Make up air is to bring air into the house to replace all the air being exhasuted by the hood. When the hood has high CFMs "make up air" is required, otherwise the air can be sucked in through your furnace, firecplace etc which can be extremely dangerous (carbon monoxide).

Be warned that you have stumbled on a very thorny topic. It's easy enough to say that you need make up air, but not so easy to say how you're supposed to get it. Residential products in this area are very limited, and HVAC contractors, who are the people who ideally should be helping you with this, are often clueless. As you say this is a custom home, I would suggest trying to enlist your architect's help on this. Be prepared to deal with contradictory advice from many different sides.

With the range you mention, its likely you'll need make up air -- even if you're building somewhere remote, where zoning rules have yet to spread, this is wise. Local zoning rules may specify how much exhaust (hood ventilation) CFMs you can have before you must make up that exhaust, depending on where you live. Also, new construction is presumably much tighter than older houses; this increases the need to make up the exhaust, since not enough replacement air can enter (as it would through the many leaks of an older home).

Thanks all! This helps and I have added this to my checklist of questions for the architect.

Cheers.

This has been an area of concern for me as well. While I don't have a super cooktop, we checked into local codes, as I wanted 600cfm.....code says greater than 400 needs make up air. DH and I decided not to even go there, we are getting less tha. 400 cfm, got a zephyr on clearance for $200, if in the future it doesn't seem like enough I'll deal with it....but I wanted to know the code before we made a decision, all I needed was the inspector coming through and then I'm out ore $$ and time on a silly MUA system.

Even the inspector thinks its a stupid code, but none the less, code is code, and I was not going to get caught in the end!

Best of luck!

It's not such a stupid code if your furnace back-drafts, filling your home with CO because you have created negative pressure in your home with a high cfm vent. The code requirements for MUA are safety based.

you might get a better understanding about Make up Aire from this link

Here is a link that might be useful: Broan Make Up Air

How about a nice window not too close to the grill. Could balance the local area btus from the cooktop and cut down the impact on the rest of the home. The alternative if the home were LEED or Passive Haus designed for energy efficiency would be to move it all to the outdoor kitchen with plenty of ventilation.

Here is a link that might be useful: Passive Haus vid

Providing MUA by opening a window is only a slightly less stupid idea than moving the kitchen outdoors. It was -20C this morning when I walked my dog, and I truly looked fwd to preparing and eating my breakfast somewhere at least 40C warmer....

Backdrafting of combustion appliances such as furnaces will only happen when said appliances are operating. If it is cool enough to run my furnace, it is too cool to open windows to feed my vent system. And, there are 3-4 months per year when the windows are totally frozen shut. If you have a totally electric home, there is not the safety hazard associated with introducing pressure differentials by venting lots of cfms. You would only have to worry about starving your system and dealing with the resultant efficiency losses if there isn't enough air in your home.

Add another kitchen outside for this type of appliance not move the main kitchen. Induction inside with its greatly reduced need for ventilazion. See.

For me, grilling outdoors in the middle of winter in a foot of snow isn't as appealing as addressing the make-up air situation during design and build.

I suggest you contact a commercial HVAC contractor or vent hood contractor that has experience providing makeup air in commercial kitchens. They can make a hood that delivers makeup air around the perimeter of the hood, so you don't get cold in the winter AND you avoid having to provide conditioned makeup air which is very $$$.

We have a 48" Culinarian and so far we are opening a window...

Billy

Here's a link to the kitchens at Hampton Court Palace. The fifth segment describes roasting chickens and other meats in front of a large open fireplace, one that burned 1+ ton of seasoned oak per day as the kitchens worked to feed 600 at Court twice a day. As a cook turned the spit the meat would be seared and juices rise and baste the outside as it turned towards the hot fire. It would then be cooled as it turned and encountered the cool air screaming past to feed the fire. This slow process produces very tender slow roasted product.
The fire acts as the ventilation.

Here is a link that might be useful: Roasting at Hampton Court kitchens

I sincerely doubt that any HVAC contractor would design a system for a cold climate residential kitchen that involved delivering the MUA anywhere near the vent, particularly if one needed over 1000cfms. Unless I suppose, one desired that wind-tunnel ambiance in their cooking zone....

My 600cfm MUA system requires heating during cold seasons so as not to overwhelm my furnace capacity, and delivers the air directly into my duct system, well away from the kitchen.

One thing to keep in mind is that there is always make up air. What goes out has to be replaced, otherwise the house would implode. If the intake is restricted, less goes out. The rated cfm of kitchen hood ventilation is specified at zero static pressure; as the pressure falls the flow rate is less. This is the nature of blower performance.

