Best WT321 hood with automatic make-up air

cindyloodOctober 24, 2010

We were all set to order our BlueStar range until we hit up against the township's "make up air" code. We are hoping that The Best WT321 range hoods which are compatible with Broan's automatic make up air components proves to be an affordable solution. Not too crazy about the mesh filter but think that is acceptable. We are committed to installing a chimney hood as opposed to a professional hood so if this approach doesn't meet code and budget requirements, we may switch to an induction range. We will have a 30 inch range and and 36 inch hood. Anyone with experience with the WT321 chimney hoods? Any other solutions that won't break the bank? Clock is ticking and we need to order the stove this week...tks!

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I have no knowledge of this Best system, but can provide some insights into make up air (MUA).

To keep from disturbing the upwelling effluent that you want to capture with a typical hood with modest overhang, it is important to minimize cross drafts. MUA should either be introduced well away from the cooking area, under the stove, or from an array of diffusing panels in the kitchen wall (not likely except in test kitchens).

The biggest difference besides total flow between commercial cooking and home cooking is that commercial outfits can size the MUA for a specific exhaust flow because the hoods run at one speed. In a residential application, there may be more than a single exhaust fan, and one or more may be variable speed. Also furnaces and fireplaces send exhaust outside. Windows may be open or closed, house leakage varies with pressure. All of these factors affect the house pressure when using the hood.

A passive system (big filtered hole to outside, with or without air heating) can adapt to this automatically, but the pressure drop in the house has to be sufficient to overcome duct and filter loss. This pressure may be lower than one would like for the hot water heater or furnace, as applicable, as well as a functioning fire place.

An active system (fan in series with the above) will raise the pressure, but even if tied to the power to the hood fan, won't nicely track the effects of other fans and fireplaces and furnaces, so the house pressure may vary a lot.

A closed-loop active system would add a fan controller that used the house-to-outside differential pressure as an input to try to keep the house negative pressure to a narrow range. This is the most complicated option and may prove difficult to tune. (I'll let everyone know when I've finished mine.)

In some cases the AHJ may accept a furnace/water heater (as applicable) kit that isolates their burner intake from the house pressure. In that case, the hazard from not having an MUA system would be avoided, although the letter of the code would still be violated. This would entail opening windows as needed for use of a fireplace. Success of this ploy depends somewhat on house leakage versus hood flow rate, as well as the hood blower's ability to work against a negative pressure.


    Bookmark   October 25, 2010 at 9:59AM
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What is the code requirement in your area? Generally, the need for make-up air is related to the number of CFM's of the hood. In most areas if you have 600 CFMs or less you don't need makeup air. If that's the case, just make sure you get a hood with 600 CFMs. Some northern locales require makeup air for anything above 300 CFMs. Switching to an induction range doesn't really solve your problem. It's true that a gas range will produce more heat than an induction range, but most of the use of the hood is to eliminate grease, smoke, odors, etc. What you cook is more of an issue than what fuel you use to cook it.

    Bookmark   October 25, 2010 at 11:50AM
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Our township's code requires MUA system for hoods with CFM above 400. We downgraded our planned purchase of the BlueStar range from the one with 22K burners to the 18K version so we believe the Best hood with 600 CFM max will be adequate for ventilating the range. After much chatting with our township, electrician, and assorted building/contractor folks, it appears experts in MUA are few and far between. The code requires the MUA to kick on when you turn the hood on -- my digging came up with the Best option. More to follow. Thanks.

    Bookmark   October 26, 2010 at 4:05PM
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Hi cindylood (or anybody else who can weigh in on this),

regarding the Best WT32I362SB hood with MD6TR or MD8TR dampers, I reviewed the Broan application note for this damper series at
and I did not see any indication of how the furnace would be turned on to automatically heat the air (when necessary) before introducing it into the house. It looks to me like the MD6TR/MD8TR dampers are intended for use in one of two ways: 1) to provide a direct connection from exterior air to the inside of your house (per sec 7.2 of that app note), or 2) to provide an input to the furnace return air stream (as per sec 7.1 of that app note).

But my understanding of the 2009 IRC is that make up air must be temperature controlled, so that the simpler solution, per sec 7.2, i.e. a direct conduit from the exterior air to the interior of the house, would not meet code. The other alternative, connecting the damper to the return air conduit of the furnace (as per sec 7.1 of that app note) also does not satisfy this requirement (at least not without additional control elements from the hood to the furnace), because there is no control to turn the furnace on until the temp at the thermostat drops below the current thermostat setting.

Am I misunderstanding the intended usage of the MD6TR or MD8TR dampers?
Or does your local building code not implement the temperature control part of the IRC requirements?
Or am I misunderstanding the 2009 IRC requirements?
Or are you planning additional HVAC modifications above and beyond what is shown in sec 7.1 of that app note, for example additionally implementing control of the furnace based on air temp at the damper?

We are in the middle of planning for a kitchen remodel and are hitting up against the exact same issues, so I would really appreciate any guidance on this topic.


    Bookmark   April 10, 2011 at 8:19PM
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It is unlikely that you will depressurize your house with a 600cfm fan unless your house is very small and or very tight.
My opinion is that these new MUA codes are complete garbage and should only apply to homes built with extremely tight construction.

