Unfortunately that will not be code compliant. I am not sure how vigorously township inspectors go after this code violation.

Well, "Whats In AName," you have opened the MUA bag of worms here in the Appliance forum, so don't be surprised at what you find inside. Perhaps first, though, the question of adequate flow rate (CFM) needs to be answered. To do that, you will need to specify the aperture size of your hood. Aperture size is the area of the hood bottom through which the rising and expanding cooking plume is to be captured. You might also specify the range size so that adequacy of this size can be ascertained.

Or you can review the legion of relevant posts here to discover all of the recommended factors.

kas

The hood i bought (Zephyr Essentials Typhoon) has a setting that you can change to limit it down to 300 or 400 CFM to pass inspection if you are worried about that. Just bump it back up after you pass. Maybe your hood has the same option.

When we redid our kitchen code required MUA for any hood over 300 CFM and our 70s house was a wind tunnel. They don't care about that. It works fine without makeup air until you end up with carbon monoxide poisoning when someone doesn't open a window. Do you plan to crack a window in the winter?

Kaseki, I did not know it was such a big deal in this subforum; so forgive my ignorance. If I understand your question correctly, the hood bottom that will suck in plume is 22X30 inch. Cooking range top area is 29X24 inch. I think hood will be installed 30 inch from the top of burners.

3katz4me- the code calls for MUA over 400 CFM. I will open a window for sure but that doe snot mean everyone will do the same considering parents visits us often. But then, as I said, our house is not not well sealed anyway.

In case the search engines aren't working (and sometimes they do not), here's a very brief precis of the literal ton of past threads mentioned by Kas.

A 400 CFM hood can do an acceptable job (note "acceptable" not "optimum") and might be better than the XtremeAir unit if:

(a) you get a hood with a canopy rather than a flat base (the canopy will do a much better job of collecting vapors/effluent/etc. and a real canopy makes as much or more of a contribution to effluent and aroma containment as CFM);

(b) get a hood as deep as possible front to back (IOW, don't get an OTR MW unit or one of those flat-bottomed designer hoods that are only 18" or 19" deep front to back --- doesn't matter how many CFMs you've got, they just do not extend far enough forward to go a good job of capturing vaporous "stuff" from the front burners when cooking on high heat);

(c) look for a hood that is 24" or more deep (preferably 27") so you get the canopy over the front burners (again, much better capture over the front burners --- and I'm guessing that you do your south Asian cooking on one of the front burners.) FWIW, that ExtremeAIr unit could well be a disappointment for you and your south Asian cooking -- the unit is only 22" deep) and looks like it has a more or less flat base;

(d) get a 36" wide hood (3" additional coverage on either side of the range --- again, better capture);

(e) get CO monitors and place them in the airflow path from your gas appliances. MUA is more about the laws of physics than regulations. Just because Code might not require MUA for a 400 cfm hood does not mean there will not be any concern for backdrafting. In theory, any vent fan can backdraft. Also, I'm not sure what you meant by seating area over a crawlspace and heating appliances, but "path," does not mean next to the heat registers. It is possible to backdraft through heat vents, but that is usually a dust problem not a CO problem. What "path" means is the route for air to flow from the exterior of those appliances through the house to the kitchen.

In addition, having a smooth (easily cleaned) backsplash on the wall behind your range, having spray on cleaner handy using it on all smooth surfaces (counters, cabinets, back-splash, stove top, floor) not long after after cooking will help a lot with lingering aromas. With curries, I find that running a spritz through the hood fans intakes, too, will help diminish the remaining oily spice aromas carried by the aerosolized grease that coats the blades and baffles.

Why are you considering a 900cfm blower? The rule of thumb I heard was 100 cfm per 10,000 BTUs. That puts you in the 400 range, and again, unless you are blasting all 4 burners at once, you probably won't even need to run it on high to take advantage of all 400cfm at once.