It is the low pressure that is potentially hazardous. Ideally, a house would be designed so that combustion appliances would have their own MUA, and only fireplaces would have a problem with negative pressure. It takes little negative pressure (around 0.02 inches of water) to backflow some furnaces and heaters. With my two kitchen fans on full, I can induce over 0.08 inches negative pressure with a single 24 x 24 inch furnace filter and heat exchanger between the interior and the exterior. Clearly, in my case, powered MUA (not yet installed) is needed to make up the pressure loss. (Yes, a 4-inch thick pleated filter would have lower pressure loss than the 1-inch filter temporarily being used, but it wouldn't be low enough.)

Also of note is that outflow is total outflow. This means that MUA has to make up bathroom fan air flow, kitchen air flow, fireplace air flow, window opening state, door opening state, etc. The more complicated the house design for air exhausts and intakes, the more potentially complicated the MUA may have to be.

Some restaurant kitchens can operate by tuning the MUA to a specific flow rate. This rate matches the single exhaust flow rate of the entire kitchen because all the hoods are run on full at the same time. The kitchen is kept slightly negative to keep odor out of the dining part of the operation. No combustion appliances are relevant other than the cook stoves being vented.

In some cases, a passive MUA is possible. In this case as the pressure falls a damper opens and lets in more air. Typically, this air has to be conditioned. The conditioning, and in particular filtering, leads to a pressure loss. Whether this loss is an issue depends on the combustion appliance situation, and on the importance of achieving the rated (or at least intended) flow rate of the hood ventilation.

To make up for pressure loss, the MUA can be itself powered by a blower. Generally, the blower has to be controlled by something, such as differential pressure. Large commercial buildings can get away with having continuous fans running for MUA where excess MUA is just recirculated in the ducting and passive positive pressure control can be used by allowing dampers to control the air bypass.

So, I concur with the advice above to find a HVAC company capable of addressing MUA unless you want to engineer it yourself.

kas

Cooksnews,

In a cold climate it is actually SMARTER to deliver makeup air near the vent. Think about it -- if you deliver makeup air at the other end of the house then your whole house gets cold unless you have heated makeup air.

Heated makeup air is the obvious solution but it is very pricey to provide 1400 CFM of heated makeup air.

The less obvious solution that I referred to is to provide makeup air around the periphery of the hood so it doesn't fill the room with cold air -- instead the makeup air goes right back up the hood instead of being distributed throughout the room before going up the hood. This design also provides an air curtain effect for smoke containment if it is designed properly.

All of which is why they use it in commercial kitchens where high CFM is standard operation. I hope this explanation helps.

Billy

Just to follow-up....

Providing heated makeup air for a 1400 CFM hood in a very cold climate could easily cost more that $10-15K, more than the cost of the stove and hood. Therefore it worthwhile considering whether it is possible to use a different approach and whether the different approach will affect the performance of the hood and comfort in the house.

Think about it -- we are putting a commercial-type appliance in our residences but we have lagged in providing comercial-type ventilation. Why not take a look at the solutions that have been developed for commercial kitchens? The obvious answers are lack of education and the fact these companies would sell a lot fewer appliances if they "scared" homeowners with the makeup air issues.

Check out the excellent link and explanation below.

Billy

Here is a link that might be useful: Commercial Hoods and Makeup Air Explanation

Thanks again everyone.....all of this makes sense.

I have to admit, it has "scared" me a bit. I can't find a single thing about cfm and make up air requirements/code on the Douglas County, CO website but did email the county. Many other cities have adopted IRC/IMC 2006-9 codes requiring make up air systems for anything over 600 cfm. I am going to assume my county is the same.

With 60 or even 48 range, the minimum is 1200 cfm making the make up air system a requirement. Will look into commercial systems by contacting a few HVAC companies (extra cost during build). I really don't want to give up on the range.

Cheers,

M

Make-up air is intended to replace the effluent laden air that one is trying to capture and contain that is rising from the pans on the cooktop. One cannot simultaneously capture and contain the rising effluent and capture and contain the make up air because, in that case, the MUA wouldn't be making up for anything.

When MUA supply is part of the hood system design, the MUA still has to be directed to flow into the area of the cooktop surface. (Preferably it should flow in a way that doesn't disturb the rising effluent.) Otherwise, some effluent will spill from the hood as it is displaced by injected MUA. This has been illustrated in Schlieren photographs of attempts to do this that were unsuccessful.

