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ventilation question - can I go with fewer CFM's?

Posted by modern_mom35 (My Page) on
Tue, Apr 17, 12 at 11:33

we are putting a Wolf 48" with double griddle into our new kitchen remodel. it has the semi-open burners. total BTU if I'm calculating correctly is 82,000. I have read lots of good info on GW about proper ventilation and talked to a couple of HVAC guys about it...and I am still really confused.

what I'm looking for advice on is straight up safety issues. while it may sound weird, we'e really not that concerned about smell issues. we have a Jenn Air downdraft right now, cook a lot, and almost never use the vent. our new kitchen has lots of windows and we can easily crack one if something smokes a lot.

I understand the safety issues involved when you get a high CFM hood (and have a fireplace, etc..) re: possible downdrafting. I'm wondering if there are carbon monoxide issues or any other safety issues involved if you just use a smaller blower than recommended??

in my state (MN), you are required to put in a make-up air unit for any hood above 300 CFM. and MUA unit must be heated. we have priced this out with two different HVAC guys and it comes in around $6-7K. while we are willing to spend the money, we are trying to figure out if that is absolutely necessary. more than one contractor we have bid the project out to has suggested using a 300 CFM hood to get around the heated MUA issue. what I can't figure out is if this would be safe.

any input would be much appreciated.


Follow-Up Postings:

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RE: ventilation question - can I go with fewer CFM's?

No safety issues. I'm thinking if there were, recircs would be outlawed and there'd be much stricter requirements and guidelines in the code book. You should definitely have functioning CO alarms but that should go without saying, even if you have an electric range.


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RE: ventilation question - can I go with fewer CFM's?

It's not safe, and it's necessary. Your range is capable of puting out a lot of heat. 300 CFM won't handle the smoke from grilling or the broiler.

I don't think you'd buy a car with an underpowered engine? Why buy a supercharged range then skimp on the accessories?

Do the right thing and get the MUA package. Think about the next people who buy your home.

I live in Mpls, and I have MUA.


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RE: ventilation question - can I go with fewer CFM's?

thank you for the feedback. I really appreciate it.

does it make a difference that we don't have the grill/charbroiler? just the double griddle. also (I should have been more specific) this is a range top, not a range, so no broiler. ovens are separate.


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RE: ventilation question - can I go with fewer CFM's?

Since you got two conflicting comments, I'd add my own. :) Strictly speaking, there aren't any safety issues as long as you are providing sufficient secondary ventilation. A correctly functional range shouldn't be producing excess carbon monoxide (CO) although it will produce carbon dioxide (CO2). CO is poisonous and is dangerous in much smaller quantities than CO2. In order for CO2 to become dangerous, it would have to displace a significant amount of air in the house which is hard to do if you have a window open. I had doctor friend once describe the differences this way. Carbon monoxide poisoning is so dangerous because it accumulates in your blood and blocks the ability for it to carry oxygen thus suffocating you from inside. Your body doesn't even know it's happening since your lungs are still taking in the right amount of oxygen. On the other hand, carbon dioxide is what triggers the autonomous breathing so as levels increase, you will feel increasingly short of breath. Typically, even sleeping people will typically wake up feeling short of breath if CO2 levels got too high.

So, it goes back to the type of cooking you do. Although it's not strictly a safety issue, if you do a lot of stir frying or sauteing, then you might end up with greasy cabinets after a few years not to mention unpleasant smells in your house. Even if all you do is boil pasta and make soup, the moisture can be an issue as well since it can lead to mold and other unpleasantness. Also, consider the resale of you house if you ever decide to sell.

You might consider an all in one unit that I've seen in commercial kitchens where MUA is built in to the hood. These are sometimes called compensating hoods. Take a look and talk to the city. There may be exceptions made for compensating ventilation systems with an interlocking MUA that will satisfy building code without a more expensive whole house heated MUA system.

Good luck on your decision.


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RE: ventilation question - can I go with fewer CFM's?

The rule of thumb is to divide the BTU by 100. In your case it's 800 cfm. If you were to buy the proper size exhauster, you would need MUA. It is possible to 'pull combustion gases' from fire places, water heaters, boilers, etc. Even with a window open.

IMO 300 CFM isn't enought for a 48" top. I don't get the high end kitchen - but forget the details mentality. Do it right and you'll have something to be proud of. The contractor who suggested you put in 300 cfm is a cheat - I wouldn't use him!


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RE: ventilation question - can I go with fewer CFM's?

Last I checked, the official recommendation is 100 CFM per linear foot of cooking surface. Yes, per Linear Foot. Has nothing to do with the strength of the burners. I believe that's a recommendation, not a requirement. If it is a requirement, then obviously 300 CFM won't meet it in this case. If it is a requirement and a recirc hood could pass inspection, then I'd say it's somewhat of a BS requirement that has nothing really to do with safety.

modern_mom brings up an excellent point re wall ovens. I haven't seen dedicated exhaust for them (and their broilers) in years. Unsafe? Some people prefer range broilers so that the smoke gets evacuated by the hood, but that's a quality of life factor not a safety issue.

