Is 6 in. vent big enough for 36 in. range top ?

johninsocalJune 29, 2012

We are considering a 36" range top (bluestar, CC, Wolf) with a grill and a 36" hood however our current vent in the wall is only 6" round and replacing or enlarging the vent doesn't seem like a possibility in our 2 story house as the vent runs along the second floor and out a wall. I am a bit worried that the 6" won't cut it. For grilling we are talking mainly steaks, I wouldn't throw fatty burgers on there which would produce a ton of smoke. Will this work with my 6" vent ?

thanks, -- john

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It may--you will need to check 2 things, but first you will need to determine if you will be able to use the current vent you have or if you will need to upgrade to a higher CFM.

-What does the manual for the rangetop recommend? The rangetop manual will tell you if you can use your current vent or if you have to upgrade.

Once that's determined, you can move on to question #2...

-What does the manual for the appropriate vent hood call for?

Hope this helps!

    Bookmark   June 29, 2012 at 9:27PM
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There are 600 CFM hoods that work with 6" ducts. That's a bit light for a grill but you can probably make due.

    Bookmark   June 29, 2012 at 9:45PM
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My 600cfm hood requires an 8" duct. For a grill, 1200cfm is usally recommended so no, 6" will not be OK.

    Bookmark   June 29, 2012 at 10:04PM
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You can make due with a 6" duct you just need more power to suck the required air through smaller area to properly vent grill and burners. It might be kinda loud.

I don't have a PhD from MIT nor Cal Tech.

Maybe Kas will chime in and help :)

    Bookmark   June 29, 2012 at 10:22PM
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Have you picked a range hood? If so, download the manual and see what the specs require.

I've got a 36" 600 CFM Zephyr Cyclone. It specs a 6" diameter vent pipe and I've never had any problem clearing the air. However, I do not have an indoor grill on my stove. The thing would likely be inadequate if I did.

For a grill, I'm inclined to agree with cooksnews that 600 cfm would not be sufficient.

If your chosen hood requires a 7" or 8" tube, you could get an adapter to neck down to 6" but deegeaux is so right about it generating a lot more noise.

If you are only grilling steaks, as opposed to fatty burgers, I wonder if you might not be happier with a griddle and doing your grilling on an outdoor gas grill. Maybe the outdoor grill would be problem for you? I ask, because I live in Montana. Half the people in town haven't locked a door in 20 years (so they don't worry about theft and vandalism of grills --- except from the resident bears). We grill outdoors even when winter temps can drop way below zero. On the other hand, we do not have to deal with rainy days like the recent deluge in Florida. (26 inches in two days? Sheesh, we're lucky to get that much preciptation in a whole year.) Our mountainous area is generally pretty light in biting bugs like mosquitos, as well. Do you need to grill indoors because of neighborhood, weather and/or insect conditions? If not, I'd suggest rethinkng the indoor grill.

    Bookmark   June 29, 2012 at 11:08PM
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I have checked out the CC hood which for 36" they say it's 1200 CFM and says a 6" or 12" duct cover but their documentation is not really clear on this. I'll make some calls to CC and wolf and Modern Aire to see if I can make due with a 6" duct. As for outdoor grilling sure that is an option here in socal where weather is usually nice, but I was looking for some convenience for the occasional grilling of a steak or some hot dogs, asparagus, stuff like that. I'll post back what information I get from the manufacturers.

    Bookmark   June 29, 2012 at 11:23PM
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Ignoring noise, getting a lot of air through a smaller sized duct is just (!) a matter of using a fan with the desired flow rate at the estimated pressure drop of the constricted path (including baffle losses, transition losses, negative house pressure loss, etc.). As the pressure drop increases for a given cfm, axial fans designed for higher pressure loss may be best.

In the supplied link is a description that may be useful to some, even if the fan shown is unsuitable for three reasons (400 Hz, 3-phase; likely a real screamer; non-monotonic fan curve.) But note the high cfm at high pressure loss that this type of design can manage.

