Need Advice - Kitchen Hood/Makeup Air

malcienceNovember 18, 2009

My wife and I are building a new house in the Pittsburgh area and we are getting close to our pre-construction meeting and plan confirmation. Our builder has said that according to 2006 IRC any kitchen hood that will produce more than 3 Pa of negative pressure needs to have a make-up air system installed with it. For our home the pressure requirement converts to 400+ CFM hood and we will have a Bluestar RNB 36" range so we want > 400 CFM. Our appliance vendor has apparently never heard this before.

I dont know anything about makeup air, our builder said it could be installed in-line with the hood (Independant INBQ) but again our appliance folks said they have never heard of it.

Could someone enlighten me as to what exactly a make-up air system entails?

What viable work-arounds exist? (or are we violating our home warrenty if we try?).

I am also going to be looking for any manufacturer sites and calling Independant to see if they can straighten me out, any other sites that could help me figure out what we need to get done would also be appreciated.


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Things to check:

* Does 2006 IRC apply to your jurisdiction?

* How did he compute 3 Pa? How does he know the physics to determine that?

Make-up air is basically a blower system that replaces the air that the hood takes out. You can see its importance, for example, if you think about a fireplace that could have the smoke "sucked" into the house, instead of going up the flue.

Another good source of moderately-priced, high-quality hoods is Prestige.

    Bookmark   November 18, 2009 at 9:19AM
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2006 IRC does apparently apply, the builder said that without make-up air the house won't pass code.

Chapter 15:
M15001.2 Indoor depressurization. Ducted exhaust systems shall not induce or create a negative pressure sufficient to cause backdrafting of naturally vented, open combustion-chamber, fuel-burning appliances, or create negative pressure in excess of negative 3 Pa. in the immediate proximity of combustion chambers of such appliances.

The builder (their HVAC engineering department) has the A/C capacity and the minimum air flow through the house, since there are minimum requirements for Energy Star certification. With this he can back out how much additional draw would be needed to violate code. 400 CFM was the number they quoted me, without seeing their model and inputs thats all I know.

I dont think the hood is the problem, we have contacted BlueStar, since they are in PA, they make hoods as well and they are the manufacturer of our range, to see what they can tell us about the regulations.

I still cant find much about make-up air systems though everything I've found is industrial.

    Bookmark   November 18, 2009 at 10:12AM
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Try Prestige Hoods -- they make the BlueStar hood and were very helpful when I called them on some ducting questions.

Your builder seems to know what will and won't pass plan review or inspection. I'm not an inspector, but as I read that, it is saying that the back-drafting or pressure drop is for other naturally-vented appliances, not the range. I don't think the range even falls into the "naturally-vented" category, as it doesn't have a vent to the exterior at all. I would interpret that requirement to apply to, for examples; gas hot-water heater, gas fireplace, gas furnace, but not the range or ovens. With the exception of the gas fireplace, I would be surprised (but not completely so, as I've never modeled it), if a hot-water heater or furnace in your garage or basement experienced significant pressure changes from a 600 (or even 1200) CFM hood on a different level and through several doors.

It might be worth a call to your local building inspection department and talk with what our jurisdiction calls a "Senior Inspector" who can give you guidance. That kind of thing saved us from having to put the gas shutoff in a cabinet (rather than behind/beneath the range) as our code recently changed from "readily accessible" to "accessible" and many local plumbers (and apparently some inspectors) were not aware of the change.

Here is a link that might be useful: Prestige Hoods

    Bookmark   November 18, 2009 at 10:43AM
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It's your HVAC guy you should be talking to about installing makeup air, not your appliance dealer. I can't give you any details but I know there are different kinds of systems - some that will heat the air if necessary so that you're not pulling in cold air in the winter.

    Bookmark   November 18, 2009 at 10:45AM
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Where is it is best for the make-up air to come in?