Personally I'd want more than 300 CFM over a 48" range, particularly if I plan to use that griddle for something more exciting than scrambled eggs and pancakes. But I don't think we're talking a safety issue, and that was the question.


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RE: ventilation question - can I go with fewer CFM's?

You don't buy a Ferrari and them put on four 10" donut spares on it. Not only does that look really funny, but it greatly compromises the performance of the car itself. You can't go 120 mph on donuts, and you can't have 3 burners and a griddle putting out heat and steam and have a 300 CFM fan keep up anywhere close. It not only compromises your ability of to be able to use your range to it's fullest, but it also will make potential buyers look much closer for other items you might have compromised on in the home. It raises a degree of suspicion that shouldn't be raised.

If you can't put in a proper ventilation system, including proper makeup air, then put in a consumer grade 30" range. It's that simple.


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RE: ventilation question - can I go with fewer CFM's?

Kitchens require 15 Air Changes per Hour (ACH) in order to be compliant with the Home Ventilating Institute's building code requirements.

Compute the volume of your kitchen and divide by 15 - that gives the minimum exchange / hr. A 20x15x8 kitchen / 15 = 160 CFM as a starting point.

Add 100 cfm for every 10,000 BTU.

So 160 + 800 is about 900 - 1000 CFM.

Read more: How to Calculate CFM for Range Hoods : eHow.com http://www.ehow.com/how_4841639_calculate-cfm-range-hoods.html#ixzz1sKTlgqyG


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RE: ventilation question - can I go with fewer CFM's?

I live much further north than MN, and there is NO WAY that I could have installed any gas range without adequate ventilation. And the code guys here look to the manufacturer's installation guides to define "adequate". Modern home construction technology requires tight shells to improve energy efficiency. Tighter construction requires planned ventilation to prevent the buildup of indoor pollutants, which can cause mold growth and other health hazards. Cooking is one of the major generators of humidity and airborne particulates in any home.

While older homes were not built to the same standards as new ones are, many undergo heating/cooling retrofits so that they are much more efficient than they were originally. In any case, major renovations must conform to current building codes.

So, do the right thing, and install appropriate ventilation.


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RE: ventilation question - can I go with fewer CFM's?

doug_gb, I cannot follow your arithmetic. If you want 15 exchanges per hour, you need to multiply the volume of the room by 15, not divide. Then you need to divide the result by 60 minutes/hr to get CFM/minute. A 20x15x8 kitchen would need 600CFM to get 15 exchanges per hour.

In your referenced link, the calculation for additional "CFM" for "static air" seems totally out to lunch, as it makes no reference to duct size. If one really cares about that, an .xls file which calculates static pressure drops as a function of diameter, bends, duct type, length and flow can be found online. The specification for the blower has a curve of output vs. static pressure, which can be used with the .xls to predict actual flow. There will be additional static pressure drops from transitions, roof/wall caps, baffle/mesh filters. If the system doesn't have make-up air, and have a tight building and large blower, there will be static pressure drops also on the intake side.

Here is a link that might be useful: air duct static pressure


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RE: ventilation question - can I go with fewer CFM's?

all really good perspectives. thank you so much for the info. I really appreciate your detailed responses!


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RE: ventilation question - can I go with fewer CFM's?

Let's also not forget that HVI is a trade organization whose interest it is to promote ventilation. To my knowledge they don't dictate code, and code could care less if a range hood's CFM is certified by them.

I'm not trying to downplay the importance of good ventilation. But if you want to hold gospel (manufacturer suggested) guidelines like 100 CFM per 10K BTU, then I'll challenge you to find even the cheapest builder grade range that won't require more ventilation than can be had in MN without MUA. (I'll save you some googling - GE's cheapest range is 36400 BTU so would require 364 CFM.)


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RE: ventilation question - can I go with fewer CFM's?

Governments, boy those guys. A nice 300cfm hood and a 300cfm Panasonic bathroom exhaust fan centered in the kitchen ceiling space offer added flexibility you can tailor to your real world lifestyle. Alternatively a Miele 36" induction cooktop will not need as much ventilation- no added heat source to remove.


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RE: ventilation question - can I go with fewer CFM's?

Broan has a residential MUA Damper available, for which I congratulate them. They also sell vent hoods.

Here is a link that might be useful: 6


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RE: ventilation question - can I go with fewer CFM's?

There are many rules of thumb for cfm, even linear dimensions of the hood edges excluding walls. One can also calculate on the basis of effluent velocity and hood aperture. (I've covered that somewhere.) But all of these will, as pointed out above, likely end up being well over 300 cfm for any hood that would work with a 48 inch cooktop.