One successful approach for which I've seen confirmed by Schlieren photos was a wall application where the MUA was injected down the back of the stove so that it could rise up around the hot stove from the floor level. This may be more useful in a commercial setting where the kitchen is very hot already and heating is not needed (although cooling is needed in the summer). Residential "cooks" may have something to say about cold air at foot level.

When hoods are tested, typically the MUA is injected into the test kitchen via diffuser panels in a distant wall to avoid any disturbance that would influence the test results. I have seen designs for commercial kitchens in which the MUA is injected via perforations in the ceiling.

My MUA is injected via a 3 x 3 foot register into a hallway that abuts the kitchen. My goal is to keep the air quasi-laminar as it enters the kitchen. Means of injecting the air below the cooktop would not have been architecturally convenient, nor would an MUA curtain around the island hood.

MUA heating and cooling is one of the largest uses of electricity that restaurants have to pay for. However, in a home setting, it is possible to heat MUA at reasonable cost. Heating my MUA at, say, the maximum possible flow rate of 2000 cfm, shouldn't cost more than a gallon or so of fuel oil per hour. How much MUA would we use in a year that was equivalent to running two kitchen exhaust fans at full power? Probably not more than 100 hours equivalent. The oil cost would be far below the imputed interest on the kitchen reno cost.

However, cooling the MUA, if necessary in other locales, would be an additional cost.

kas

The commercial hood link provided above by billy_g is a good summary of much of the MUA information I've gleaned since starting my reno in 2007.

kas

Yes...it is a great resource. I am building new so all of this can be addressed upfront rather than a retro-fit down the road. I'll just need to budget more to account for it.

Really appreciate the detailed responses and links.

Cheers,

M

We have a 1,500 cfm heated MUA system, here in Minnesota. The MUA supply is in the ceiling, right by the stove. The air is heated by an electric duct heater, with a three posistion switch: off, low, high heat, we can also control the fan with a speed control. Parts + installation was about $4,500.

IMO, if you can't afford the MUA - then you can't afford a high end range!

Doug,

I never stated I could not afford it. I am trying to educate myself on the concept, requirements and solution options. It is helpful to have a basic understanding prior to discussing with the architect, builder and HVAC sub. I appreciate all of the helpful and detailed responses from the GW community as well as reference links to further my research.

The home is $2M with appliances currently at $50K. Another $10-15K for MUA isn't going to blow the budget. With the right knowledge and solution, I may even be able to add a second range for catering staff.

Cheers.

M

Great that money is no object for you. For those that do have to live with a more modest budget, here's another thought. I just skipped the make-up air altogether and got a modest CFM hood. I have an old house that's not air tight and that just seemed like the dumbest thing to spend money on. That was six years ago and I'm so happy I made that decision. I absolutely detest the fan sound of the hood and use it as little as possible.

The paper at this link provided by another member in a different thread may be a helpful summary.

kas

Here is a link that might be useful: CA energy board paper

Sorry, but the above message got pasted to the wrong thread.

kas

Hi,

I had been planning to install a barometric device (such as the Broan bd4 or bd6 in my furnace room. Now I am a bit worried by kaseki's figures: if it only takes a few hundredths of an inch of negative pressure to present a backflow concern, then it seems a barometric device is useless. At least the Broan ones. They take tenths of an inch of negative pressure to allow MUA flows in the range of hundreds of CFM according to the spec sheet linked below. Does this seem right to you experts?

(Just a few facts about my situation in case it affects your responses: I am putting in a 625 CFM hood. The house is from 1930, although I have endeavored to tighten it up as much as possible. I have steam heat, so no air registers. I have a direct-vent tankless water heater, but a conventional furnace (boiler) vented through a flue and using room air for combustion.)

Any thoughts on my MUA strategy?

Thanks! A_D

Here is a link that might be useful: Broan bd6 spec sheet

I know that I could backflow my oil burner furnace at -0.08 inches. As a result I had a separate MUA source installed for the furnace. Now, even with ducting from outside to the oil blower assembly, there is still some slight leakage at that pressure level, probably from around the pressure balancing damper. Note: all measurements taken with a BAPI differential pressure gauge/transceiver measuring between the edge of the kitchen and a reasonably well externally ventilated attic.

Somewhere at home I have a reference that describes the negative pressure limits of different combustion appliances. Sorry I can't provide it now.

One option is to enclose the furnace and heater and supply passive MUA to this enclosure from outside.