At the hood (e.g. make-up air enters the room at the front of the hood, the exhaust air is drawn out at the rear of the hood)? Somewhere behind/under the range? Elsewhere in the kitchen?

I'm asking because, in a cold climate like PA, I wonder if the need to pre-heat the make-up air (complication, cost) is affected by where it enters?

Also, is it possible that having make-up air enter at the hood, close to the exhaust vent, might make the system less effective at exhausting vapor/grease from the range top some 30" below? As in, the exhaust simply removes the make-up air, leaving the cooking vapor behind?

    Bookmark   November 18, 2009 at 2:37PM
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As weisman said, this isn't the venue of an appliance dealer. A good dealer will help you to figure out how much CFM you need to properly vent your range, but the whole home ventilation design, including makeup air, is your HVAC person's responsibility to design and implement. In your climate, you will probably also need a heat exchanger as well, and the whole shebang can get to be pretty darn pricy. If it's rolled into your home HVAC quote, it'll be less of an "ouch" though. HVAC is an ouch enough on a new build, but once you reach a certain point, there's little % difference in a 25K quote and a 30K quote if that 30K includes everything you need HVAC wise. Don't forget bathroom and dryer ventilation to talk about with your HVAC person.

    Bookmark   November 18, 2009 at 5:15PM
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Your HVAC guys are definitely the ones to talk to regarding all your make-up air questions. Appliance dealers have a vested interest in NOT knowing anything about MUA, since in your jurisdiction (and mine) it could easily mean lost sales for high powered vent systems and ranges when installation costs get included (my $2000 VAH cost over $7K to install).

MUA does not have to come into the kitchen. Mine enters the furnace room and straight into my duct system.

BTW, is 400cfm really enough for a 36" Bluestar???

    Bookmark   November 18, 2009 at 8:21PM
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Our MUA system was installed by the HVAC crew in our new build. No we couldn't pass inspection without it even though I was just willing to keep a window open. Anyhow, the outside air enters through a main vent and comes into our kitchen through 5 small outlets. The intake passage which branches into 5 is surrounded in a space blanket type insulation and each outlet has a heated coil because air coming in on cold days no longer makes the home heat system efficient. There is a sensor that makes the coils fire up when the outside air is colder than 65 F. The juice this system sucks up is unbelievable, it makes the lights in the kitchen and even the interior fridge light dim and pulsate. I had my electrician set the MUA to come on only when my hood was on the 4th and most powerful setting. Thankfully I don't use that setting very often. Unfortunately, last summer when it was really hot, the switch to make it come on would trip when it was at the 2nd powersetting which is the one I use most often. That managed to blow hot outside air into the house which otherwise would remain reasonably cool. Summer was over by the time that got fixed

    Bookmark   November 18, 2009 at 11:19PM
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If you have a gas furnace, or if this will be a tight house, the HVAC guys will be adding make-up air anyway.

    Bookmark   November 18, 2009 at 11:49PM
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Thanks for all the info folks! I really appreciate it, I am going to have a talk with our builder, again, to have them look at the additional HVAC work. I will give an update as the excitement unfolds.

I do have to say that I am kind of steamed that the appliance people are either disingenuous or ignorant about MUA. Had they let us know in the beginning that this would be an issue, they would have earned some points in our book and we probably would have gone with whoever they suggested for the installation (probably earning them a nice referal fee).

If I hadnt chased this down before the construction started I would have had to change everything and pay an amendment fee on each change to the original plan (nearly everything), which would have cost thousands just on the change fees.

Cooksnews - the 400 CFM definately wouldnt work with the BlueStar, that is why I need to chase this down... if I cant get the MUA worked out I will need to change the range hood to something

    Bookmark   November 19, 2009 at 7:43AM
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There is a lot of research on the best way to add makeup air that has been done for commercial Kitchens. If you want the details ask and I will dig up some links, but it is techie stuff aimed at commercial applications. My interpretation of the research is that the best arrangement is to introduce the makeup air behind and below the heat source. I designed mine to bring air in below and under the rangetop. The recommendation is to bring in ~70% of the makeup air at the heat source and pull the rest from vents in the room. I am not currently tempering the makeup air. I'm sure I am losing heat, but I use my grill a lot and so far the cold makeup air tempers the radiant heat from the gill and makes it more pleasant to work close to the range top.

