sizing a hood

A good source for wisdom on these question is the appliance forum at gardenweb

 

http://ths.gardenweb.com/forums/appl/

 

look in particular for Kaseki's contributions e.g. in

 

http://ths.gardenweb.com/forums/load/appl/msg0820244318868.html

 

Bottom line: think of a frying pan or wok on your stove putting out smoke and grease and whatnot.  Call that "effluent." 

 

As your column of effluent rises, it expands sideways -- an upside-down cone.  Kaseki figures that the cone's overall angle at 40-50 degrees, so 20-25 degrees as a half angle.

 

I found it helpful to get out my graph paper and make drawings.  Start with how high above the surface of the stove the hood is going to be.  Something like this http://www.csgnetwork.com/righttricalc.html will handle the trigonometry.  For example if your hood is 30" above the cooktop, and you figure conservatively on a 25 degree half-angle for the effluent cone, the answer will come back that the effluent column will be about 14" wider at the top than the bottom.  Fortunately the effluent cone is not starting right from the edge of the stove, and this is where a drawing is helpful, depending on the actual configuration of your stove.

 

To be effective, the hood canopy needs to capture the rising effluent column.  For most installations this will mean at least a 6" overlap on the sides, and you probably want a hood that is deeper than the stove (you can also bump it out a little from the wall).  You will also notice that most residential installations have ineffective hoods.

Bigger hoods can operate efficiently with lower fan speeds which results in less noise.

 

Personally, whether for home or business, I'd prefer a hood to be too large over one too small.

 

If my stovetop was 36" wide and 30" deep, I'd want a hood that was a minimum of 48" wide and 36" deep.

You've probably already got this set, but wanted to add my experience here.

 

We've got a similar setup - Wolf gas range and ventahood.  Went over size by 6" on both width and depth for the hood, went overpowered by a bit (don't remember CFM), and bumped the hood out from the wall a few inches as well to bring it farther forward.

 

Ventahood's generally specify an insanely low mounting height of 24" above the counter or something like that.  I'm quite tall and that really wasn't going to work. 

 

With the extra power, width and depth I mounted it 40" above the counter and I still get very good smoke and fume collection from it (as long as I keep it clean).  It's also got a wok burner under it and I've used that to burn off oil under the hood without it escaping into the rest of the house.  All that and I don't bash my head reaching for the back burners.

 

As usual, your mileage may vary.

 

 

For most home applications you do not need, or probably even want a hood that's larger than your range with a 36" or smaller unit. That can change significantly with larger ranges. Don't get hoodwinked (pun intended) by branding claims. 600 CFM is just that. Larger hoods in home applications usually require blowers with more CFM. If you want to keep the noise down you can use a blower system that's not mounted in the hood itself (depending on brand). If you have doubts you may want to consult a qualified installer before you purchase. I'd suggest some one that does NOT work for who ever you are buying the appliances from. Remember that when you calculate your CFM requirement you loose a significant amount of air flow for every turn your venting takes as well as the length of the run.  That's a far bigger factor and it gets over looked frequently. There is an industry standard of CFM loss for each for 90 degree turn and each foot of exhaust length etc. Island cooktops like the person quoted by the OP can be a very different beast. It's probably a good idea to keep things in perspective and think about how a larger hood is going to look in your home. I'm regularly in homes that sell from 1-10 Million and even at those price points it's rare to see a range with a hood larger than the cooking surface. Those that do have larger hoods are almost always installed with 48-60" ranges set into an alcove or an island cooktop.

Either way enjoy the new kitchen!

 

Dave

Edited by DuckFat - 3/22/12 at 1:18pm

Duckfat, the trophy kitchens in multimillion $ houses are lousy guides to how to do hoods -- they're usually made to suit the eye of a kitchen designer, not for functionality.  I've seen expensive kitchens with hoods set *back* a foot from the front of the stove.  I've seen highly impractical hood designs used just because they look cool.

