Passive MUA Supply Location

whallydenJune 28, 2012

We're roughing in HVAC at the moment and a little torn on where to locate the fresh air supply for our passive MUA system. I'm lobbying for under the free-standing range. The HVAC contractor has suggested a ceiling vent.

Please share your experience.


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Are you planning on building a platform under the range to accommodate the ducting or do you have access through your floor (i.e., a basement)? And what size range?

And you have to consider the obvious factors like cold/hot outside air flowing on your feet while your standing in front of your range, aesthetic factors regarding the louvered grill at the end of the fresh air duct, raising your cabinet/countertop height if you build a platform under your range, and service/maintenance accessibility (sliding the range in and out of its cabinet opening).

But, more than likely, the reason the HVAC contractor wants to mount it in a ceiling vent is because of the limited space he will have to work with under a free-standing range. You have to consider whether or not he has the space to mount the motorized damper and the fresh air intake near the range without getting into a completely custom application (which will drive your cost up). And the replenishment air you draw back into your home will be cleaner if you are able to mount the fresh-air intake at a higher location on an outside wall and not close to ground level.

And if you do decide to mount your system under your range, check with your local building inspector to make sure there are no restrictions for mounting it near the gas valve and electrical receptacle.

Hope this helps.


    Bookmark   June 28, 2012 at 4:19PM
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I'd do what your HVAC guy recommends. If it enters your duct work, it will be filtered to keep outdoor particulates out of your house. And it will control drafts, which you don't need in your kitchen. Air moves freely enough that you don't really need it proximate to your vent hood to work. As long as it enters anywhere within your house, you will achieve the desired movement up your vent.

    Bookmark   June 28, 2012 at 4:57PM
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You should look at the instructions for your range. The Capital Culinarian specifically says you shouldn't put a vent underneath it.

    Bookmark   June 28, 2012 at 5:45PM
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The MUA should enter the kitchen in a way that doesn't disturb the rising effluent between the stove top and the hood aperture. This is easier said than done, but if a ceiling diffuser is used in the kitchen, it should be configured so that the air is directed away from the direction of the stove. Then the air should flow down one or more walls and approach the stove in a less turbulent manner and not aimed in a direction that makes it flow against the rising effluent.

Diffusers are available with many configurations of diffuser blades allowing one to aim the air where he wants it to go. The diffuser should be significantly larger in area than the duct feeding it to minimize pressure losses, particularly if the MUA source is passive.

Note that a diffuser is not a register. Instead of a grill or a series of tiny blades, it uses large curved blades similar in size to those of '50s Venitian blinds.


    Bookmark   June 28, 2012 at 6:12PM
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Does the MUA have to enter in the kitchen? Could I have the MUA enter in the basement for example?

    Bookmark   June 28, 2012 at 6:34PM
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The fresh-air intake needs to be in or very near the kitchen for the pressure to balance before it draws air from unintended locations (i.e. combustion vents). Air will always take the path of least resistance, so the duct that brings in the outside air needs to "cut-off" any potentially hazardous points of infiltration or leakage.

So if you put the point of entry for the replenishment air in the basement then your house will draw air from any open or unsealed location between your hood and the basement intake. Which defeats the purpose of installing a MUA system.

    Bookmark   June 28, 2012 at 8:51PM
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That makes a ton of sense, thank you.

    Bookmark   June 28, 2012 at 10:39PM
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@Nate: No platform planned, the 48" Bluestar range sits on 3" legs. The kitchen is over a basement and we used open-web trusses -- so HVAC contractor has plenty of room to work.

SeaKoz: I double checked and found this, "Any openings in the back wall or the floor of the installation site must be sealed."

The theory being discussed is very interesting, but I was hoping for real life examples of successfully designed systems. My interest in under stove mounting started with the videos linked below.

    Bookmark   June 29, 2012 at 1:20PM
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Both of the videos show custom applications specifically designed to bring in replenishment air from under the range. And both use completely different engineering for the same effect. I have not had an HVAC contractor or specialty installer mount the fresh air intake at floor level, it has always been at ceiling height. But that does not mean your theory will not work, because it will. It is just going to require some customizing of your MUA system. For example, if you ducted your fresh air intake through the floor and basement, then you would have the third example of a completely custom design for mounting the MUA system under a range.

The two methods that I am familiar with are a passive flow system mounted in the ceiling near the range hood and a forced air system connected directly to the air circulation fan motor with the replenishment air entering through the return air plenum. Both of these fall squarely into standard HVAC ducting locations and installation practices. Your HVAC contractor is probably trying to avoid the costly learning curve mistakes that can result from building a custom system.

I like the concept of bringing make-up air in from under or behind the cooking surface but that does not always translate into a successful system. For example, in the first video the two rectangular vents under the range are about 18" apart and about 10" wide. That is around 3 feet or more of unsafe flooring to slide or roll a 500-700lb four foot wide range into place after the cabinets and countertops are installed. So everything will need to line up perfectly at the end of the build job for them to be successful.

So mounting your MUA fresh air intake under your range is a feasible application, it just doesn't fall within normal residential architectural designs.

