Help! Venting hell!!

mpasquarApril 12, 2013

Let me start by saying I live in Michigan. My wife and I are building a dream home but with a builder that doesn’t do custom. Think of it as you get-what-you-get situation. So we’re going to do a minor kitchen renovation before even moving in. I always wanted a 36” range or cooktop and am willing to do quite a bit of work to make it happen. And of course you need a nice vent hood to go with it. And that is where the complications start.

I don’t care for the builder’s kitchen vent option. Their design has a downward pointing 90 elbow right, square 3.25”x10” ducting between the 2x4 walls, then into basement, and out the side of the house… seems like a bad design to me. I’m thinking some soffits, vertically up, horizontally over, and out the garage wall (or roof) is better. And then I get to pick whatever size duct I want.

But local code MUA requirements are really throwing a wrench in our plans. I’d like to avoid it as best as possible because of our cold winters. I heard someone call this ‘venting hell’. Sounds spot on to me.

So here is my best stab at selecting a cooktop, vent, and ducting:
- Viking 6 burner 36” cooktop VGSU164-6BSS, capable of 66k BTUs
- Viking 36” W x 24” D wall canapy vent hood VWH36481, but pair it with a 300 CFM internal blower (I actually surprised to see that Viking offering such a blower ��" model number VINV300)
- 10” round exhaust duct work

I think the benefit to this combo is threefold:
- I get the 36” cooktop with the much desired real estate
- The vent hood has good 24” deep geometry that will help the capture smoke and grease, albeit with lower max CFM output
- Duct in 10” round for universal vent hood/blower changes in the future. If I need more CFM and are willing to pay the cost for heated MUA system, then I just have to add the MUA system.

My only hesitation at this point is the 10” duct with a low 300 CFM. Are there any adverse effects? Sure seems unusal… but in the interest of avoiding really cold MUA in the dead of winter I think this might be the best compromise. Thoughts?

I welcome any and all comments from the forum! Thanks for reading!

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300 CFM is NOT enough for any of what you propose. You are going to either have to bite the bullet and do the required make up air system, or go with a plain jane consumer grade range. With a new home build, it's tight enough that just having bath vent fans together with the consumer grade vent can trigger MUA requirements. It's the total of the whole home's CFM output, not just the range hood.

    Bookmark   April 12, 2013 at 10:51AM
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Thanks GreenDesigns for your comment. What is the rational for saying 300 CFM is not enough? Is that because of the large 10" duct size? Or because of the 66k BTU cooktop max output?

    Bookmark   April 12, 2013 at 11:22AM
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mpasquar - I feel your pain - I live in Michigan also and our town is strictly enforcing MUA.
I think with the gas stove you are proposing - your ventilation is weak.
We ended up going with induction and a 350(but does boost to 450 shhh) ventilation fan. It is OK with our induction but I think you need more.
There are other posts that talk about how to calculate your needs.
I don't like the idea of all of the bends in the venting - It will make it even less efficient. My DH is a HVAC engineer and when we redid our kitchen we improved our AC/Heat by more straight runs. He talked about how each bend decreases the efficiency.
BTW - there is not a requirement to put in ventilation - just if you do, you can't exceed the 440 CFM - crazy!)

    Bookmark   April 12, 2013 at 11:54AM
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I don't think there is any harm in an oversize duct, I don't think there is any particular velocity requirement for residential kitchen exhaust. The air will move just fine (better than in a small duct), just more slowly. If you plan to never upgrade the big duct is a bigger path for cold outside air to backdraft into the house.

If I were in your situation I'd put in a big hood, a big duct, and a little in-line blower. Once the inspections were signed off I'd replace the blower with one in the 6-1200 CFM range. The heated MUA requirements for residential cooking are simply absurd. I cook a lot and probably don't run the hood on high much more than an hour a week in 5 or 10 minute bursts. For those brief times I can pop a window open. Who cares that I'm pulling air in from little cracks around the house? Now, with more energy efficient furnaces, water heaters etc that are sealed and use outside combustion air even the old argument about pulling combustion gas back into the house is specious (assuming your house is so equipped). If the house is well sealed the hood just won't work very efficiently and you'll be reminded to open the window by the smoke escaping under the hood edge; that's how it works for me. I think this is an excellent example of a time for a little civil disobedience.