    Bookmark   November 19, 2009 at 11:08AM
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There are several ways to get makeup air. You can use passive vents, that will open when needed (I believe through any neg pressure).

You can also use HVR (Heat Recovery Ventalators) or ERV (energy Recovery Ventalators). These exchange air so that in the winter months the warm air going out helps heat the cold air comming in. In the summer vice versa. The difference between a HRV and ERV is that an ERV also echanges humidity. In a cold weather climate the warm moist air in the house will humidify the air that is coming from the outside. In the summer the the humidity will be stripped from the hot air comming in from the outside.

These systems can either be ducted to your forced air system or be independent. Typically you would place vents to capture outgoing air from humid places like bathrooms, Laundry rooms & Kitchens and you would want the fresh air to enter in places like bedrooms. This keeps helps maintain proper humidity as well.

    Bookmark   November 20, 2009 at 2:13PM
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The steps you or your HVAC supplier need to follow are:

Determine the air flow needed to properly vent whatever you are going to cook on the stove, given your hood size. This may be well above 400 cfm.

Determine the duct size that will keep the air flow, when venting at full power, between 1000 and 2000 feet per minute in the duct.

Determine the exhaust fan that will pull this flow against the negative pressure of the hood and ducting when the house is at the negative pressure you intend to let the house get to before the MUA kicks in. (This may require a blower that is _rated_ 1.5 - 2 X the flow intended to go up the hood.

Design an MUA system that will supply this much air at that negative pressure. The MUA system can be a far more daunting project than choosing the cooking ventilation system.

First, one wants the air warmed in the winter, and maybe cooled in the summer.

Second, the air should be supplied to the kitchen so that it doesn't disturb the upward flow of cooking effluent into the hood. Air that flows down a hall to the kitchen, or is introduced under the stove, or via an opposite wall may be suitable. Turbulent air when it reaches the cooktop is your enemy.

Third, ideally one wants only as much MUA as one is pulling through the hood, bathroom fans, fireplace, etc. This may be the mother of all residential "tuning" jobs to achieve.

What I have planned and partly fabricated to do this is as follows, but I think your HVAC guys can probably integrate yours with your air conditioning. Or if you have the funds and roof space, you might get Greenheck to design and build one for you based on their commercial designs.

I will use a heat exchanger (think car radiator for looks) above a ceiling diffuser that has a 30 x 30 inch opening. The heat exchanger is fed hydronic heat water from my oil burner via a large 013 Taco pump. Pump control will be based on thermostats set to keep the air under the heat exchanger at around 72F AND the air over the heat exchanger above 40F. (I live in NH.)

Electric units are obviously easier to plumb and control, but do require a significant power feed, as a serious kitchen ventilation system may require 100,000 BTU/hr be put into the MUA when the outside is at minimum temperature.

The blower I plan to use for the MUA airflow will be the largest Fantech in-line unit. It will supply air against its duct losses to counteract the probable 1000 cfm I can pull through a Wolf island hood using a 1500 cfm rated exhaust fan, plus the probable 600 cfm I can pull through a 1000 cfm rated NuTone (Broan) exhaust fan pulling air from ceiling registers located above my wall ovens, plus whatever the bathroom fans are pulling at any time.

MUA fan control is _planned_ to be based on the differential air pressure between my attic (open to the outside) and my kitchen. A proportional/integral/differential (PID) controller should be capable of adjusting MUA flow to keep the differential pressure between 0 and -0.02 inches H2O (forget how many pascals that is).

