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Installing ductwork for a range hood

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Posted by: from New Hamburg
7/15/2012 at 11:23:49 AM

Hi,

I will be installing a 36" range hood and I need to run two ducts directly out of the back of the range hood. It is a 1200 CFM blower with dual blowers and I can't vent it up so I need to go back.

1) What is the best way to cut the holes through the brick on the side of my house with minimum repair to the outside.

2) Will I loose any power by running two 6" ducts instead of one 10" duct?

3) Any other advise would be great.

Thank you,

Erik

REPLIES (6)
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Date/Time7/15/2012 at 1:26:59 PM

1) you can rent a masonry drill from Stephenson's. They are very helpful and you can ask them what tool will be the best to use in your situation

2) if the manufacturer's instruction are for one 10" then that is what I would suggest you install. The duct work can be order through a number of duct suppliers that are noted on the web or the Yellow pages of your phone book. If you are able to use the 2 6" ducts then you still would need a transition section of duct to go from the unit to the duct work and also a cover on the exterior of the wall. If you must order the transitions pieces then I would recommend to order the 10" duct work. My concern would also be with the manufacturer of the hood if there are any issues with the unit or if it does not work then you may void your warranty if you did not install it as per the installation instructions.

Just as a note if you go straight back without any 90 degree bends there could be a fan noise issue. Please discuss your proposed installation with the fan supplier or the installer of the duct

Lou Frustaglio

Dreambuilders

416-736-6828

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Erik in New Hamburg
Date/Time7/15/2012 at 3:29:28 PM

Thanks for the quick reply.

The hood does come with a 10" transition piece but I don't have room to go up and there is a stud that I would need to cut in order to go out from the rear. If I use two 6" ducts I avoid cutting the stud. The instructions do show that I can vent either vertically or horizontally but it doesn't specify the size of the duct for the horizontal option. With the horizontal option it shows the duct without a 90 degree bend.

It is an outside wall stud and I'm guessing load bearing so I'm not sure if I can cut it or not.

Thanks,

Erik

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Date/Time8/14/2012 at 10:30:49 AM

Hi Erik,

I am not sure if you have already gone ahead with this or not but the installation of your exhaust fan is not the big concern here.

The issue is the 1200cfm. While most appliance stores will recommend the biggest and best and bigger always sounds better it often is not.

In this case, it definitely is NOT!

The problem is not with the size as a 36" range and corresponding range hood (which will look and function great!) but with the flow of the range hood.

Here is why: A 1200 cfm rated exhaust fan is evacuating exactly that, one thousand two hundred cubic feet of air per minute from your home (a 40' X 30' bungalow with 8' ceilings contains 9600 cubic feet of air. It would take 8 minutes to empty). Unless you have a gaping hole built into your house, or a tropical rain forest producing oxygen, you will essentially be rapidly evacuating all of the air from your home, this will in essence create a vacuum. If your home is air tight you could actually suck all of the oxygen out of the house. This is not good for living inhabitants or combustible utilities like gas water heaters etc.

The reality is most houses leak air and unfortunately most houses leak a lot of air. This will avoid the previously potentially fatal scenario above but can be even worse for your home.

With the fan running at full (1200 cfm) you will quickly create a negative pressure environment in your home, to make up that air, your home will draw in air from anywhere it can. That means an open window, door or every single tiny crack in your house around windows, your attic hatch, unsealed utility penetrations, etc. If it is raining out or humid (which we experience here in southern Ontario) you will be drawing all of that moisture into the building envelope as well. This moisture will condense in wall cavities and lead to mold and premature rot in the buildings framing components. It will also greatly increase the humidity level in your home which can affect hardwood flooring and other finishes.

If you do install a unit of this magnitude (many restaurants use 900cfm) open a window when in use or better yet have an HRV (heat recovery ventilator) installed to provide make up air for that super sized vacuum over the stove.

Just a side note: The most powerful 6.5HP Shop Vac flows 185 cfm

Sorry to scare you, but this is a serious concern. I hope this helps

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Erik in New Hamburg
Date/Time8/14/2012 at 2:43:17 PM

Thanks for the info.

I have read about this when researching range hoods and I don't intend to use the full power of this blower. It was a free promotion with our range, otherwise we would have gone with a vent-a-hood with 900cfm. If I do turn it on full I will open some windows but I'm hoping I won't need to do this. We have a dehumidifier running but I'm not sure if this would effect moisture in the walls or not.

So far I have not used it, the installation is slow going due to the lack of a ladder but I'm hopping to test it out soon.

Do you know of any building code that requires a HRV to be installed with a range hood of this size?

Thanks,

Erik

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Date/Time8/16/2012 at 1:34:25 PM

Hi Erik,

That is an interesting question.

You will not be in violation of the building code if you install that size blower without an HRV, however interestingly, if you were submitting for a permit for a renovation or new construction that included a Heat Loss / Gain calculation and design and that blower was specified it is quite probable that an HRV would be required by the Engineer in the calculations to provide the make up air. If that were the case and you did not install the HRV and did install the blower, you would be in violation.

Good luck with your purchase. Just remember to open a window (regardless of time of year) if you fire that thing up to full power!

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Date/Time3/26/2013 at 12:16:29 AM

I've installed one of those in a new Bellini kitchen. Great fan. Owner no longer uses it because of the noise, even though it's a rolling cage. Also, be aware of the CO2 gases being pulled towards it if it's an older house with non direct venting on the water heater and furnace as mentioned in the previous post.

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