Most of us would agree that if financially feasible, a properly heated, pressurized, balanced MUA system is needed, and, (in my view) the affordability of a 48-inch Wolf range should include the MUA cost in the project. However, there may be ways of circumventing some of the costs.

One way (won't work for fireplaces) is to have any other combustion appliances (furnaces and hot water heaters and gas dryers) in a room that is isolated from the kitchen (air flow wise) with its own not necessarily well heated minimal MUA (after all, the flow rate for these appliances will be small). Then the safety issue is accounted for, even if the inspector won't care.

Then a passive MUA design (a damper in a duct) passing outside air to a heater would be sufficient to pull in sufficient replacement air. The kitchen pressure would be low (lower if filtering is used), the hood flow would not be what one might expect, but the code would be met and safety achieved.

A 10-inch duct with pressure sensitive damper flowing air (due to negative house pressure) past a heater of some type (think 100 kBTU/hr order of magnitude) would be the type of heater required, maybe larger in MN. You would save the cost of the big blower needed for pressure balanced MUA, and the cost of a proper pressure control system and its tuning.

kas


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RE: ventilation question - can I go with fewer CFM's?

wow -- I am overwhelmed by all these detailed responses (in a good way). thanks so much for taking the time. you have all given us a lot to think about.


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RE: ventilation question - can I go with fewer CFM's?

I'm in the process of going through the install of a Bluestar in MN so let me tell you what I've learned about Minnesota's Byzantine MUA codes. (Take this all with a grain of salt as I haven't been inspected yet. I'm going off what my HVAC guy is telling me.)

1. Yes they limit to 300 CFM, but the give you another 300 CFM for free so the limit is really 600CFM.

2. You don't need any make up air IF you have no gas appliances from INSIDE the house. So it might be cheaper to get a new furnace, hot water heater, and a dryer than it is to get heated a MUA unit. Especially if you might need them anyway. ( I priced out MUA, and if I remember right it was at $7K per 650 CFM. There's a place in Burnsville you can get them from. I can look that up if you want.)

In the end I went with a Vent-A-Hood 600 CFM unit. Vent-A-Hood "claims" they are as good everyone else's 900 CFM units. (Let the Vent-A-Hood bashing begin.)

Minnesota's codes on this are horrible... You can't just call City Hall and ask if what will work. (I have a funny story about calling the Saint Paul inspector about that.)

JP


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RE: ventilation question - can I go with fewer CFM's?

I would really like the focus to be on the shape of the canopy that captures the rising air (steam + smoke).

I would feel comfortable with a lower CFM than the number proposed by the average post in this forum, or any industry average.


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RE: ventilation question - can I go with fewer CFM's?

So, David-- what is the most (and least) efficient shape?


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RE: ventilation question - can I go with fewer CFM's?

The most efficient shape is a large volume overhead canopy that overlaps the edges of your cooking surface by at least 3" per side. The overlap serves to "catch" the cooking smoke and grease that otherwise will "plume" outward as it rises. A large canopy catches the effluent better and the idea is to have a big enough blower to be able to evacuate the capture area faster than your cooking skills can fill it.

GW member clinresga posted a picture of what I would consider to be at or near the ideal configuration for optimal ventilation in a residential kitchen.

Photobucket

It manages to look great while being superbly capable of thorough ventilation. Of course, we do not know anything about the guts of the system pictured, but assuming they did the blower and ductwork as well as they did the hood setup my guess is it is near flawless.


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RE: ventilation question - can I go with fewer CFM's?

Clinresga reported on the components at some point, 2009 maybe, but I don't have time right now to search the data base. IIRC, he used mostly Fantech components, other than the hood itself.

kas


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RE: ventilation question - can I go with fewer CFM's?

IMO, when you buy a "professional range / cooktop" it gives you the POSSIBILITY to be a better cook / more eventuring person. So you can buy it to look nice, and cook ramen, Kraft dinners, canned soup, or you can look outside yourself for more creative ideas.

There are folks here who say "well if it's electric or mag, then you don't need a hood at all!" Others claim a bathroom fan can act as another source of exhaust.

Heat is heat, and grease is grease. If you would like to stir fry a pound of meat, in a very hot wok, with a 300 cfm fan - no matter what the heat source - good luck.

The second item are the posts from the people from Minnesota. I'm from the east coast, but I've lived in Minneapolis for 30 yrs. From personal experience (as an engineer - not a civil engineer) I have observed that the quality and upkeep of homes is nowhere near other metropolitan areas in which I've lived. IMO, many things are shortcutted in buildings and homes. A lot of these new homes I call "sheet rock mansions." Every corner is cut with the cheapest materials and craftmanship.

This post is not intended to be an attack on anyone in this thread. Just my general observations.


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