On another note:

kas, did you see the 4 ninety-degree turns in the exhaust duct in the first video? And unless he is going with a roof cap, then he will have a 5th ninety to reach an outside wall. He would need to add a high CFM in-line booster fan (900-1000 CFM) after the second juncture to maintain pressure and velocity, wouldn't he?


    Bookmark   June 29, 2012 at 5:46PM
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a forced air system connected directly to the air circulation fan motor with the replenishment air entering through the return air plenum.

I never understood how these work. Does the hood now need to be wired to turn the furnace or A/C on, as well as the damper? If not, aren't you just pumping icy cold/slash/hot & humid air into your kitchen?

    Bookmark   June 29, 2012 at 8:05PM
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During operation of a high CFM range hood, unconditioned outside air is necessary regardless of the method used to bring it into the home due to mass balance considerations. Furthermore, the replenishment air will require heating or cooling to the thermostatically programmed internal temperature of the home whether brought in by a make-up air system or through natural air infiltration. The HVAC system will heat and cool during operation of the make-up air system as needed by measuring the internal air temperature of the home.

So essentially, if you evacuate air from inside your home then the replacement air must come from outside. If not by a residential make-up air system, then by your garage entry, front or patio entry, by backdrafting through fireplace/furnace/dryer/bath vents, or any crack or crevasse the air can seep through to balance the internal pressure.

And it is important that you do not compare the make-up air system to your regular HVAC system. Range hoods are not used more than 2 or 3 times a day, if that (more like 2-3 times a month - Lol). So there will not be a steady flow of cold or hot air gushing into your home and driving up energy costs.

Hope that helped explain it a little better.


    Bookmark   June 29, 2012 at 9:13PM
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"The fresh-air intake needs to be in or very near the kitchen for the pressure to balance before it draws air from unintended locations (i.e. combustion vents). Air will always take the path of least resistance, so the duct that brings in the outside air needs to "cut-off" any potentially hazardous points of infiltration or leakage. "

Sorry, but I totally disagree. Air moves so freely within a house that the entire interior will always be at the same pressure, regardless of where the air is entering or leaving. It is quite OK to have MUA entering the basement - if enough is coming in to balance what is leaving through the kitchen vent, it is impossible for combustion appliances to backdraft.

    Bookmark   June 29, 2012 at 10:10PM
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Cook: You are correct if there are no obstructions, e.g. an empty house with all doors propped open.

If the makeup vent is mounted in the basement, the resident can close doorways or obstruct the vent with furniture, and the airflow is interrupted by the stairwell and hallways.

This is the same reason your HVAC return air is located in an open area near the center of the house, and not in the basement.

A good rule-of-thumb would be to say there should be a clear line-of-sight between the hood and the makeup air vent. This would ensure proper airflow, and minimize the chances of a resident or builder obstructing the vent with doors, furniture, appliances, hallways, and so forth.

- Saul

    Bookmark   June 30, 2012 at 10:12AM
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Several thoughts here that are worthy of discussion.

As I have noted previously, just as the hood ventilation fan needs to overcome pressure losses, so does the MUA path. A passive path cannot have much pressure loss or one is back into the state where the house pressure is potentially dangerously negative when the hood fan is at the higher range of its flow rate. If the passive path is is filtered for objects smaller than dragonflies, there will be pressure loss, probably higher than the 0.03 inches that will start to backflow a combustion water heater.

If there are no combustion appliances or fireplaces, then safety is not an issue, except to robotic inspectors, who might argue that there could be such appliances in the future.

So, if the MUA is to have pressure boost to avoid this, control then becomes an issue. If the hood fan is variable, other fans are on or off, a fireplace may be used while cooking, etc., the MUA fan has to be variable. Then one needs to have a means of controlling it, and a measurement of house negative pressure to drive the control. It should be obvious why most don't want to deal with this complexity.

I don't have a simple solution, other than using passive to the kitchen, and isolating all combustion appliances in their own room with its own passive (unfiltered) MUA.

I have separate MUA to my oil burner, but without MUA I can run the house sufficiently negative that combustion odor can escape into the basement via the pressure equalizer damper in the oil burner exhaust stack. I'm going to fix this sooner or later, even though the MUA will be pressurized.

On the topic of how close to the kitchen the MUA should be supplied, my thought is that the path should not be seriously constricted because air has inertia. Normally one should turn on the vent hood before making a stink, so the MUA path should get moving in concert with the hood fan powering up. The path through the basement and up stairs, past a door that could be closed, to the kitchen seems to be unnecessarily tortuous.

However, using basement air is a potentially unsuitable choice for other reasons, such as practically any project that a handyman might perform. One produces chemical smells, dust, and fumes doing home projects. Laundry activities can produce odors or even dust. Air in the basement may also have a path to an attached garage which will potentially include gasoline odor.

If at all possible, I would use an attic for a path from rooftop intake to diffuser in the kitchen, attached hall, or other room with a wide and certain path to the kitchen.

Did I forget to address a topic?


    Bookmark   June 30, 2012 at 10:51AM
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