The tricky part in this design is to tune the PID controller parameters so the dynamic response is good without overshoot or oscillation. Simpler control designs, such as scaling the MUA fan power to the hood fan power have their own issues. In my case, with three bathroom fans and two kitchen fans, scaling the MUA fan to the hood fan is not appropriate.



    Bookmark   November 21, 2009 at 2:41PM
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Thanks for all of the information and support folks. I am in roud 7 of resolving the makeup air issue, but I think (hope!) we are close to closure. Here are the tidbits that I've picked up talking with inspectors, hood manufacturers and our builder.

The new residential code IRC 2009 Chapter M15 REQUIRES that any hood ventilation system now have a makeup air system. This may come as a suprize to people doing a remodel and expecting to get their inspector to sign off.

For EnergyStar certification, you cannot have air that is just brought in from outside as the standard makeup air system because of the loss of heating/cooling efficiency in the home.

sfjeff suggested I talk to Prestige. I originally tried to call the people we would have been buying a hood from, never got a call back after leaving a couple messages. When I called Prestige, they put me on with one of their technical folks and we have been working together to design a hood that works.

Their two best options were:

Using the basement as an air resavoir and ducting from the basement to the hood.

Develop a residential self-compensating hood (as has been suggested above). This sounds good but its pretty expensive just for the fabrication, and I dont believe this includes installation.

    Bookmark   December 16, 2009 at 1:40PM
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I don't know the answer on this but I had the same problem - couldn't have an exhaust fan greater than 300 CFM. In my case they would have brought the make-up air in through the basement somehow - with a system priced out at $4,000. That was 4.5 years ago. This was a remodel in an energy inefficient wind tunnel home built in the 70's so I said no thanks. I got a 300 CFM fan - really happy with that decision since I can't stand the fan noise and use it as little as possible. Fortunately I didn't have my heart set on a range with a lot of BTUs - also no regrets there. I guess I was fortunate to have an appliance dealer and a contractor who were familiar with the issue.

    Bookmark   December 16, 2009 at 5:51PM
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Does anyone know if a kitchen addition/remodel in Maryland will run into this requirement? I purchased a new 600 cfm thermador hood on craigslist but really don't want to deal with putting in makeup air.

    Bookmark   December 16, 2009 at 11:03PM
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You'll have to talk with your local building inspection department -- "local" codes are generally closely modeled after the national or "international" codes, but there may be many years' lag between the two, as well as sections not adopted or otherwise overridden for your state, municipality, or other level.

(As an example, San Francisco has had rather bizarre requirements on water and DWV piping -- nothing non-metallic; no ABS, no PVC, no PEX -- claimed for EQ safety, believed by most to be a strong union's "contribution" to the codes.)

    Bookmark   December 16, 2009 at 11:31PM
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Our 1980's townhouse certainly isn't energy star efficient :) We were required to provide make up air but were allowed to keep it simple. We have a register built into the toe kick on the island that we can open for make up air when the hood is on and close when it's not. We vented it right through to the outside...pretty basic, a simple vent from one side of the room to the other. No fancy heat make up goober etc.

Honestly, we don't need the make up air...the house is NOT tight, but in a new build it might be a very important thing. I'd certainly follow code.

    Bookmark   December 17, 2009 at 4:22PM
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To keep things simple, my city has the same codes for new builds and renovations. While it meant a big charge for MUA in my 40yo leaky house, it also means I don't pay ongoing taxes for a huge fleet of inspectors who have to know how to assess every single house/project for air exchange.

    Bookmark   December 17, 2009 at 5:40PM
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Its been a while, but we are in the final throws of finishing the house and so I thought that I would post about what we found/have had to deal with for those out there who may be in the same boat.

