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Range Hood CFM Reducer

Posted by DCJersey (My Page) on
Tue, Feb 26, 13 at 21:26

I recently purchased a 600 CFM Kobe hood. Unfortunately my local building code requires a make up air unit for hoods over 400 CFM. Unfortunately installing a make up air unit is not within my budget. In doing some research I noticed CFM reducer kits for Broan and Faber hoods. I tried to see if Kobe makes such an accessory but was unable to find anything. Is it possible to reduce the max CFM's on Kobe hoods? I would really rather not buy a hood under 400 CFM as the selection is lacking.


Follow-Up Postings:

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RE: Range Hood CFM Reducer

how exactly is the code worded, and how is your hood fan controlled?

If it has speeds of low/med/high and your hood fan curve specs state that, for example, low and medium are within the cfm range that is acceptable to not have the makeup air, then it might be possible to wire the controller to not go to high. Even if this is possible, it may or may not meet the code requirement if this is done.


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RE: Range Hood CFM Reducer

Do you know if the code is actually enforced?


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RE: Range Hood CFM Reducer

I have spoke with my contractor and according they didn't enforce it for the longest time, but in the past year they have become more vigilant.

Montgomery County, MD uses the 2012 International Residential Code which reads as follows: "

Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system."

I guess if we rendered the hood incapable of going over 400 CFM then we would fit within the code. Am I correct?


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RE: Range Hood CFM Reducer

I'm not an expert, but mine is reduced (not by choice) by the size of the duct.

Because I had to put the duct going out in between the 1st floor ceiling and 2nd floor flooring, I could only use a 7" duct. The 42" Kobe hood I got uses an 8" duct, so my max CFM is reduced due to lower air flow (by what amount, I can't say).

Amanda


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RE: Range Hood CFM Reducer

Code inspectors are very unlikely to accept any restrictor on the original vent as satisfying either the letter or the spirit of the code. They will want the vent itself to be less than 400 CFM. And, the range's requirements also factor into this as well. If your range manufacturer says you need 600 CFM, then that is by definition incorporated into the code, as are all appliance manufacturer's requirements. You will be required to have both the 600 CFM and the makeup air. This isn't just to make renovation difficult for you. It's a health and safety issue that could possibly save your life.


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RE: Range Hood CFM Reducer

I understand the reasoning for requiring a makeup air unit. We will rarely be running the hood at its highest rate all that often and when we do we can simply crack a window. Our house is not that tightly sealed that the extra 200 CFM on the hood is going to cause a huge backdraft from the fireplace (especially given that we can close the flue and we have a big wood burning stove insert (which is never used).

I doubt our stove will affect our make up air requirements. We will be using an induction range not a gas stove.


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RE: Range Hood CFM Reducer

I'd probably bite the bullet and return the 600 cfm Kobe for a 400 cfm vent hood. Kobe has a nice reputation for being more quiet as well as aesthetically pleasing. If you don't like their 400 cfm hoods, try one of the many other manufacturers. Or keep perhaps what you have?

Induction should be wonderful to use. I considered it, but we loose electricity too often in our neighborhood.


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RE: Range Hood CFM Reducer

I'd probably be looking for a barometric or electrically controlled damper to put the MUA into my furnace. Someone here put a system in fairly cheaply (few hundred) during his remodel.


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RE: Range Hood CFM Reducer

We have a Kobe 400 CFM hood it is perfect, and no need for makeup air...


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RE: Range Hood CFM Reducer

MUA is not just a safety issue for the current homeowner who installs a system, but also for any subsequent owner/inhabitant. If I bought a home with a 600 cfm vent in it, I would expect that it would work as designed AND meet all code installation requirements.


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RE: Range Hood CFM Reducer

Any chance of installing your old hood and slipping in the new one after the remodel is done?


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RE: Range Hood CFM Reducer

Our code is strictly enforced but the code does not require a hood at all.
There are safety reasons for the code and if you have a tight house, you can cause a backdraft. Older houses are less likely to have this issue.
In our case, we installed after the inspection as a precaution. Ours runs at 350 cfm but can boost to 450. The only time I ran the boost was while cleaning the oven and I opened a window. In 20 years, when we sell, if the code is the same, we will swap it out. For example, if I have a 450cfm, it would make sense that I only need to deal with the extra 50 vs the 450.

So- if 400 is ok, should the mua system be the delta between the actual and the 400 vs a total system.


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RE: Range Hood CFM Reducer

Have you made a decision? Do give an update on what solution you used and how the inspection went (once its happened) TIA


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Broan MD8TU 8" Make Up Air Damper ~$200.00

The Broan Make Up Air Damper was the first residential affordable MUA attempt by any manufacturer. Kudus to them (no affiliation). Their latest model @ ~ $200 uninstalled.

Broan MD8TU 8" Universal Make-Up Air Damper

Broan MD8TU 8

Here is a link that might be useful: Broan MD8TU installation guide

This post was edited by SparklingWater on Thu, Feb 28, 13 at 20:05


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RE: Range Hood CFM Reducer

Zypher had a nice universal MUA diagram showing how the damper in the vent hood interacts via low voltage communication with the return on a gas furnace to bring in fresh air. You do have to run a 6" or 8" duct to the outside (with specifics on where it can be located) for the fresh, "unconditioned" air. Conditioning, treating the fresh air to be within 10 degrees of room air, is typical in commercial applications but thankfully not in residential as DCJersey's code wording shows.

I found this information very helpful. Communication between the vent hood damper and the return air may be through an X10 system. I am trying to determine this.

http://en.wikipedia.org/wiki/X10_(industry_standard)

X10 is an international and open industry standard for communication among electronic devices used for home automation (domotics). It primarily uses power line wiring for signaling and control, where the signals involve brief radio frequency bursts representing digital information

Can anyone comment on if the X10 works in this system? May be a separate low voltage system, idk.

Here is a link that might be useful: Link: Universal MUA (Zephyr)


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RE: Range Hood CFM Reducer

Exhaust fans are free air rated, these ratings do not include filters, backdraft dampers, ductwork, exterior wall or roof vents. your actual exhaust volume will be less than 400 cfm.


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