My solution will be to buy a ventahood with 2x 300cfm blowers and remove one blower when the inspector shows up.

It is also code in my area that mua is required for anything over 300cfm. My house was built in 1981 and has a total living space of 4500 sq feet. There is no way a 600cfm fan will depressurize my house so I plan to skirt the code rather than pay $7000 for a heated make up air system and probably $10 in electricity costs every time it kicks on. (heated make up air system has a 10,000 watt electric element)

    Bookmark   April 10, 2011 at 8:49PM
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Whoops, I'm really sorry - upon doing additional research, it seems I was incorrect in my post above when I said that according to the 2009 IRC, make-up air needs to be tempered (or at least heated as required). The tempering requirement seems to be the case only in certain areas of the country, and is not part of the 2009 IRC per se.

I do believe I was correct that the 2009 IRC requires MUA for any kitchen hood rated over 400 cfm, however - I have now read that in a large number of online references.

Sorry for any confusion,

    Bookmark   April 11, 2011 at 6:04AM
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Just wanted to follow up my earlier posts by pointing interested parties over to another thread where the MUA code issue for California was clarified with help from a more educated GW'er than myself. The mini-summary is that MUA does not seem to be required in the 2010 California building code after all - although YMMV according to your local codes.

Please see the thread linked below for more details.


Here is a link that might be useful: GW thread re: MUA and California building code

    Bookmark   April 13, 2011 at 7:35PM
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In a residential environment, I agree 100% with ribs1: MUA is a fix in search of a problem. Other than the following caveats, it is ridiculous overengineering: (1) you have an incredibly tightly insulated, very SMALL house, or (2) you have a fireplace or woodstove very close to the hood. In a normal situation, turning on even a powerful hood will cause a slight decrease in air pressure in the immediate area near the hood and a draft of self provided make up air from the rest of the house (and of course from penetration from outside). Whether you provide the MUA from a dedicated system or simply though penetration, the air will be untempered, unless you spend the extra money for needless equipment that will do the same thing as your existing HAVC system.

Only in the case of a nearby fireplace or woodstove would I consider the installation of a passive MUA system. We rely 100% on our woodstoves for heat and run them from October through the beginning of May. Contrary to what most people think, smoke does not naturally rise. Only heat does. Especially in the case of a dying fire, there is the possibility of smoke being sucked into the room if there is a decrease in pressure within the room (ie, the fire will draw its air from the chimney and exhaust into the room rather than vice-versa). Because our kitchen woodstove is about 20' from the hood I installed a passive MUA in the kickboard of the island across from the hood.

Even that I think was overkill.

    Bookmark   April 14, 2011 at 12:29AM
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Generalizing negative pressure development or lack thereof on the basis of the size of the house is too inexact for my taste, given that the risk is combustion product back-flow into the house. One should measure the pressure, or test for back-flow, or both. This requires that the house not be under construction.

It is possible with high cfm exhaust fan capability to develop enough negative pressure for back-flow even with an open, but screened window. (Notice the generalization here neglects to mention the nature of screen, the size of the opening, and other factors, thus making it only a warning and not a rule.)

I think it sufficiently conservative to plan for MUA and then test to see if it is needed once the construction is settled enough that the house leakage is representative of its finished condition. If the leakage is insufficient, then one has to adapt the MUA to the external conditions.

Don't forget that house leakage only occurs when the interior pressure is negative relative to the outside, and balance will occur between leakage in and air flow out. In other words, as the vent fan spins up the house pressure will go negative, the leakage will increase, and the steady-state fan cfm capability will just equal the leakage at that negative pressure.

One approach to evaluating the scale of MUA required is to find someone, often associated with energy saving organizations, who has the equipment to measure the pressure differential as a function of the operation of combustion sources and kitchen ventilation. Another is to buy a suitable sensor and do the experiments oneself.

In a low dust and pollen area with temperate weather, a big hole with a damper would be sufficient. Otherwise, a filter is needed. The pressure drop across a good furnace dust filter can easily exceed 0.1 inches of water, depending on air flow and filter size. If there is insufficient supplementary leakage to keep the pressure drop low, as is assumed here, the system will need a fan to boost the flow because 0.1 inches of water will cause back-flow of most combustion heat sources.

If the environment is cold, then MUA heating is needed. This can be by electric coils, or by a hot-water heating system heat exchanger, or by introduction of outside air into a hot-air furnace system or into an air conditioning system where the cold air ends up being introduced into widely dispersed areas that might have sufficient room heating for the task. The solution will very much depend on the house configuration and its heating system.


    Bookmark   April 14, 2011 at 11:49AM
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Do not waste your money on this range hood - heat sensor is too low - so you constantly run fan on over-ride; i have replaced three sensors and still no relief. Also, if you use internal fan - exhaust - do not install the damper - it will hang up on the over enigneered exhaust flex pipe - further complicating temperature issues. I am almost ready to send this unit back - its unfortunate, looks so nice. J

    Bookmark   July 19, 2011 at 1:42PM
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