We basically had to dicker with the building company for essentially 4 months, delaying the start of the house, because they were not allowed (internal regulations), to build a house that did not meet ComfortHome EnergyStar compliance. We ended up needing to request Prestige Hoods to design a one-off self compensating hood for us. The hood performed its regular function and in addition had a seperate set of ducting and 'chamber' that pulled air back into the hood from outside inlet duct ~10' from outlet duct to avoid spitting the exhausted air back into the house. Getting this designed/built/installed has run 4-5x what a regular 1200 CFM hood/blower would cost.

On the other hand, the rest of the kitchen... minus the missing wall oven, has come out great, and so we're looking forward to kicking on the hood at the start of our first meal in the house. Though I'm afraid its going to sound like a train chasing a banshee.

Thanks for all the posts and help!


    Bookmark   May 17, 2010 at 9:23PM
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And thanks for the feedback. You are a little ahead of me as I don't yet have all the parts of my MUA installed. Does your MUA have a blower in it for replacement air, or does it use a passive connection to the outside air?


    Bookmark   May 18, 2010 at 9:45AM
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I don't have an intake blower, but that was my choice, Im on an outside wall and so there is ~8 feet of ducting for each the intake and exhaust exits. There is an option to install an inline intake but we opted out, I don't think well miss it, if you aren't on an external wall then I think I would go with the extra intake blower.


    Bookmark   May 18, 2010 at 12:23PM
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I know my question won't apply to the OP and he's in a different state subject to different codes...

But I just bought a BS 36" and I plan to just crack open a near by window (even in winter) when I crank up the Wolf 42" hood I also just purchased. Ignoring the code side of it, will this plan work (and avoid any issue with fireplace or furnace air reversing?). Thanks

    Bookmark   May 19, 2010 at 12:26AM
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That should work perfectly well... thats the real rub of the new code set. There isn't anything wrong with whatever size hood you have if you have a ready source of external air, and a cracked window fits the bill.

You probably want to crack a window on the other side of the kitchen pretty much as far from the hood as you can. This will cause the airflow in the room to be drawn into the hood, better confining smoke and odors.


    Bookmark   May 19, 2010 at 7:10AM
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Schleiren photography shows that the most important aspect of introducing make-up air is to keep down turbulence around the cooktop that will disturb the uprising effluent that one is trying to get into the hood. Hood testing is performed in laboratories with large perforated diffuser panels planted along the bottom of a wall well away from the system under test.

For our more limited kitchen spaces, an ideal would be to introduce the air under all the cabinets so that it enters the kitchen through all the toe-kick spaces, but that would be a project all of its own, and the basement likely would be cluttered by ducting. If the air is entering from an attic space via a roof vent or eve vent, a diffuser that is large and well away from the hood is probably best, preferably in the ceiling of a nearby hall or in another room open to the kitchen.

No matter how the air is introduced, unless one lives where the ambient temperature is always ideal, it will have to be conditioned -- at a minimum warmed when it is cold outside. This adds complication.

If the MUA is passive (no fan), the house pressure will fall until low enough that the MUA flow through the pressure losses of its ducting, plus house leakage at that pressure, equal the hood flow that will also be limited by the negative house pressure. This is a non-trivial problem to analyze for MUA fan requirements. There are MUA systems for furnaces, so that safety issue could be overcome without a kitchen MUA, but the code is the code. Otherwise, a passive MUA must have a pressure loss low enough that the furnace doesn't backdraft.

Another approach is to do the minimum that is required by code and then open a window to supplement it when necessary. Air heating is still desirable, of course, so the window should be where the incoming air has some chance of being tempered before everyone present gets to sample the ambient temperature.

In all cases, avoiding backdrafts of furnaces, water heaters, and fireplaces is important, and the allowable negative pressure will depend on which of these is present. A fan powered oil burner will be a lot more tolerant than a fireplace that depends for its initial draft on the small (0.02 inches of water, I recall) pressure difference between a typical room in a house and the pressure at the roof.


