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clasact 06-19-2007 07:03 AM

vent hoods
I am putting in new vent hoods over my range,it is a commercial type range in my home.The new hoods come with 6 inch duct work but I dont think that is large enough for the amount of air they can pull 900cfm.It is a 60inch rang so I am putting two matching hood over it right next to each other.I have checked with pros to see if this will work and was told it should be fine also checked for any codes for this sort of application but cant find anything that really applies in my area except for resturants and that calls for even more things like fire suppression systemy and so on.So what I need to now is should I replace the duct work with a larger size if so what.I would like to run them above the ceilling and T it together then out the roof.Got any suggestions

skymaster 06-19-2007 03:16 PM

Since the Mfg bilt em with 6" then 6" is no problem. My thoughts are that you run 2 runs of 6" and then TY NOT T into a transition that meets the 1800 cfm or a tad more into the vent hood of that size.
If you look in Yello pages you probably can find a sheet metal supplier and they can size it for you and supply it also.

bigMikeB 06-20-2007 10:57 PM

If it is a commercial high btu type range then you should use a commercial type hood. There are guide lines for the height of the hood from the range top and even the depth of the hood itself to properly pull heat and greasey smoke from over the stove. Keep in mind that the temperatures that a commercial range makes exceeds the rating for a residential hood and the possibility of a grease fire in the vent piping is where you will get yourself in trouble. You will need a dead air space between teh hood itself and the wall material behind it that is one of the big differences with a commercial hood. Also the vent piping will need fire retardent insulation. I think you should talk to your local Fire Inspector to see what local codes call for in your area.

clasact 06-22-2007 05:51 AM

I talked with the local fire inspector yesterday and according to him I don't have to use a commercial hood system since it will not be used for commercial needs ( it wont be in continuous use and not a lot of volume like in a restaurant) The codes say it must be 3 inches from combustible material and at least 3 feet above the cook surface which I already figured in also for new construction anything above 650cfm should be more then 6inch duct from the top of the hood to the outside and must protrude a min of 10inch above any outside I am thinking I should run 8 inch from the top of the hoods existing duct to a y which will connect the two and then out the roof if I think the blower is to loud I can then put a external blower on the outside sound reasonable

troubleseeker 06-24-2007 05:41 PM

If the manufacturer has the unit set up for six inch duct, that is all that is required. Unless the installation instructions say that you can transition to larger duct, you may actually hurt the performance of the hood by increasing the duct size and thus decreasing the velocity of the air. If you bring the two duct runs together, use a wye fitting and increase the outlet to 8 inches, not a tee. If you suspect you may end up with an external mounted blower, check their specs first, many of of the ones I have seen actually require a 10 duct to connect to at the wall or roof.

clasact 06-24-2007 06:01 PM

thanks troubleseeker I didnt know that about external blowers or hurting the vents proformance

fireguy 06-24-2007 10:50 PM

Section 506 International Mechanical Specialty CodeThe hood shall extend at least 6 " on both ends and the front of the range. The hood & duct needs 18 " of air space to combustables. using a layer of 5/8 sheet rock ( 1 hour rated)will reduce clearance to 3 ". Duct/fan termination to be 40" above roof. All joints and connections shall be welded. Do not use silicone, it will deteriate with time and heat and grease. There is more, but the building inspection office will have what ever code may be enforced in your area. Most Fire Departments have absolutly no idea what you are talking about, so go the building inspection office. Don't forget a permit.

The exhaust hood should have come with detailed instruction for mounting. If each hood (why 2 hoods?) has an exhaust duct, do not use 90 bends unleess absolutly required. Shallow bends donot decrease efficiency of air flow as 90 bends do.


clasact 06-25-2007 09:58 AM

the reason for two is to cover the overhang needs and cost.If I had a custom hood made it would run somewhere between 2500 and 3000 dollars and restaurant equipment is running close to the same for hood even used ones.As far as the other requirements I have left a 6inch space on both sides and behind the range the framing will be coved with cement board up to the hood and that will get covered with a stainless steel panel above and behind the hoods will also be cement board and above the hoods will be stone(only about 18 inch from hood to ceiling. The outside wall is brick so the only combustible part are the studs and insulation I had to order the hoods so I don't have the instructions for installation yet but if I have to take them both up through the roof thats ok but if I can I still want to Y them together to one (rather have one hole in the roof then two).As for the inspector my area uses a third party inspection company with whom I have talked to on this subject and many other areas of this house renovation.Basically what have been told is for my county their are not many rules for a renovation unless I am adding on or adding or upgrading electrical or gas lines.For all my renovations I have been trying to follow the codes for new construction and even overbuilding in many areas(when putting in new windows and door I have put 2x10 as headers and sills also tripled the studs around them) so its not like I am trying to jack leg any of this it just that getting the right info around here is somewhat difficult and trying to to stay in budget do the work right and safe plus have it functional and looking good has been no easy task either since everything I open up creates a new can of worms

fireguy 06-25-2007 06:41 PM


Originally Posted by clasact (Post 50392)
is somewhat difficult and trying to to stay in budget do the work right and safe plus have it functional and looking good has been no easy task either since everything I open up creates a new can of worms

In other words, it is the typical remodel? Good luck and post any more question you might have.


clasact 06-26-2007 03:06 PM

yes I guess it is but I still like to ask the pros so the house dont fall down around me or burn it up .It may be just a typical remodel to most of you but to me its a big investment and my home.Not that ya all dont see that but I think sometimes their may be so many questions and some of you do so much of this kind of work that that point get lost a bit but thanks for all the info it is of great help to everyone

slakker 06-26-2007 03:21 PM

I know in BC Canada, there's code that regulate the amount of air return that must be put in to accomodate that much air being removed. In my friends renovation, they didn't past inspection until they were able to put in new cold air return in the kitchen space so as not to create a vacuum in the house.

pjpjpjpj 07-02-2007 11:41 AM

Commercial codes not only require welded joints, but typically require stainless steel duct of a certain gauge, and require the duct to be sloped back towards the hood (so that any accumulated grease would run back). Typically the code will give you a minimum feet per minute (FPM) to ensure "grease capture" so that would drive the size of your duct, both in the single runs and in the conbined portion.

Just things to think about - if your code guy says it's not "commercial", then that's a whole different story.....

bigMikeB 07-02-2007 05:14 PM

Where is this code
I have been doing hvac in the US for over twenty years and have never seen a code requirement for stainless ducting. The ducting for a commercial hood is made from black steel with welded seams and flanged or welded joints and wrapped with synthetic asbestos insulation jacketing.

pjpjpjpj 07-03-2007 06:14 AM

Sorry, bigMikeB, you are right - I meant black steel, not stainless steel. Had a brain fart. :huh:

But nonetheless, not pop-together ducting you buy at HD or Lowe's.

MechanicalDVR 07-03-2007 10:37 PM

Not to burst anyone bubble here but, homeowner hoods aren't designed to carry the heat from a commerical stove, they have no provisions for air spacing from the rear wall and the small ducting and gauge of the sheetmetal couldn't contain a grease fire. Props to fireguy and bigmike on this one.

Clasact, the house and life you save maybe yours, by doing it to code and not by price. Don't drive a hummer if you can't afford the gas.

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