 

Rising effluent is rising effluent, and columns of rising effluent expand.  No matter how much money you spend on your house, you can't buy a different physics.  

FWIW, for maximum capture of vapors and smoke, assuming no side walls, the hood should extend 1/3 of the height of the hood from the cooking surface, i.e., hood edge is 36" above the cooking surface and the cooking surface is 36"x30", the bottom of the hood opening should be 60"x54". That would be for an island installation.

 

If the bottom of the hood is y" above the cooking surface and x" is the amount of overhang of the hood, x"=y"/3 is a good rule to follow.

I put a 42" hood over a 36" rangetop. It's as deep as the rangetop. Still get a little smoke that will escape when using the front burners. I have a 600 cfm motor

It's important to get the hood going before you start cooking to get the air flowing

Colin;

From my perspective those homes usually have far more engineering put into them because most actually had.... an engineer and a designer involved instead of just a salesperson and builder trying to maximize profit. If you want a larger hood on a 30-36" range go for it and enjoy!  Will it work more effeciently assuming the run isn't full of turns and installed properly? I'm sure it will, However it's hardly necessary. I'd venture a fair guess that I've cooked on more brands with different venting systems in various homes than the average bear and I've yet to see a 36" hood over a 36" range be a problem. I will say I have never, ever seen a hood set back a foot from the front burner. If that happened it would be a moot point if you had a 60" hood on a 30" range. Improper installations really don't help with any comparison.  A 6" set back from the front of the stove with some brands is fairly common but also irrelevant as the burners are set back due to the design of most ranges. 

The residential hood systems we're talking about are already waaaay ahead of the curve in performance, especially when you consider the norm is now a combination microwave/vent in the vast majority of new homes. 

Lets not forget it's a home, Not a professional kitchen. The range is not going to be running full tilt 12 hours a day, 7 days a week. As long as it is removing food odor and grease particles it is doing the job. To give you some idea how carried away this can get what I would suggest is taking any item that's smoking or steaming in order to give you a visual aid. Turn on a 30-36" hood and hold the smoking item at burner level. Now back it up at that level 12" out side the hood. Even a full foot away with a 600CFM unit on a properly installed run it will draw the vast majority into the exhaust. The real problem is that the exhaust run of so many hoods gets installed improperly. Poor draw, Poor exhaust, No matter how big the hood is.

Most manufacturers build ranges and hoods so there is a reason they match 36" ranges with 36" hoods and it's not because they want to save consumers $$$ by selling a smaller unit.  I think the best bet is to try to find an installed unit and go see it in action before creating a home kitchen with an over sized hood unless you just want a larger hood. IMO any one building a new kitchen or going through this process would be well served by consulting a professional installer not associated with the store you are buying from before placing your order.

 

 

 

Dave

 

Edited by DuckFat - 3/24/12 at 1:38pm

An extraction system has two parts.

 

1. The canopy captures the effluent column as it rises.  Effluent columns expand, and the canopy, to do its job, needs to be roughly as large as the expanded effluent column when it reaches the canopy.

 

2. Once the effluent gets into the canopy, the fan needs to make enough air flow to vent it out.

 

So you need *both*.  If you have 1 without 2, the effluent gets into the canopy, but then spills out again.  If you have 2 without 1, much of the effluent never gets into the hood and just mixes with the rest of the air in the kitchen, and then your fan is effective only to the degree that it is exhausting air from the kitchen in general.

 

The error a lot of people fall into is fixating on cfm alone, and imagining that a powerful fan can somehow reach down to the pan and tell the effluent where to go.  It can't.

 

Duckfat (or Dave?) claims: "Turn on a 30-36" hood and hold the smoking item at burner level. Now back it up at that level 12" out side the hood. Even a full foot away with a 600CFM unit on a properly installed run it will draw the vast majority into the exhaust."   I invite him so demonstrate this, in a residential kitchen, and post the video.  Preferably, using something that puts out effluent at the rate of a hot wok.