    Bookmark   May 19, 2010 at 10:25AM
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Have there been any recent developments on this issue? I called BS and Signature Hoods today--the BS tech I spoke to never heard of MUA, and the Sig Hoods tech said they don't recommend MUA. It seems no progress has been made there.

We will be installing a 48" BS with 1200cfm hood in a super tight new home. One advantage is that all appliances are sealed combustion, and the woodstove and fireplace have their own supply air.

Some questions about current MUA thinking:

1) Where is the best location to locate the MUA airstream? Under the the back of the the backsplash on either side of the toekicks?

2) It seems a waste to heat MUA only to have most of it be exhausted. Is it really necessary to warm or cool MUA in extreme climates, if the airstream is properly located?

3) Does it make a difference if MUA is boosted with a fan, or passive, but in either case with a solenoid damper?

Thanks for the help. --Mark

    Bookmark   September 28, 2011 at 5:13PM
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I'm interested in converting from a downdraft vent to an over-range hood. Does anyone have advice or experience converting an existing downdraft duct to a makeup air source? It would seem to provide laminar flow at the rear of the stove and not require such a heating requirement, as it would mostly exhaust. Here in northern Minnesota, heated makeup air is a significant energy use.

    Bookmark   October 30, 2013 at 1:27PM
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In a study I read once, successful MUA introduction without significant disturbance of the rising air into the hood was obtained by injecting downward flowing air from the roof onto the floor behind the stove and allowing it to spill out and flow up around the stove. A downdraft duct could probably emulate that well enough.

Care would need to be taken to ensure that the uprising MUA did not disturb the flow into the hood. You are not trying to inject MUA into the hood, and in particular are not trying to displace rising effluent with MUA, but are trying to replace the air in the space where the hood collects effluent, the cooking surfaces and the boundary around them up to the hood aperture.

A major, if not the major use of energy in restaurants and commercial kitchens is ventilation. If you want to keep grease off the walls, and odor from penetrating the rooms of the house, then for the time when cooking is ongoing power will be needed to supply MUA and condition the MUA.

Someone on this forum published a photo a few years ago showing the ducting in his basement with inline fan and heater coil section. Electric coils should add heat with less back pressure than a radiator style heat exchanger. The electricity cost for the time spent frying something will probably not compete with the imputed (or real) interest for the cost of the upgraded kitchen requiring MUA.

I suggest reviewing, at a minimum, the Greenheck reference listed in my My Clippings.


    Bookmark   October 30, 2013 at 9:38PM
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How about introducing makeup air at floor level behind the refrigerator?

Also, is there a huge benefit of fan powered makeup air? If a house were leaky enough, or if a window were open, it sounds like adequate makeup air would be available. A window or leaky house are passive supplies, so why would a passive 10" or 12" duct [for a 1200cfm hood] not be sufficient?

    Bookmark   December 11, 2013 at 11:06PM
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A big duct with a damper that opens when the hood is turned on probably won't need a fan. However, if you put a filter in this duct that will catch mosquitoes or dust and lint, then there will be a pressure drop across it. This pressure drop depends on the filter performance and its area. A two-by-two foot 4-inch ripple filter pack at 1500 CFM can cause a 0.1 inch of water pressure loss. If the pressure loss exceeds 0.02 in w.c. for fireplaces, 0.03 for combustion appliances, or (forget exactly) around 0.06 for blown oil burners, there will be back-drafting, which is bad. In these cases a fan is needed.

If there are no running fireplaces, and combustion appliances, if any, are built with their own MUA, which will be very modest in ducting, then a filter should not be a problem. If you heat the air then there is a further pressure loss through the heat exchanger, depending on design, which may or may not require some boost.


    Bookmark   December 12, 2013 at 11:35AM
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Thanks for the reply. What are your thoughts on introducing air behind the fridge, to help to isolate and possibly warm the makeup air a little? In this case my fridge is about 16" from the range.