Comparitively, Wolf recommends 900cfm for their 36" ranges, and they put out 85,000 BTU on full blast. Many people don't even use 900 cfm hoods on those, because it's pretty unlikely anyone will blast 6 burners at once.

http://www.todayshomeowner.com/how-to-calculate-kitchen-range-hood-fan-size/

Just a point of clarification -- the rule of thumb is 100 cfm per 1000 BTUs.

jwvideo, thanks a lot for so many good points. Problem is, I could not locate a 400 cfm canopy style hood which extends beyond 24 inch depth and has baffle filters. If you find one, please let me know.

Micheal- Those suggestions are from posters in Chowhound. Personally, I do not know if 400 cfm is good enough. Problem is, I just can not buy another new hood if 400cfm is not good enough to serve the purpose.

We have a 600 VAH and a 6 burner BS. MUA was not code, or really common knowledge even here back when we built 11 years ago. I crack a window, yes even in the harshest New England winter cold, when I fire up the hood for serious cooking or coffee roasting. That said, follow code if it requires MUA and save the headache if you get nailed.

A better rule of thumb, in my opinion, is achieving around 90 CFM per square foot of aperture. For the numbers provided, flow rate for the hottest cooking should be 90 x 22 x 30 / 144 = 412, so 400 CFM actual is fine.* Except! Except that no ventilation system has zero static pressure loss, so the rated static pressure performance of the blower has to be higher than 400. I suggest with good (low pressure loss) MUA, a factor of 1.5 (for baffles and clean mesh filter hoods with some ducting and an exterior cap), which brings us to a 600 CFM blower. If you were to try to pull an actual 400 CFM from outside through cracks in the plaster and around switch-box covers, then the rated performance would have to be much higher than 600 CFM to succeed.

Do you have any combustion appliances in the house that have an interior air path to the kitchen? By combustion appliances I mean furnaces, hot water heaters, dryers, etc. If so, then the interior air pressure should not fall below 0.03 inches w.c. to avoid back-drafting (0.06 for blown oil burners). This is difficult to achieve even with MUA systems, so it is best to isolate these appliances so they take their air from outside.

Alternatively, you could have the house tested to ensure that leakage is adequate at the pressure drop you will allow.

kas

*Note that the 100 CFM/1000 BTU rule of thumb, as well as the X CFM per linear foot of [commercial] hood rule of thumb, are also for actual airflow in commercial circumstances. More rated CFM is needed to account for pressure losses. Any competent commercial ventilation system supplier would address those effects in the design.

I saw a This Old House episode where the homeowner moved the stove to the island. Needed to remedy removing heat and smoke. They installed a vent over oven rante and it went into the attic and out the side of the house. Had to cut a hole in the siding. I recall the contractor said, you need to move flow of heat and smoke out at an mmediate point' over the stove. Hope this helps.

Yes, closer is better in capture efficiency, physical size materials efficiency, and blower power cost efficiency, but can conflict with sight lines and with painless head motion. The commercial standard of seven feet from the floor is a compromise in this respect. Most cooks can deal with more modest hoods 30 to 36 inches above the counter (5.5 to 6 ft from the floor) in residential settings.

Kaseki,

I've learned a lot from your posts on this subject, but the more I learn the more I realize how little I know. It is amazing how complicated venting can get! Can you elaborate on the below comment of yours? What is meant by an "interior path" to the kitchen? I have a furnace and hot water heater in a basement, a fireplace in the kitchen and neighboring family room and a gas dryer in a neighboring room to the kitchen. How can I confirm if they are isolated or not? What is meant by isolated? They are pulling air for combustion from the outside?

Do you have any combustion appliances in the house that have an interior air path to the kitchen? By combustion appliances I mean furnaces, hot water heaters, dryers, etc.

Kas will likely come back with more precise information, but in the meantime, you can start thinking about "interior air paths" in terms of lines of sight and doorways. "Isolated" isn't necessarily the issue or the only way to deal with backdrafting.

The idea of MUA for a hood (and the other appliances) is not so much that you need to isolate them but that you need a source of air that draws more easily than from the rest of the house so it will not backdraft the other appliances' combustion venting while they are running. In an old house, that may or may not be easy.

