# Vent Hood CFM's



## Dragon (Aug 1, 2006)

*Hood CFM*

I recently built a house and installed a hood from Broan in the kitchen. The model line is called "Best by Broan". I went over kill and installed the 900 CFM blower fan. It is an exterior model blower unit that is mounted on the roof. Very nice unit all around.

Now that I have had the unit for a while I realize that the 600 CFM blower unit or maybe even the 300 CFM would have been just fine. Oh well. 

The nice feature is that this blower is that it is controlled by a sliding switch, similar to a dimmer, that allows you a lot of control over the blower speed. After having this I will never go back to a High-Medium-Low only type of switch. This switch also helps with the fact that I oversized my unit as I simply leave the speed on the lower end and I don't utilize the full power available from the unit. That is unless I burn something and really need to vent the smell out of the house.  

I would also recommed getting a unit with the Exterior mounted blower or even an In-Line blower. Do not get one with the motor and blower mounted right in the hood unit. These are the noisiest as the blower/motor is right there in front of you at head level. My personal recomendation is to get the smallest unit that allows for exterior or in-line mounting. Those usually start at 300 CFM and up.

With my unit I can just hear the blower in the distance through the duct tubing, noticable by not annoying like the ones mounted in the hood.

Previously I have only owned a house and had apartments that recirculated the air through one of those over the range microwaves. Those things are useless unless the unit vents to the exterior. Make sure yours vents to the exterior.

Other things to keep in mind. The hood unit itself should be as wide as the stove below it and preferable a few inches wider. The front edge of the hood you choose should come out as far as possible from the back wall. Preferably it will come out almost as far as the edge of the counter or stove. The reason for this is that a lot of hood units don't extend far enough to capture the stuff coming up from the front two buners. So if you are boiling water the hot moist air will rise up towards the hood but a good portion will esacape into the room if the hood is too small. The bottom line is that the more area of the cooking surface covered by the hood the better the venting will be.

I also run the unit any time I run the gas cook top, even if I am just boiling some water for pasta. This ensures even that small amount of moisture and exhaust gases get sent out of the house.

Good luck selecting a unit.


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## wease (Dec 5, 2006)

Thanks for the great information. Do you have any ideas for manufacturers who make a 300 or 600 cfm "chimney style" vent hood with an external blower?


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## Dragon (Aug 1, 2006)

Check out http://www.bestbybroan.com

They have many different styles. Not every model is compatible with an exterior blower. Select ones that look interesting and in model descriptions it will list if an exterior blower is an option.

Many appliance stores can order these for you or you can find a dealer online.

Good luck. Be sure to come back and post what you selected.


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## troubleseeker (Sep 25, 2006)

wease said:


> I'm getting a new range and vent hood in my kitchen and I'm wondering what is an acceptable cfm rating for the hood. Code says I don't even need one.
> 
> I'll have a gas grill on the range, but I don't cook for a large crew all the time. Just wondering what CFM I need that isn't overkill. If I get too much, then I need to install makeup air systems and I don't want to do that.


Be aware that the high cfm units with exterior blowers may require you to upgrade the size of your existing vent pipe. Do you have access to do this? The typical hoods require 6 or 7" inch duct so that is probably what you have, whereas it is not uncommon for the big external blowers to call for 10"


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## wease (Dec 5, 2006)

troubleseeker said:


> Be aware that the high cfm units with exterior blowers may require you to upgrade the size of your existing vent pipe. Do you have access to do this? The typical hoods require 6 or 7" inch duct so that is probably what you have, whereas it is not uncommon for the big external blowers to call for 10"


I'll be replacing the whole works. Access is pretty simple. Still looking for a reasonably prices 300 cfm exteral blower model that's a chimney style in stainless. Wow.


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## troubleseeker (Sep 25, 2006)

You should be able to find that style between Broan, Vent a Hood, Viking, and even the GE upper series. But I'm not sure you will find an external blower option on the smaller cfm units. 

"WOW"..I guess that is a reaction to prices you have received.


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## wease (Dec 5, 2006)

troubleseeker said:


> You should be able to find that style between Broan, Vent a Hood, Viking, and even the GE upper series. But I'm not sure you will find an external blower option on the smaller cfm units.
> 
> *"WOW"..I guess that is a reaction to prices you have received*.


You bet. For what it is I'm surprised.


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## STEPHENWANGEL (Feb 19, 2007)

*Cutting through the confusion...*

Let's go back to square one and figure out what you REALLY need in terms of CFM's. The general rule of thumb is that you take the total output of ALL burners, including the grill and divide by 100. Add extra CFM-S for a long run or more than two elbows. Don't forget that the grill will generate a  -load of smoke. Surely, I'm thinking minimum 900-1100 CFM. Now, don't check in here at DIY central, and forget about the big-box stores. Talk to a reputable appliance dealer who must stand by the information that he gives you. Anything that you do in the 900+ range should be an exterior blower so that you don't hear the motor noise. I' have specified Best by Broan numerous times (oh, yea, I'm a professional kitchen designer). Vent-a-hood is also very good along with Thermador and Viking.

Happy shopping:thumbsup:


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## cmorris (Mar 14, 2008)

*vent hood CFM's*

We have a gas stove and 48" GE Monagram vent hood. We have had 2 years of our HVAC trying to bring in fresh air. This has been a major problem that I think the vent hood companys should answer. If anyone has any suggestion on this problem be our guest!


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## stan 41943 (Feb 27, 2008)

troubleseeker said:


> Be aware that the high cfm units with exterior blowers may require you to upgrade the size of your existing vent pipe. Do you have access to do this? The typical hoods require 6 or 7" inch duct so that is probably what you have, whereas it is not uncommon for the big external blowers to call for 10"


My 48" vent hood does not have an external blower & recommends 6" ducting. Seems to me that a 6" + hole in my roof is just begging for trouble (leaks etc.) How is the end of this system booted & capped?


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## vlad_mi (Apr 17, 2008)

Can I get a quote too? I'm looking for 36 wide hood with external blower. 

Thanks


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## STEPHENWANGEL (Feb 19, 2007)

*Range hood vent*

The ball-park rule for range hoods seems to be 1 CFM for every 100 btu's of burner output.

Use caution when using this rule. The longer the run is to the exterior and how many turns are in the run can increase back-pressure and lessen the effect of the blower. Also, be sure to use the minimum size duct recommended for the hood.

Finally, consult a professional before determining blower rating, or whether to use an internal/external/mid-line blower.


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## Roddymacroddy (Apr 19, 2012)

I've often wonderd whay these venting solutions don't use some kind of venturi design. Simply, for a 6" vent pipe, you could install an inline fan to a larger exhaust with the fan suspended in the middle of the air flow. The expanding air would have to gain speed to get past the fan, creating a low-pressure on the 6" side. This would draw more air, and be quieter too.


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## Jacques (Jul 9, 2008)

It would bang when it shuts off.


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