High Cfm Range Hood Make Up Air MUA

10 years ago

I WILL START OF BY SAYING I AM NOT A HVAC PROFESSIONAL THIS IS SIMPLY THE INFORMATION THAT I FOUND IN MY QUEST FOR A MAKE UP AIR SOLUTION IN WISCONSIN THAT MY INSPECTOR HAS APPROVED FOR MY NEW CONSTRUCTION HOME AND WANTED TO SHARE WITH THOSE OTHERS WHO ARE SEEKING A PATH THROUGH THE MAKE UP AIR DILEMMA

First you have to contact the health and safety official of your county and speak with with the inspectors, Do not go directly to HVAC contractor, there job is to sell you products to the best of their knowledge which may or may not satisfy the inspector, or may or not be needed. Sometimes they get it wrong and I am not willingly to give away money for what they think I need to pass code.

Read STATE code regard ventilation (Wisconsin was pretty simple to find) since this is easiet to find and the countys all follow state code, to prepare your self when talking with the inspector.

Simply tell the inspector you have a Xcfm range hood and wanted to figure out if make up air is required and how should I do it to get through inspection. Listen to there suggestions and compare it to the local codes you have read and work together, by asking questions

From the above process I found out the following for a 1266cfm blower in Wisconsin that my inspector said he is fine with passing.
When using passive MUA, WI codes states that only 40% of exhausting mechanical ventilation has to be made up from all sources that have interior intakes and air fultration is considered part of MUA, when house has no appliances that take combustion air from inside the house; ie fireplaces older water heaters and furnaces. All my appliances have air intakes on the exterior of the house

Blower door test should be done to determine air infiltration

Formula from working with inspector.
(Range Hood CFM * .4) - Air Infiltration - Range Hood CFM Allowance + Bath Fans = MUA requirement

In my case
1266 * .4 - 0(blower door test not done yet) - 300 + Bath Fans = 206.4 CFM + Bath Fans + Dryer

The most about of air that I would have to bring into my house would be 206.4 CFM + Bath Fans + dryer, which I can get with a 4" duct direct from outside with a damper and floor vent. I probably wont even need the intake duct, depending on the leakage that I find from blower door test. But will still do one. Inspector say I could introduce the amount using HRV also

***TIP = Keep bath exhaust fans small. Atleast for inspection calculation purposes. Chances are youll never have every single blower running in your house at the same time.

The catch - The inspector specified the air intake ducting must be insulated.

CONCLUSION MAKE UP AIR IS NOT THAT HARD, MAY NOT EVEN BE NEEDED TO RUN, EVEN THE HIGH CFM RESIDENTIAL KITCHEN RANGE HOODS.

On topic part of post.Anyone have recent review or stories about Proline Range Hoods

Comments (4)

eyezonz
Original Author
10 years ago
Math was written wrong

Range Hood Only
1266 * .4 - 0(blower door test not done yet) - 300 = 206.4 CFM

Range Hood, Bath Fans, Dryer
1266 * .4 - 0(blower door test not done yet) - 300 = 206.4 (CFM + BathFans + Dryer) * .4
weissman
10 years ago
Not only do they oversell make-up air, but they oversell CFMs. Unless you have a grill or a very large range, 600 CFM is usually enough for most people. The formula they use for CFMs by totaling up the BTUs on the range assumes that you're running all burners on high most of the time, which no one ever does.
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foodonastump
10 years ago
I also can't help but think that in anything less than an "ideal" setup/environment most hoods aren't drawing as much as their inflated specs claim. Think my Kobe really moves 720 CFM through 6" round? I'd want my inspector to prove THAT before telling me what sort of MUA he wants me to install. (Of course arguing with inspectors is easier said than done, and not always advisable...)
kaseki
10 years ago
I suspect the 0.4 factor is intended to account for the fan/blower performance in realistic conditions, particularly conditions that correspond to the small amount of MUA that the formula requires. My comments here have usually assumed a factor of 0.67, but that approximation assumes that imperfect MUA won't lower the interior pressure more than 0.1 inch w.c., there is another tenth lost in the baffles, and a commensurate amount in the hood ducting.

One-tenth negative pressure, however, is too much for combustion appliances with air intakes that aren't isolated from the kitchen, or for fireplaces. Note that the Wisconsin formula listed by the OP is for cases without such combustion appliances.

If there is an intent to filter the MUA duct for anything smaller than a bat, I would at least taper out to a largish filter so the filter pressure loss will be minimal. A smaller sized furnace filter would do, or a window screen if large enough (perhaps two square feet).

When arguing for a four-inch duct because it can move X CFM, one has to consider what pressure loss will occur at that CFM. I think six-inch would be better in this case unless the duct length will be very short.

kas