    Bookmark   April 12, 2013 at 1:38PM
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The first question was flow rate versus duct size. In general, one doesn't want to be sending greasy air through a duct too slowly, because it will settle on the duct walls. There is an entire ASHRAE study on this and the answer somewhat depends on the temperature of the duct and the number of bends, but lower than 1000 feet per minute long term at full fan power when grease removal is most important is probably a bad idea. So if you intend on 300 cfm long term, a smaller duct is recommended.

The plan to ignore the MUA code raised in the last message appears to have some merit for the liberty minded, but ignores the original reason for the MUA code. If combustion appliances, such as hot water heaters, are used in the house and not fed separate MUA, they can back-draft at some house negative pressure. This can be hazardous.

For some appliances, back-drafting starts at a negative pressure of only 0.03 inches of water column, IIRC. So, for the liberty minded, I suggest a differential pressure sensor tied to an alarm to remind that there needs to be adequate MUA when the post-inspection replacement fan is trying to move the 600 or so actual cfm that serious cooking would require.pass through a 36-inch hood. When enough windows are opened, the alarm would cease.

It is possible to build alarms so obnoxious that Pavlovian response will lead to opening the windows first, assuring MUA.


    Bookmark   April 12, 2013 at 4:22PM
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"But local code MUA requirements are really throwing a wrench in our plans. I’d like to avoid it as best as possible because of our cold winters."

I'm not understanding your line of reasoning here. Cold winters are precisely the reason why you WANT heated MUA for high cfm vents. Since you must have extra air introduced into your home, to prevent combustion appliances from backdrafting AND to allow your vent fans to operate optimally, you must heat that air during cold weather to keep your indoor temps from fluctuating uncomfortably downward. Opening windows in the winter is a totally silly suggestion, for the above reasons, plus the fact that they may be frozen shut during the depths of the heating season. Oh yes, it also doesn't meet code....

If you want high powered cooking appliances, understand that you need appropriate venting capacity, and that doesn't come cheap. Any quick-&-dirty work-arounds will compromise the health and safety of the home's occupants, and allow the spread of grease, smoke, and vapours all through your brand new dream home.

    Bookmark   April 12, 2013 at 7:20PM
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Opening a nearby window for the few minutes a day that we operate our 1200 cfm vent does nothing to cause uncomfortable temperature fluctuations. We have a perfectly adequate furnace that quickly corrects any change in temp, but for the most part there is none, as there is a short and direct path from the window to the hood. I usually welcome the bit of cool breeze when the cooking is intense. Why pay for a second very expensive heat source when you already have one that will work just fine? Here in Massachusetts they have not adopted these MUA requirements, so our installation meets code. It is the same in this house as the last one, so I can tell you from 16 years of experience that it works fine. For most low exhaust settings the two energy recovery ventilator units appear to provide adequate makeup air, on the high settings the window comes into play. Sealed combustion on the water heater and furnaces keeps them from backdrafting, which they could theoretically do by turning on a couple of the bathroom fans. Redundant, expensive heating capacity does nothing to improve health or safety.

    Bookmark   April 12, 2013 at 9:04PM
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Thanks everyone for your comments.

Kaseki- I had not considered the slow air speed that won't carry the greasy air. Great comment. I'm going back to the drawing board.

Cooksnsews- All I'm saying is that 98% of the time I expect to have the fan on low and would like to find a solution without have to purchase a $7k commerical heated MUA unit. It is frustrating that the code is written such that I would have to have a duct open to 10F winter air drafting into my kitchen for 8 hours while I simmer chicken stock. There has to be a better way.

I like the window analogy... i.e. the window has to be open while the vent is running (either for illustrative purposes or pre 2009 ASHRAE standards). Makes sense. But it sure would be nice to only open the 'window' as far as needed instead of full open full time.

Perhaps I could place two dp sensors in the exhaust duct. The first dp sensor would open a MUA damper sized for a blower on low. I looked at some of the Wolf 1200 CFM remote blower curves. Low is approx 200-250 cfm. So maybe a 6" MUA duct would properly replenish at that low blower setting?