 

--

 

2 additional points, having just been through some of this with a kitchen remodel myself.  One is that neither kitchen designers nor salesfolk are much use on these issues.  It's even hard finding an HVAC person with residential kitchen experience.  If you want to test someone, ask them about makeup air requirements (after reading up on the subject yourself) and see if they give you an honest or even coherent answer.  Salesfolk in particular have no incentive to point out that buying a cool-looking high-BTU range may get you into thousands of dollars of *additional* expense and require an unsightly hood -- especially since many of their marks are buying for bling rather than function.  Two, overall venting needs depend a lot on how you cook.  If you sear meat, if you do wok cooking, if like me, you burn stuff on a regular basis and your loved ones don't like smoke, then you want effective venting and that requires a big hood.  You can't fight the physics.  If your cooking is less smelly, you can get by with less.

Colin do you have a wall range in Your kitchen with a hood larger than the range? If so did you try what I suggested?

I think we agree on the majority of points but I'm just not on board with the notion that a 30" hood over a 30" range is insufficient, or 36" over 36". In my own kitchen I've had a Viking with the same size hood for the last 12 years.

I see dozens of homes every year with Island cooktops that either have no hood at all or downdrafts, which are dismal in comparison. These family's still manage to cook like most of us. Not every one is insisting on searing or Wok cooking on the front burner. If I was that serious about Wok cooking I'd consider one of the Euro ranges with the center Wok or larger burner. That would simplify the issue for dedicated wok cook venting considerably.

If we were talking about Island hoods I'd agree with you all day. (Actually I did)

Either way if you would like to PM me and talk about mutually making a video I'm open to that. I've never done a video but I'm willing to give it a shot if you are but lets talk and coordinate the paramiters so we both put up a video that can be used as a valid comparison.

 

Dave

I'm not sure what happened to Colin but I did have the chance to go to the other site he mentioned. I have been a member of that forum several years however I rarely post there. In either event instead of wasting time showing that a hood can indeed draw from outside the physical dimensions of the hood I thought I'd post a link to a thread on the same forum talking about hoods drawing fumes across the kitchen. I would note that even the person Collin was quoting did weigh in so this may be of some interest.

The one piece of advice I will always maintain is talk to a qualified installer that can physically look at your home and where you want to place your hood before you buy. All too often I see some one post that they have spent thousands of dollars on a piece of equipment that can not be returned only to discover that it won't work in their home. While those of us who have been through an installation like this can offer some advice no one on the Internet can offer absolute advice as every home and every installation is different.

 

Dave

 

 

 

 

 

Thanks Dave,

 

It's definitely worth looking up Kaseki's contributions on the gardenweb appliance board.  

 

Here's a nice video https://www.youtube.com/watch?v=kMw87dfSFYo which I think illustrates both points: one that you need adequate suction out of the hood, as you have rightly pointed out -- you can see the effluent on the left spilling back out of the hood because the fan doesn't suck enough.  But you can also see even the hood on the right, the one that the video is made to advertise, is not wide enough to capture all the effluent even though it has better cfm.  You can quite clearly see how the column widens, and some of it gets past a hood that is the same width as the stove.  If that's greasy smoke, some of it is depositing grease on the ceiling.  This matches my own experience with stoves and hoods.

 

So the hood canopy's job is to grab effluent from the stove before it mingles too much with the rest of the kitchen.  My own observation is that the hotter the effluent -- and this applies especially to anything involving hot fat -- that more vigorously it rises and expands.  As the video shows, you want to be able to deal with volume.

 

Note that the standard pro hood spec is 6" wider http://www.acitydiscount.com/Restaurant-Grease-Heat-Hood-Sizing-Guide.31.1.htm -- and that's with high cfm *and* makeup air systems that do a good job of directing airflow.  Indeed the standard pro hood depth is four feet, though they're generally mounted higher than residential hoods.  