    Bookmark   December 12, 2013 at 6:54PM
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If it were an older refrigerator with coils in the back and the air were cold, then there would be some thermodynamic advantage.

The main issue is not disturbing the rising cooking effluent, so any way that the air can be ducted into the room avoiding blowing at the cooking surface should be satisfactory. Large CFM introduced behind a refrigerator might see some pressure drop from the path it has to take; I can't guess about that from here, but otherwise should be OK. If the air is very cold, standing at the refrigerator door in bare feet for long may not be pleasant.

Generally, introduction at floor level or via a long path to the hood from a ceiling register should work. Matching the laminar introduction schemes of hood testing labs is not going to be very easy for most kitchens unless a spare wall is available that wasn't needed for cabinets.


    Bookmark   December 12, 2013 at 7:29PM
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Good point about the fridge. In this case it is a newer unit, so the coil is on the bottom. As you said, it might be quite chilly standing in front of the fridge in the winter. Can you say more about the cause of potential pressure drop? I'm trying to understand what's occurring in such a case.

Do you find that introducing makeup air at ceiling level is optimal both in terms of comfort and performance? Or as far as comfort goes, does the introduction of cold makeup air make for a cold room no matter where it's introduced?

    Bookmark   December 12, 2013 at 10:02PM
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Any time the flow of air (or part of the air) has to change its velocity vector (direction and/or speed), even slightly, work is done on the air, and this requires either heating or force times distance. Pressure is force over area. Velocity change occurs not only when a duct bends, but when there is turbulence in the duct or at the duct walls, or when the air is forced through a filter, around baffles, etc. Thus, there are many ways that pressure drops occur along a flow path. If not, one could blow through a straw without having to build up pressure in one's mouth.

I use a large (three-foot-square) vaned ceiling diffuser to introduce MUA into a hallway adjunct to the kitchen. The diffuser is about 18 feet from the edge of the hood. This configuration was the best option relative to where I could take the air from the roof area. It is my expectation that by the time the air gets to the vicinity of the hood it is flowing relatively smoothly and low in velocity due to the size and length of the hall.

Certainly other approaches would work for other kitchens depending on where the air is taken from and how a house is architected. Test kitchens use a porous diffuser panel in an adjacent wall to the hood under test. I am sure that from a functional point of view, MUA flowing out from enough toe-kick area would also work, so long as it wasn't somehow directed at the stove. For example, if a cooktop were on an island, and the entire toe-kick area were grilled using a set of registers, the MUA would flow out and then up in a way that did not disturb the rising effluent from the pans. Even down flow behind a refrigerator can work if the path behind and underneath the refrigerator is relatively open (several times the area of the supplying duct.)

Never forget that perfect is the enemy of good enough, and what we are trying to accomplish is good enough capture of grease, moisture, and odor. Good enough is also user dependent, so not everyone needs to put up with the compromises that a nearly perfect MUA system would impose on a house.

So, my best advice is to find a way to introduce the air that doesn't blow at the cooking zone and disturb the natural rise of the cooking effluent.


    Bookmark   December 13, 2013 at 10:45AM
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Thanks Kas. What size blower would you recommend for makeup air for a hood with a 1200cfm blower? One of equal size?

Is there much issue with using flex ducting instead of solid for the MUA?

    Bookmark   December 15, 2013 at 10:45PM
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Given a house interior at the ideal of zero differential pressure to the outside such that there is no house leakage, then the maximum MUA required would equal the flow that the hood can manage (less than 1200 CFM due to baffle and duct losses) plus the sum of all the flow rates that bathroom fans and combustion appliances with access to the same air would use. If one knew the pressure losses of the hood path, and of the MUA path, and knew the fan curves for both fans, then one could calculate the zero static pressure rating that the MUA fan had to have for any particular hood fan.