(A non-monotonic fan curve can cause hunting if one tries to operate part power within the reversed slope region.)

In the realm of best is the enemy of sufficient, if the baseline fan is in the hood, using a booster fan at the exterior to compensate for the pressure drop could be solution. An "exotic" solution would not be required.


Here is a link that might be useful: AMETEK Rotron

    Bookmark   June 30, 2012 at 10:19AM
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john, is it a 6" duct over a few feet length, or over a far longer distance?

    Bookmark   June 30, 2012 at 5:43PM
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If you are going with a BS range top you might want to look into their matching hoods. I purchased a few years back I think I recall their 600 CFM hoods using a 6" duct. They are pretty nice hoods (aka Prestige). I have the 1200 CFM motor with a 8" duct run and only one turn. It is noisier than I would like but most of the noise I have found is the air going through the baffles so likely not related to the undersized duct (10" would have been better). However, it pulls like crazy. Good luck!

    Bookmark   June 30, 2012 at 5:58PM
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Ok I did a little more research. My current vent goes into the wall with a 6 inch round however that is just a cover, inside the wall is a 10 inch by about 3.5 inch square duct that runs up the wall to the second floor (about 3 feet) and then this has a 90 elbow into a 6 inch round duct that runs 12 feet in the second floor to the outside of the house. So this is probably not an ideal layout but this house was built about 16 years ago and this vent was just for over the counter microwave on top of a standard range.

I could make no changes and use the current duct work but with this small 10x3 section I am worried about air flow. There is also a possiblity that without too much trouble I could go up to my second floor with a chimney hood and then 90 elbow and run along the second floor in the kitchen room to the outside wall only about 6 feet if I go this direction. I believe the joists run the proper direction in this room, in my dining room they run the other way which is why I suppose they close to vent it that direction and be hidden in the wall. Here is a picture of my current vent

    Bookmark   July 3, 2012 at 3:41PM
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The standard rectangular duct is 3"x10." Slightly larger cross section than a 6" round duct but generally regarded as interchangeable. The builder probably converted to the 3x10 rectangular to fit into the relatively shallow wall space.

For a 600 cfm vent hood, your present run is adequate. Not great, because of the length and having two 90�bends. (Lenght and bends reduce the effective airflow but). As is, it and a 600 CFM hood would probably work reasonably well for most everything other than that indoor grill.

Shorter runs with fewer bends being better, your idea of going up and out with a single 90� bend would improve air clearing even if you stuck to 6" vent ducting.

    Bookmark   July 3, 2012 at 6:29PM
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From your description of the ductwork, I would estimate an equivalent 40 feet of 6" duct, counting bends and adapters. At 600cfm, that gives 1.2 inches of H2O pressure drop. The performance curve for my Broan rooftop mount 600 CFM blower doesn't even go that high, but some blowers can still perform with significant static pressure. With my blower and your duct, I would expect about 420CFM as installed. With a different Broan 600CFM blower and your duct, I would expect about 500CFM. If you have a performance curve for the blower in question (CFM vs. static pressure drop), see where the curve lines up,

600CFM 1.19" static pressure drop
500CFM 0.83" static pressure drop
400CFM 0.53" static pressure drop

    Bookmark   July 5, 2012 at 7:02PM
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j in socal, some blowers cannot push against the pressure that your ducting will generate. Some can. This deserves attention now

    Bookmark   July 5, 2012 at 10:25PM
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David has a great point. There are vent hoods on the market that have the ability to detect push-back and then ramp up their CFM to overcome a reasonable amount of static pressure. That would be the easiest solution to get past the rectangular plenum and 90 degree turn. And kas suggested adding an exterior ventilator to work in conjunction with your interior unit. If you buy the units from the same manufacturer then you may be able to sync the fan speed controls. That might reduce the overall noise level of using your hood on high as well.


    Bookmark   July 6, 2012 at 9:10AM
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