Another is to use a differential pressure transceiver such as the BAPI (there are other options) to control a damper.

Honeywell may make more sensitive (smaller differential pressure) barometric controlled dampers.

In any case, the Broan spec sheet appears to me to be informing the viewer of the pressure loss through the assembly as a function of the flow rate through it, not the pressure drop that causes it to open. Most residential furnaces do not pull high cfm. If this were used to feed a furnace as a closed assembly, the furnace would still work, it would just need to be recalibrated to achieve the desired mixture ratio (soot setting). Weil-McLean and other manufacturers should be able to supply suitable mating assemblies for their oil burners. Your oil delivery company may be able to install them.

In other words, if the only cfm being made up is the furnace, than a device like the Broan should work fine if ducted to the furnace, or fed to a closed room. The problem is that if this device is intended to supply all the cfm being pulled in the kitchen, then indeed the pressure drop will be high and the furnace will still backflow. A much bigger passive assembly would be needed for the higher cfm rates of kitchen ventilation systems.

kas

Billy said "The less obvious solution that I referred to is to provide makeup air around the periphery of the hood"

The California Energy Commission paper referred to above disagrees: "Do not use short-circuit hoods ... Use caution with air-curtain designs."

Kas: thank you for your thoughtful and insightful comments. I appreciate your taking the time to write that up. It will take me a while to digest this, and I may be back with more questions.

In any case, the Broan spec sheet appears to me to be informing the viewer of the pressure loss through the assembly as a function of the flow rate through it, not the pressure drop that causes it to open.

I read this, and said "aaah!" and nodded my head. Then I thought "Wait!" I am not at all convinced these are different things. It seems to me that these are two ways of saying the same thing. I will think about this....

If you think about the damper above a vent hood, it will usually open (against gravity and any slight springs it has) with the fan control on low. The flow needed is usually small, or more precisely, the pressure drop needed for opening is small.

Another way of looking at it (for hood dampers) is that two metal pieces with at total area of pi D^2 / 4 have to be lifted by differential pressure. Let diameter D be six inches and the weight of the metal be one ounce (it is probably less). (After the damper cracks open, progressively less differential pressure is needed as the weight is transfered to the hinge. Very little pressure drop from the damper parts results when it is fully open)

So, in this example we have 1/16 pound over about 28 square inches. This requires 0.0022 psi to move. Sea level pressure of 14.7 psi is about 34 feet of fresh water column (IIRC), or 408 inches. So, 14.7 psi = 408 inches. This is 28 inches per psi. Thus, 0.0022 psi is equivalent to 0.061 inches (if I didn't make any mistakes).

A vertical damper doesn't work against gravity, but is either motorized to work with the fan it is opening for, or works against whatever spring force is set for it. I don't know what Broan has their 6-inch damper set for.

kas

Peter,

I was the person who posted the California Energy Commission report. It has far better scientific advice than anecdotes from individuals (although we like that too!) My comment about the short circuit hood was based on an anecdote from someone on Gardenweb who had a short-circuit hood built by a commercial hood manufacturer and said it worked well. I also read the report and saw they do not recommend it as the only makeup air source, although it is OK to use for a small percentage of the makeup air (but I'm not sure that would be worth it). So thanks for pointing this out.

The point is the closer the makeup air is to the vent hood, the less effect it will have on the conditioned air in the entire house. But as the California report states there are problems when the makeup air source is too close to the hood and interferes with the airflow around the hood.

I like their suggestions of ways to reduce the hood CFM and thereby reduce the makeup air requirements. Some of these strategies are extending the hood overhang, lowering the hood, including side-shields, and building the hood into an alcove with side walls.

I also like the makeup air strategy of releasing the makeup air below the cooktop. Perhaps this could be done through cooktop registers or probably something larger? This means less unconditioned air throughout the house but it would mean the chef gets blasted with unconditioned air.

There are many tradeoffs here.

Billy

Above I meant "toekick registers," not "cooktop registers."
Billy

countertop gaps would make it an air curtain.

60 inch range in a house...for what. How could one ever use that many burners or an oven that big? I thought 48 inches was big.

We have a 48 inch cooktop and if I had my way it would have been 60... a 24 inch griddle would be great with 6 burners or with 4 burners and a 12 inch grill.

