Tuesday, January 26, 2010

Maximum Suck

After writing about the Death Star, it occurred to me I haven't discussed one of my favorite appliance-related topics: Ventilation. Ventilation is one of the most overlooked aspects of kitchen design and yet it plays such an important role.

A number of years ago, a friend of mine wrote a terrific article on ventilation for the food website Dave Scantland's article was so thorough I asked him if I could use it to help my clients understand the importance of the subject.

(I've edited the original article slightly for brevity.)

Your home is full of holes.

Buildings must breathe; occupants need fresh air; combustion in furnaces and appliances requires oxygen. Since most of us don't live in pressurized capsules, we take it for granted that the air pressure outside the home will equal the pressure inside. And this is the job of holes. When air is expelled via one hole, new air rushes in through another to replace it. So your home is full of them.

Try this. Take a shoebox and seal its lid with heavy tape. Cut a hole in the side just large enough to accept the hose from your vacuum cleaner. Stick the hose in just a little way, seal the edges, and turn on the vacuum. Your results may vary, but one thing will be clear: the box is subjected to considerable negative pressure and threatens to collapse. To a lesser degree, it's what happens when you punch the "on" button on the exhaust hood over (or behind) your cooktop. The air it expels must be replaced.

Since the energy shocks of the '70s and '80s, buildings have been constructed with tighter joints, more insulation, more efficient insulation, and greater attention to weather-stripping and air-tight glazing. All of this means fewer holes.

An increasing affinity for high-output ranges, downdraft cooktops, and the idea that the way kitchens look is as important as how well they work, have led to an increase in home ventilating power.

In the US, at least, there is no standard for how much ventilation should be applied for a given size or output of range (Arne's note: same applies for Canada). In a way this is understandable. Ventilation fans should remove smoke and odor, but the size of your stove, the BTUs it generates, and the amount of smoke and odor you create when using it are not necessarily related. Even if you're a serious cook, the relationship is tenuous. You can simmer a stock all day and not have a problem. But neglect three measly bacon strips -- whether the pan is on a five-inch electrical coil or a multi-ring 18K BTU burner -- for just a moment, and you can fill the house with the airborne effluent of carbonized fat and flesh that will linger for days.

Like many suburban houses, ours came with a built-in exhaust fan over the stove. It is white; its 30-inch width is the same as the width of the range. The fan is 5 inches in diameter and is mounted behind a removable, metal mesh filter. The filter and fan are in a 10-inch square casing set in the center of the hood. A label inside the hood says that the fan moves 160 cubic feet per minute (CFM).

The range is 25-3/4 inches deep, but the front edge is about 27 inches out from the wall. The edge of the largest burner is 24 inches out, and the hood is 17 inches deep. Is it reasonable to assume that the fan can remove smoke from that far away? How strong a pull is 160 CFM?

According to the NuTone Group (, the world's largest manufacturer of residential ventilation products: For conventional electric and gas ranges or cook tops, we offer a complete line of hoods that can fit your cooking needs with features you desire. For high output gas ranges or cooktops, the minimum rate of 1 CFM of ventilation per 100 BTU is recommended.

The first part of this statement is not very helpful. The second part, however, provides some sort of guideline. Find out the BTUs, divide by 100, and you have a starting point.

The range is electric and, like the vent hood, it came with the house. My first stop is the owner's manual. Unfortunately there is no list of specifications. Looking the model up on the manufacturer's Web site, I find no mention of BTUs. Instead the burners are rated in watts.

Can watts be converted to BTUs? Since they are both units of energy, yes (we'll skip over the explanation that energy output ought to be expressed as watt-hours or BTUs per hour, but never is). One kilowatt-hour is equal to 3,412 BTUs per hour. So the 2,500 watts of my nine-inch burner is equal to 8,500 BTUs; the 1,500-watt burners are 5,118 BTUs; and the 2,000-watt, eight-incher is 6,824. All told, it comes to 25,560 BTUs. If I want to plan for a worst case, I need 256 CFM. Or do I?
I rarely, if ever, have all the burners cranked all the way up at the same time. If I had the two larger burners running at full blast, the 160 CFM supplied by the present fan should be sufficient. But the fact is that I can fill the house with smoke using only one burner turned up maybe 75%. The output of the burner is not really the issue; the issue is smoke and odor.

Let's look at it another way. My kitchen is 11 feet wide and about 14 feet long, and the ceiling is nine feet, making the volume of my kitchen 1,386 cubic feet. At 160 CFM, the air in my kitchen should be completely replaced in a little over eight and one-half minutes -- actually somewhat less, since a lot of volume is consumed by cabinetry and appliances. So what gives?

I assume that the manufacturer's label isn't lying. But maybe the rating is for the fan alone, without regard for the filter between the fan and the smoke, and the resistance of several feet of ductwork between the fan and the outside. Maybe the answer is to crank up the CFMs. Lots of people do -- it goes along with the fad for commercial-style ranges. Given the popularity of commercial-style cooktops featuring six or more burners rated at 15,000 to 18,000 BTUs, you might assume that these ranges require heavy duty ventilation -- up to 1,200 CFM. Indeed people do install such systems. Downdraft fans for small island cooktops start at a minimum of 600 CFM (partly because they have to fight convection). If a measly 160 CFM won't do the job, multiplying it by five or six ought to and probably will.

Imagine what happens to a 2000 square-foot room when you turn on a 1200 CFM ventilation fan. With eight-foot ceilings, the room would have a volume of 16,000 cubic feet, and the fan will revitalize the room’s air in 13 minutes. Where is the clean air going to come from? Answer: through the holes -- gaps in the floor, walls and windows.