The second sensor would be for med/high blower settings. That would trigger a second independent MUA duct, such that it combines to make the the total MUA needed. Perhaps a 8" MUA duct?

Most of the commerical-style vent hoods worth their salt have a 10" round exhaust. Seems like most MUA designs size the MUA duct w/ to match the exhaust duct. Interesting enough, I'd have the same 10" cross sectional area by using a 6" + 8" MUA system. i.e. 50.24in2 + 28.26in2 = 78.5in2.

To say it again a little differently, only the 6" MUA would be open while the blower is on low, but the 6" and 8" MUAs would BOTH be open while the blower is on med/high. Of course I'd need a HVAC professional to check the sizing and set up everything properly... the numbers above are just for illustration.

What does everyone thing of this idea? Feasible? Or am I missing something?

    Bookmark   April 12, 2013 at 9:27PM
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One more point of clarification... my township requires MUA to be in the same room as the kitchen and automatically triggered when the vent is on. But the MUA system can be passive, i.e. no MUA blower required... draft-only from the vent hood is acceptable.

    Bookmark   April 12, 2013 at 9:40PM
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At first blush one could say that 1200 cfm would require a 10-inch damper controlled MUA duct. But it really depends on the pressure loss through the MUA duct, or more exactly, the negative pressure developed using the system that you build. A duct one-foot long will have a lot less loss than one that meanders around getting to where the air is injected into the kitchen.

At very low cfm, the house leakage should suffice for MUA, but this needs to be confirmed by testing. Any filtration in the duct will cause a pressure loss higher than 0.03 at significant flow, so if dust is to be removed, then a boost fan is needed. A similar requirement exists if a heat exchanger is used in the duct path.


    Bookmark   April 13, 2013 at 10:21AM
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You may ask to see what the HVAC guy or your city is doing to calculate your MUA requirements. I dug out the code, found the table and found a workaround. The calculations often take into consideration what kind of furnace, water heater, and fireplaces you have.
I was looking at a bill for over $4000 to install heated makeup air, but then realized converting a wood burning fireplace to a direct vent gas fireplace would eliminate my need for MUA at all as it changed the calculation substantially. So for less than the price of heated makeup air, i could get a new, more functional fireplace. Sure I traded ambiance of a wood burner, but the thought of spending that kind of money on a device (MUA heater) that I get zero pleasure out of....

So do your research. This is a complicated matter than many in the field are still reeling about. It may help to call your local city inspector in person and discuss your options. The other thing I learned on talking to him is my city doesn't require a hood at all! I have a 48 inch culinarian rangetop, and they could care less if I had no vent....

It may also help to ensure all of your gas appliances are direct vent, since you are building I'd make sure this is the case (probably code now for new building).

    Bookmark   April 14, 2013 at 3:39PM
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Residential has gotten a pass on hood requirements for many years.

Unless you try to use a gas oven or range for heating the house they rarely run long enough to be a real CO issue.

The CFM for a range hood also depends greatly on your coking style.

How much smoke & grease do you produce?
If you had a wok burner you would want a high capacity hood for everything except steaming.

Have a grill on the stove?
Do you really want all the smoke & grease all over the inside of your house?

Sear something before braising?

The oven is usually not all that bad, but searing before baking will throw up plenty of smoke and grease.

    Bookmark   April 14, 2013 at 4:11PM
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Some jurisdictions, like mine, do require external venting above any kitchen range, for new builds. The reasons - new build standards require very tight building envelopes, with limited specified air exchanges with the outside, for energy conservation purposes. It has become evident after many years of such tight builds, that toxic compounds can accumulate within the home, triggering a number of health issues. For example, the generation of steam and water vapours can result in mould formation, which cause respiratory problems for many people. Home cooking is one of the main sources of indoor air pollution (even if folks never sear or sir-fry, many still burn stuff occasionally), so some venting is mandated to provide a safer environment. I'm not sure what the minimum CFM requirement is here, but it is way below the MUA trigger level, and is consistent with so-called builder grade hoods and appliances.

BTW, there is no specific venting requirements for gas vs. electric ranges, as both produce the same amount of CO, which is none. The confusion likely arises because it is CO that gets backdrafted into homes through furnace chimneys when more air is vented out than is supplied by leakage and/or MUA systems.

