I have been at a complete loss trying to sort out a hood for my upcoming Capital Culinarian 36" rangetop, 6 burner. I have read your posts and many, many others on venting issues over many months and still don't know what I'm going to do. I know I will do a 42" hood. Calculations tell me that I need a lot of cfms, but I'm having trouble finding one with the style I like, the cfms I need, baffle filter, and a good brand name that doesn't cost a fortune. I love the barrel look of that great Modernaire' but it's completely out of my budget.

Insult was added to injury last week when the city inspector told me that MUA is required for any vent over 400cfms. If you believe most of what you read here, 400 cfms isn't even adequate for a tiny electric range let alone a monster gas CC.

I don't have a contractor to sort through this mess for me. I am the GC. I don't even know who can help me figure this out. An appliance salesperson? Do they even know about MUA? An HVAC guy?

I'll watch for the rest of your posts on the subject while my head continues to spin. I know I'll learn something!

Try working through the Greenheck guide using the size of your range and hood. My guess is it will say something around 600-800 cfm.

The guide is for commercial equipment so it assumes commercial overhead type hoods (overhangs the cooking equipment by at least 6'' on front and sides, mounted at least 6' 6'' above floor) and also assumes the equipment is going full blast on all burners and ovens all the time, and also that there's no option of opening a window or door when things get smoky, probably lots of other differences too from a residential situation. But I figure it is probably a good start, and likely more informative than the average appliance salesman.

Boy, I stared and stared at that calculation page, but can't make heads or tails of the math in these wee hours. I'll try to find someone who can do this math for me. I just want a new kitchen. I didn't realize I'd have to learn calculus to get it, ;)

Once you decide on a cfm requirement (I won't address that in this message), you need to determine the pressure losses from the hood transitions in and out, its baffles, the ducting AND the residual negative pressure that you will have in the house with the intended MUA. These pressure losses are then used to look up on the fan curve the actual cfm. For this you need to choose candidate fans and get their fan curves (from Broan's website, Wolf's customer service, Fantech's web site, etc.).

Hood and baffles may be worth a tenth of an inch of water by itself. The house may be a half a tenth negative. The duct losses will vary with length and bends. For example, achieving 1260 cfm will likely require a fan that, at zero static pressure, is rated somewhere between 1500 and 2000 cfm.

As you add MUA (accounting for house leakage when there is residual negative pressure), the pressure loss in the ventilation fan decreases raising the cfm exhausted. This is clearly a recursive analysis unless you separate the functions by establishing a specific household pressure you are aiming for.

At a specific pressure and outward flow rate, and given an estimate or measurement of the house leakage at that pressure, the MUA can be separately engineered. Don't forget the effects of any other household exhaust fans, dryers, furnaces, etc.

As I recall noting before, household MUA is more difficult to engineer than restaurant MUA, and is more like large building MUA where the inlet and outlet flow rates, and even wind flow pressure effects, are always changing.

Likely points of control are the MUA motor or a bypass flow path motorized damper. If the furnace has a separate (passive) MUA and is installed in a sealed room, then a passive MUA system for the rest of the house may be acceptable. Unlike a building, you probably won't have a problem opening a large exterior door against a negative pressure.

kas

Within living memory, humans deemed 120 to 250 CFM to be sufficient. Anthropologists can confirm this. Also, anyone who walks around the aisles in a middle-class kind of big box hardware store can confirm this. Dinosaur products of this ilk are still on the shelves and in the displays. In fact, it has been reported that sales of these products still take place. People caught in the act usually mumble something about an apartment they rent out. No responsibility is admitted for the hearing loss that their tenants will suffer, if the fan is turned on.

Fireplaces often have an air tube; it's a pipe bringing cold air into the firebox where extra air is needed, to give oxygen for combustion, and also to provide volumetric filler to the exhaust stream, otherwise the fire consumes house oxygen and makes cold air seep in through any possible means.

I think a large CFM exhaust could benefit from the simplest possible setup: a pipe that one opens when one wishes. A pipe positioned anywhere.

John,
I apologize for the tangent here, but I want to say that I regret not having planned and installed a commercial deep fryer (and designed the ventilation system to accommodate that) when our home was designed and built. Have you considered that?

Somewhere in all these calculations there has to be a fudge factor for cooking style. If you stir-fry in a wok every night and need to exhaust smoke and grease, your requirements are different from someone who only wants to remove steam from boiling pasta or the smells from a baking fish.

If I were ambitious enough to do the calculations, I'd likely find I need >1000 CFM. But since I'm the second cook above, I'm doing fine with my 550 CFM hood, and usually run it at less than maximum speed.