 

I don't think "manage to cook" is the most useful standard.  The kitchen I just renovated was built in 1941 and had *no* ventilation at all.  Not even a wall fan.  Boggles the mind, but clearly the folks who lived there before we bought it managed to cook for 70 years.  People put up with all kinds of things!   We agree that ventilation needs vary -- for example in addition wokery we do a lot of Indian stuff where you're quickly frying up things like finely chopped green chili peppers, and that creates unpleasant fumes.  It's nice to be able to get that out of the house.  I rarely deep fry, but when I do I really want that effluent gone.  So yes, it's important to assess your needs - how smelly and smoky and oily are your fumes, how sensitive the inhabitants of the house are, and probably also where the kitchen is located.  If it's a closed room at the back of the house, you can probably get away with less.  If the house has an open plan and the kitchen is central, as tends to be the fashion now, ventilation that captures stove effluent before it mingles might be more of a priority.

 

 

EVERYONE WORRIES ABOUT VENTELATION AS TAKING OUT HEAT . Worry also about return of fresh air which is just as important. In any event ,to big is better then to small.  Ifyou have a charbroil and a battery of fryers you will need more hoot coverage  then just for a stove and oven . It goes by cubic feet of hot air that can be removed and then air returned, I can tell you it if figured by linier footage is the most expensive part of building a kitchen.(including a fire suppressant unit Ansul) BIG BUCKS

Colin,

With all respect that video is very poorly done. The larger hood on the left doesn't seem to be functioning at all (notice that the video has no sound?) and the smoke is coming out the front and the side. I don't think that really shows us much other than perhaps that's a brand to avoid? Instead of looking at an advertisement with out any idea of the CFM or set up lets get back to my offer to each make a video so we can help others see what the real world difference would be with known variables and maybe both learn in the process. If still photos would be easier perhaps we could do that instead of video?

I do agree that how open your kitchen is will have some impact on what you choose. I just think that balance and design in relation to the size and value of the home need to be compatible with the investment you make in a kitchen. What I would like to see is manufacturers start offering hoods that are standard width (30-36-48) with deeper dimensions as an option. That would be useful to more consumers with ranges than wider hoods IMO.

If Professional hoods are only 6" wider I think that's a fair indicator that most home owners don't need a hood wider than a range. Most of the professional hood systems I have worked with have been so much larger and higher there wouldn't really be any valid comparison for a home owner. I would however like to have the self cleaning hood feature we had at Disney. Push button, louvers come down, hood system powerwashes itself. Very cool.

I also agree the "manage to cook" standard is not the best but every one has different needs and wants. It is surprising how many large homes I enter with an Island cook top and no ventilation at all in a white kitchen. The home owners always seem happy with their kitchen.

Chefedb We are talking about home systems so hopefully not that much deep frying going on inside. When I do deep fry I use a 8Qt heavy stock pot and my hood has no trouble keeping up.

I do not use a Wok or blacken in the house as the BGE works so much better and no ventilation worries.

Makes killer Paella as well!

 

Dave

Dave, I responded to your " "Turn on a 30-36" hood and hold the smoking item at burner level. Now back it up at that level 12" out side the hood. Even a full foot away with a 600CFM unit on a properly installed run it will draw the vast majority into the exhaust."  with the suggestion that you supply evidence.  That's all.  I'm not going to be drawn into a larger project, and I've said all I need to say on this thread.

All you need to test what I posted is a steaming pot and a hood. It's very simple to try.

If you don't physically have one of the hood systems we are discussing and are just working a theory based on what you've read on another forum that's fine. I was just under the impression that you had installed a larger hood over a range in your new kitchen and were speaking from hands on experience.

The project you suggested to provide "evidence" should be of no greater effort for either one of us to supply.

I remain open to the project If you change your mind in the future.

 

Best Regards,

Dave

 

 

Edited by DuckFat - 3/30/12 at 8:28pm