    Bookmark   April 14, 2013 at 5:56PM
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mpasquar, in your hypothetical 6" mua draw (on low) and 8" mua on high ducts, would it not be less complicated to pick one or the other depending on vent motor rating (materials, labor, and protection to house from new duct to the outside)?

Also, if passive draw mua direct to kitchen, what are you thinking when you say "draft only from vent hood is acceptable".

I'm in the same boat with a new range (my first), and the city code office just informed me passive draw is okay for residential. Prior to being told this, I was told non-treated, but active mua. Totally confused on what to do. I have a brick wall 10 feet straight from and connected to the kitchen. I wonder if I could set something up using it.

Where will you be placing your mua duct?

This post was edited by SparklingWater on Sun, Apr 14, 13 at 19:44

    Bookmark   April 14, 2013 at 7:41PM
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Hi all, I first want to thank everyone for their posts. I've learned very important facts from each of you. Great technical discussion!

I want to ask a general question to the forum and then circle back to SparklingWater.

In the more recent by kaseki, I think he suggests measuring for when MUA is actually needed. I think it would be interesting to discuss his comment in context to the IRC code. I copied the 2009 IRC code from GreenBuildingAdvisor: IRC provision, which is found in section M1503.4: “Exhaust hood systems capable of exhausting in excess of 400 cfm 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.”

is there some wiggle room with this? Or does the MUA system need to full open whenever the vent is on, regardless of desired blower setting? Is it possible, assuming I have data from a backdrafting test, to only trigger my MUA duct above a given threshold??

I pose the question because, again, I go back to my chicken stock example. Sure would nice to not open the MUA duct at low ventilation needs. But I want (and need) large amounts of fresh air coming in when I'm cooking up a storm and along with 2 ovens going.

I could look at it this way:

1) My house is already being built with a damper tied into the central HVAC cold air return system. For instance, the builder offers a kitchen vent rough-in option (10" x 3.25" rectangular). And so therefore, some level of MUA is already designed in. I'm confident, and I think this is reasonable, that my house can sufficiently handle ~ 300 CFM kitchen exhaust without going too negative... otherwise it would never pass the cert-of-occupancy inspections.

2) So then, in this example, above 300 CFM is the trigger for a secondary MUA system to kick in. I'd have to size it according to the blower curves and ducting and actually test when it was all said and done. Sure would be nice to have it open only when the powerful kitchen exhaust is overwhelming the central heating system.

So what does everyone think? Can the code be interpreted this way?


Back to SparkingWater:

- I'm tearing up a few walls to do all this. Yeah, it would be more expensive to rough-in two MUA systems but I'm already going to be knee deep into it anyways. If it keeps me from having a 10" duct open at low ventilation requirements, then I'm willing to do it. 10" duct is 78 sq in. That would be like opening a 2ft window 4 or 5 inches, not to mention the negative draw of air into the house. Who would normally do this in the dead of winter when boiling water for pasta? No one would. You'd turn on the crappy over-the-range-microwave vent and not think twice about it. MUA would be coming from someplace, but it sure wouldn't be an open window (or in my case a duct). And you'd still live to eat dinner with the kiddies that night.

- my comment of "draft only from vent hood is acceptable" is meant to describe the draft created via negative pressure from the exhaust blower running. I'm trying to describe a passive MUA system. Let me know if you still don't get what I'm saying.

- I'm looking at 3 ways to run the makeup air. None of which is ideal. I'm thinking this is the penalty you pay for an open floor plan. a) straight into a 2x4 wall and then down in the basement and out the side of the house. b) up into a soffit then over and under the top stair of a staircase, then into the garage and out the garage wall. c) straight up and then into the master bedroom above - either in the m/b wall or making a new chase - and then into the attic and out of the roof. Gonna suck no matter what I do.

    Bookmark   April 14, 2013 at 11:11PM
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"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.”

As always, the AHJ has the last word, but if there is a gravity close damper on an input duct, not only will it "open" as soon as the house pressure goes sufficiently negative, but it will allow proportionate amounts of MUA in by opening according to the flow/pressure difference. This is the scheme used for boiler furnaces for years to bypass the furnace so chimney drafts did not cool off the interior boiler housing that was providing stored hot water.