Billy

There is someone who has commented on this forum about his floor level discharge MUA system. It is ducted through his basement and, IIRC, has electric heating and motor boost. I don't recall how its flow was controlled. He provided a photo.

kas

For depressurization standards: Go to the site below and click on the "Building Analyst Professional" pdf link. Go to page 14 of the document and recall that 1 pascal is about 0.004 inches of water. Note that there is a big difference in allowed negative pressure between a natural draft water heater and an oil burning furnace with its own draft fan.

kas

Here is a link that might be useful: BPI Approved Technical Standards

Hi we are just in the finishing stages of our new home build in Northern California - we are using the Broan BD6 damper as a 6" passive air inlet for our MUA source. The damper will automatically open when any of our bath and laundry exhaust fans are on. In addition, we are wiring the chimney hood with a current sensing relay to open the same damper when the island hood fan is turned on.

Our solution to make up air "conditioning" is to run the air intake to the back of and top of the refrigerator cabinets. The built in style of freezer and refrigerator have a top grill for air flow, and we have the MAU inlet piped into the back of this area. We are in a very temperate climate, so we don't have to worry about below freezing air often or ever...so it is a good solution for us.

We were required to have automated MAU, or a HRV, for our bath fans anyway...and we just piggybacked in our 600cfm kitchen hood to it.

We aren't living there yet but it should work as planned.

montel:

It may be worthwhile to measure the achieved negative pressure when all fans are running. Depending on your combustion appliances (if any) you may be ok or not. If you don't have any combustion appliances then your MUA will help your kitchen hood flow, even if it doesn't get the kitchen to ambient pressure.

kas

The air is heated by an electric duct heater, with a three posistion switch: off, low, high heat, we can also control the fan with a speed control. Parts + installation was about $4,500.

Doug - Am I correct in interpreting this to mean you need to manually control both the heat and the airflow? If so, how do you know how high to turn the fan? I have absolutely no basis for comparison, but $4500 sounds like a ton of money for a system that doesn't automatically regulate air pressure.

It may be worthwhile to measure the achieved negative pressure when all fans are running. Depending on your combustion appliances (if any) you may be ok or not. If you don't have any combustion appliances then your MUA will help your kitchen hood flow, even if it doesn't get the kitchen to ambient pressure.

kas

In our case we have no combustion appliances in our home - so we are fine with all of our MUA requirements - at least according to our building department and the inspections we have had. To many of the trade workers this stuff is so new that many don't know all the ins and outs of the requirements yet.

The 2009 code in many cold weather states require MUA systems for over 400 CFM
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."

"Makeup air required: Exhaust hood systems capable of exhausting in excess of 400 cfm (0.19 m3/s) 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."

A2 is strictly enforcing this. I decided to go induction and stay under the 400 CFM. I was leaning toward induction and this just tipped the scale. I am probably going with a Broan evolution hood @ 350 CFM and 4 halogen bulbs using theater lighting. The zephyr genova is very discreet, but concerned that the single fan on the left might not do the trick (only 250 cfm). The ventahoods have a true HEPA filters. I figure A2 is pushing me into the future. If you go too high on CFM, there are dangers, so I plan to avoid the problems! Good luck!!

PS - I plucked the Wolf induction on sale. Love the appearance of the surface and had no trouble with the controls - I used my knuckle to tap the power up and down when my fingers were slimed! I liked the large burner and the set up of the burners and only one cook as my husband only knows how to order pizza and not cook. I only tested it at the appliance dealer, so can't wait to the kitchen is done and I actually can really test it.

I wonder how the word "capable" is interpreted in these jurisdictions. It is very unlikely that a 400 cfm hood would actually move air at that flow rate in any real duct system, and certainly couldn't if the household pressure were reduced due to lack of MUA.

I have to guess that the value was somewhat arbitrarily set using assumptions about house leakage and fan curves.

Otherwise, one would hope for a requirement that was relevant to the actual issue -- one that specified the maximum allowed house negative pressure when all exhaust fans were on full power, with a value that depended on what combustion appliances were present. This would require the building inspectors to carry a manometer along with their hot water thermometer and GFI tester.

kas

It also seems interesting to me that a 399 cfm fan needs no MUA but a 400cfm fan requires 400cfm of makeup air. It would have made more sense to me if the MUA required was that in excess of 400.

Kaseki - you are right that the rule is subject to interpretation and most systems don't move the amount of air stated. Unfortunately, our local inspectors are holding the line based on the specifications of the unit.
weedmeister - Great idea! Now how can we get that rule implemented!!
I plan on purchasing either the Zephyr Genova (so cute but only 290) or the Broan Evolution 2 or 3 (3 does boost over 400, so might have to get the 2 @350 max). The theater lighting is quite nice and I think it is quieter than most other hoods (except those with external fans)