, and makes good sense!

I'm going to start off with taking a look at getting some custom duct work done that will increase the flow - it's definitely not optimal now - some funky turns, and combo of flex tube and rigid.

Cheers

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