In this case, one might have a motorized damper that opens when the hood fan starts, and a second gravity or very weak spring damper providing the flow "metering." The furnace dampers used an adjustable weight to set the pressure drop that would begin opening the damper.

Civilization functioned before electronics was invented, I am forced to admit.


    Bookmark   April 15, 2013 at 9:39AM
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"there is no specific venting requirements for gas vs. electric ranges, as both produce the same amount of CO, which is none."

While true fr electric ranges, it is NOT true for gas stoves.

If you see anything besides blue in the gas flame you can be assured CO is being produced.

Put a large heavy pan on a burner set on high ad you will make a small amount of CO until the pan heats up.

It is a small enough amount that it is not normally a hazard.

    Bookmark   April 15, 2013 at 11:01AM
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kas- I want to thank you a TON for the automated MUA damper + spring damper idea. Genius! And simple! Awesome just awesome!!

I hope that people will come across your suggestion if they are looking to 'proportionately' control MUA if they, like me, live in cold climates.

Thanks again. This will be a big undertaking this fall, but I will be sure to post info on my MUA system along the way.

    Bookmark   April 15, 2013 at 8:53PM
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Just don't be surprised by anything that flies, walks, crawls, or slithers through an unfiltered MUA duct when it is open. :) This type of unboosted MUA only works without filtering. However, the larger sizes of wire cloth may have low enough loss to work and still keep out the larger critters if your duct is large enough.


    Bookmark   April 16, 2013 at 9:58AM
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You can get inline vent heating units for a few hundred bucks.

    Bookmark   April 17, 2013 at 11:21PM
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These would need to have low pressure loss to the passing flow. Electrical coils hanging in the flow might be ok; a car radiator heat exchanger would not, as the pressure loss would probably exceed 0.1 inches w.c.


    Bookmark   April 18, 2013 at 11:01AM
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Glad our experts seem to have helped mpasquar find an answer to proportional mua. Have to admit, I like the idea of dual ducts, each drawn from based on cfm need to put it crudely.

" the larger sizes of wire cloth may have low enough loss to work and still keep out the larger critters if your duct is large enough"

kas- I'm still planning an 8" universal mua damper tied into my return through low voltage wiring from the range hood (Wolf). I looked at spec sheets: it appears these are run from the rangehood down to the transformer near the damper? So the wires run through the wall behind the hood, down in front of the drywall through the floor to the furnace air return? Or is this remote turn on?

I'm showing my ignorance tonight for sure. But my GC says I'm one of the first considering putting this in. Even code now tells me it can be passive not active mua at one's own risk.

To make matters worse, I am hearing hints on HVAC experts site that these mandatory MUA requirements in the IRC adopted by most states may soon be deleted due to all of the confusion.

SOrry OP. MUA is indeed "venting hell". Glad you found your solution.

    Bookmark   April 18, 2013 at 10:27PM
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I am not familiar with your specific parts, but expect for the damper that there is a direct connection from transformer and damper to the sensor measuring flow. Current-wise, one leg of the transformer goes to the damper motor and from there to the flow switch from which it returns to the transformer.

This connection has to be run whichever way is safe and compliant with NFPA 70 (the NEC). You should download the installation directions first to determine what you have to do, wiring-wise. A remote control setup can be built, but is unlikely to be part of an inexpensive unit.

Long drill bits of appropriate diameter are your friends for running cabling. A Milwaukee right angle drill is often a critical tool to supplement regular straight-line drills. Avoid drilling near the power cables (BX, NM, etc.). 26 Vac cables cannot terminate inside boxes containing 120 Vac cables without an approved separator. Etc.


    Bookmark   April 19, 2013 at 12:27PM
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Thanks again kas. I'm actually not doing the job, thankfully. Just trying to understand the wiring aspect (my GC has the specs to the Broan 8T mua install).

    Bookmark   April 19, 2013 at 7:06PM
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Low voltage wiring can go inside the wall like regular wiring, just not necesarily in the same place.

    Bookmark   April 19, 2013 at 9:02PM
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