Generally, we never get to know all of that. Baffle loss is rarely reported, and then only for commercial units. MUA duct losses and filter loss can be estimated. If the baffles are 0.1 inch w.c. at whatever flow occurs for full fan power, and a suitable pleated filter for the MUA duct is also good for 0.1 inch w.c. at the MUA fan's full power, and the ducting is comparable for the two paths, and the bathroom fans are trivial, etc., then matching zero static pressure ratings of 1200 CFM for each path's fan would be reasonable. How the MUA is heated, if it is, would need to be considered in the pressure loss balance. Restrictive heat exchangers (radiators) might require a more powerful MUA fan than hood fan.

Controlling the MUA fan is an entire subject by itself. Perhaps matched fans and losses would allow driving them both with the same voltage and avoid any need for pressure measurement and a control system. Y$MV


    Bookmark   December 16, 2013 at 9:51AM
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Great reply.

In your opinion, considering 1200 CFM of unconditioned MUA introduced along the upper wall of a kitchen with 9.5 ft tall ceilings to assist with a 1200 CFM hood exhaust, would it be just minutes before the kitchen (and possibly the adjacent rooms) becomes uncomfortable (too hot in summer, too cold in winter)?

    Bookmark   December 16, 2013 at 11:23PM
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Well, an approximate answer can be calculated, given some parameters we don't know. Suppose that the house had perfect mixing (one big room encompassing the house volume (V) with a circulation fan to provide mixing). In reality, the other rooms will change temperature more slowly and the kitchen more rapidly. It would probably require computational fluid dynamics to deal with the temperatures at all points in a house with imperfect mixing.

The following ignores subtle thermodynamic effects such as changes in density of the outside air as it initially enters the warmer house. Flow rate in volume units per second is denoted by (f), e.g., cubic feet per second, which is 20 cu ft/s for 1200 CFM.

Assume that the temperature of the air outside is T_2 and the air inside to begin is T_1. Let T_1 > T_2. Let the difference of these T_1 - T_2 be dT. I would expect the interior temperature at time (t) after fan turn on (assumes instantaneous speed-up to full flow) to follow this function:

T(t) = T_1 - dT*(1 - exp( - (f*t/V)))

where exp denotes the base of natural logarithms e; e = 2.718...
and the asterisks denote multiplication

The time constant to where the temperature has fallen by 1 - 1/e of the difference between the two initial temperatures is V/f. With minor changes this formulation can deal with hotter outside temperatures.

I can't divine what temperatures you would find uncomfortable.


    Bookmark   December 17, 2013 at 10:23AM
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I can grasp some of your formula, but I certainly can't wrap my head around its application. I suppose what I am wondering is this....

In perfect conditions, it appears a fan running at 1200 CFM will empty the air out of a 2,000 sq foot home with 10ft ceilings in 20 minutes. If that air is replaced with unconditioned makeup air that is below 32 in the winter or above 90 in the summer, it seems like the space would be uncomfortable rather quickly (approx 20 minutes).

HOWEVER, when the MUA is introduced at ceiling level in relatively close proximity to the hood (e.g, approximately 15 feet away), is the majority of the MUA exhausted by the hood, thereby lessening the impact of the frozen or hot unconditioned air?

    Bookmark   December 17, 2013 at 11:54PM
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The goal is not to have the MUA be exhausted by the hood, but to have it replace the effluent laden air that the hood is supposed to exhaust. The MUA is, of course, eventually exhausted too.

The short answer is yes the house will fairly quickly become uncomfortable unless the heating and/or cooling system is designed to deal with that much air "leakage."

If the air is introduced close to the hood but in a manner that does not disturb the rising effluent, then the hood function won't be disrupted and the zone around the hood will chill or heat more quickly than the rest of the house due to imperfect mixing. Care must be taken that the flow of MUA does not end up replacing the effluent that is supposed to be collected by the hood before it is collected. In that case the effluent laden air would spill out into the kitchen and the hood would remove the clean MUA.


    Bookmark   December 18, 2013 at 10:56AM
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