Frankly, unless you have unusually good dampers for your chimneys, your two fireplace chimneys are already probably pretty leaky and functioning as MUA for the kitchen and the clothes dryer. (Hopefully not for the basement water heater and furnace.) You just do not want to be having fires burning while running a range hood without its own MUA.

The simplest way to see if you have a problem is to get a couple of CO monitor/alarms from the hardware store and put them in the paths between the appliances. If the alarm goes off when the appliances are running (say, when your dryer and range hood are running at the same time) then you definitely need some professional MUA assistance.

Thanks JW - fortunately I am still in the planning phases of my kitchen redo, and though my house is not new (built in the 70s) I am planning on having MUA given that I will be going with a fire breathing 48" range and thus need pretty powerful ventilation. The expressed concern around even MUA being challenged to achieve a certain air pressure given the presence of other combustion appliances gave me another variable to consider, though sounds like I should be ok.

By interior air path, I meant that there would be a way for a cat or a butterfly to get from the vicinity of the air intake of the combustion appliance to the kitchen.

If a combustion appliance has an air inlet path from outside, and a reasonably sealed exhaust path to the outside, and lives in an area that is more or less sealed off from the kitchen, I would call that isolated. Isolated combustion appliances would not be expected to backdraft due to the kitchen ventilation.

All other such devices can backdraft when subjected to lowered house interior pressures, which is what happens when a big blower is used to pull air from the kitchen without sufficient MUA.

As hinted at above, keeping the pressure drop low in the MUA path can be challenging, as large flow rates across even large filters can have a significant pressure drop.* So, unless the MUA has its own blower, filtering has to be minimal -- somewhere between sparrow restriction and mosquito restriction.

Blower MUA systems have their own complexity (potentially a lot of it), as unlike many commercial operations where ventilation rates are held constant and the MUA can be tuned to a fixed rate, the hood blower likely has several speeds, there may be bathroom exhaust fans that can be on or off, and fireplaces can move a lot of air, but tolerate only a slight pressure drop to maintain sufficient draft.

This is why I suggest where feasible that combustion appliances have some combination of their own MUA and some isolation from the kitchen (via a basement door that stays closed, say, or better, a closed room within the basement). My oil furnace has its own intake duct to the outside using a kit manufactured for this purpose. I'm not familiar with modern gas dryers or hot water heaters to be able to comment on equipment of this sort that might be available for them.

In other words, minimizing the need to keep the house pressure difference to the outside within 0.03 inches may be a lot easier than incorporating a sufficiently sophisticated MUA system.

There are some simple ways of controlling MUA that may appeal in some cases, and I'll try to get to that subject in the near future.

kas

MUA approaches:

It looks like Fantech has recently embraced MUA systems and information thereon that I haven't had time to read. However, here is a link:

http://www.fantech.net/enus/Solutions/cook-residential/

Another scheme that might be considered where refinement is not critical is an MUA duct into the house with a damper that opens when flow is detected in the hood duct. Some screening is advised, and this approach may not be sufficient for the most sensitive combustion appliances. I think Broan has such a damper and hood duct sensor.

An approach used by some big buildings is to run a MUA blower at full speed pumping air into the building, but include a bypass around the blower that is back pressure controlled. When the building internal pressure gets high enough, a damper opens and the excess blower air is routed around the main blower. This reduces the amount of filtering required compared with a blow-off damper that releases pressure above a particular level. For kitchen applications, this scheme might be turned on only when the hood is on. Ear "popping" might be an issue if one goes for too much pressure.

Otherwise, complex computerized closed-loop servo schemes can be employed to drive an MUA blower to achieve a particular differential pressure vs. the outside.

In general, if protection from combustion appliances back-drafting is needed, and the house air pressure can vary widely depending on vent hood power, other fans on or off, fireplaces, house leakage, and window states, then some sophisticated electronic, pneumatic, or mechanical scheme is needed.

Blowers in MUA systems are necessary when filtering and/or heating are needed and these devices by themselves would drop the house pressure below the back-draft limit if the duct were passive.

For larger applications, Greenheck might be a potential source of hardware and systems expertise.

kas