If the doors and windows are open, air movement is easy. But if it's recently built or remodeled housing, it has openings that are well weather-stripped, and a heating/ventilating/air-conditioning (HVAC) system that gets its air directly from the outside. In this case, air comes in through places where it isn't supposed to.
Air comes in through holes where it ought to be going out such as the fireplace, the furnace flue, and the water heater vents. This is air laden with exactly the sort of things you don't want in your house: carbon monoxide, oxides of nitrogen, and smoke particles. You might even spot a wisp or two of dryer lint along with anything that might be stuck to the inside of your furnace filters. A few pieces of lint are fine, but the rest is potentially very dangerous. It's called backdraft, and people die from it.

The easy solution is simply to open a nearby window (or two -- a 1200 CFM fan needs an opening about 24 inches in diameter), but this defeats the whole point of making the house tighter. While that fresh air (called make-up air in HVAC circles) is coming in, thirty years of advancement in home design and energy efficiency is leaking out -- just so you can rid yourself of a few cubic feet of noxiousness. We have to rethink this.

Home kitchens aren't subject to the same regulations as commercial kitchens, and there are good reasons for this. A commercial kitchen must vent everything, including aerosolized grease, or risk creating significant fire hazards (or a stampede of patrons). One of the first things commercial kitchen novices remark upon is how noisy the lines are, and it's no surprise, given the 1 CFM/1K BTU guideline. Twelve 20K BTU burners means 2,400 CFM ventilation (and the required make-up air). It's like working in a wind tunnel.

Although change appears to be coming, few residential rules are included in the government regulations, and most are vague guidelines. One of the few specific directives available is from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers ( The applicable document is Standard 62-1989: Ventilation for Acceptable Indoor Air Quality. It prescribes an absolute minimum of 100 CFM, or 40 CFM per linear foot of cooking area. On that basis, a 30-inch range needs the minimum. OK, it's specific. But in the end it's not helpful, since my 160 CFM unit, at 60% above the requirement, is not doing the job.

The problem is that neither Broan nor ASHRAE address the real problem: you've got a source of smoke sitting in a specific spot, and you simply want to direct the smoke out of the house. So it's not an issue of power, it's an issue of control. One way to exert control is to make sure that the vent hood covers the entire cooking area, and take advantage of convection. Smoke, along with the hot air, will rise naturally to the fan intake. Island hoods do this job very well, since they usually match or exceed the cooktop in size. This coverage means that they can sit higher than the standard under-cabinet vent hood, which is usually 27 to 30 inches above the cooking surface or 63 to 66 inches above the floor. If the perimeter of an under-cabinet hood matched the cooking surface, anyone of average or above size would be banging their head against it with painful and annoying frequency.

The fact that a typical under-cabinet fan does not cover the entire cooking surface may not be significant for most cooks. It works for my Mom. A colleague of mine was surprised that I used one at all -- hers seems to be most useful as a microwave platform. But if you're still reading this, you are not most cooks. You stir-fry at extreme temperatures. You blacken fish in cast-iron skillets -- on purpose. You sear the edge fat on 2-inch-thick New York strips. But because you sometimes play with vacuum cleaners and shoeboxes, you have also learned that to remove smoke, brute force alone will not avail. The only avenue left is finesse. You have to make the smoke want to go where you want it to go. And as with children, spouses and co-workers, sometimes gentle pressure is most effective. (Arne's note: There is another option: increase the depth of the hoodfan canopy. Most manufacturers offer "commercial depth fans. Nowever as noted in the "Death Star" article this creates its own problem.)

We learned from our shoebox experiment that negative pressure pulls things in (in the case of our shoebox, it tried to pull the walls in). This is how an exhaust fan works -- it creates an area of continuously regenerated negative pressure by removing the air in the vicinity of the fan.

We can also infer the corollary: positive pressure pushes things away (imagine reversing the motor on your vacuum cleaner while it's still attached to the shoebox). So if we could create an area of positive pressure between the housing of the fan and the outer edge of the cooking surface, we could nudge the rising smoke into the negative pressure area, and it would go up the chimney. Luckily, we have a way to create a high-pressure area: we have a fan right there in our vent hood. With a little duct-work engineering, we can push air out on one side and suck it in at the other. As it turns out, Broan actually makes such a contraption -- the Allure. (Arne's note: Gagganeu and Miele also make such a fan, as do others.)

And so we come to my original plan for this article. I am no Norm Abrams. I am not Handy Andy or Ty Pennington. I'm not even Tim Allen. My first attempt at appliance upgrade left us without a working toilet for three and a half days. That was twenty years ago, and I haven't improved much, except to recognize -- sometimes -- when it would be advisable to call for professional assistance. However, in fulfillment of my manly duties (and so I could go one up on the Magic Board), I set out to install a new vent hood. I expected the installation to be a disaster, but anything that involves power tools, hot air, and a legitimate chance to use duct tape for its intended purpose is too grand to pass up. At the least I would have a great farce for the Webzine. But much to my surprise (and probably that of the rest of the household, though I am too proud to inquire), it works, and I accomplished the feat in less than a day.

I admit that I wasn't confident enough in my own grasp of physics to resist springing for the top-end, 400 CFM boost unit. But we don't use it. Score one for science.

Dave Scantland is an Atlanta-based writer and graphic designer.
Photos and illustrations by the author.


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