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9 Helpful?

Makeup Air for Range Hoods

If your kitchen has a powerful exhaust fan, it may be pulling air down your chimney or water-heater flue

Posted on Nov 19 2010 by Martin Holladay

UPDATED on January 8, 2018

Most homes have several exhaust appliances. These typically include a bathroom fan (40-200 cfm), a clothes dryer (100-225 cfm), and perhaps a power-vented water heater (50 cfm), a wood stove (30-50 cfm), or a central vacuum cleaning system (100-200 cfm). But the most powerful exhaust appliance in most homes is the kitchen range-hood fan (100-1,200 cfm).

Every time an exhaust fan removes air from your house, an equal volume of air must enter. The air that enters cracks in a home’s envelope to replace air that is exhausted is called “makeup air.” Two trends affecting makeup air are causing increasing problems for homeowners: homes are getting tighter, and range-hood fans are getting more powerful.

So where does a powerful range-hood fan get its makeup air? If the house doesn't have enough random air leaks around windows, doors, and mudsills, the makeup air is often pulled backwards through water-heater flues or down wood-burning chimneys — a phenomenon called backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney.. Since the flue gases of some combustion appliances can include carbon monoxide, backdrafting is dangerous. In some cases, it can be life-threatening.

Range hood manufacturers can’t answer homeowners’ questions

Most residential Q&A columns get regular questions from homeowners asking how to solve backdrafting problems or how to provide makeup air for exhaust fans. Yet range hood manufacturers are notorious for failing to provide installers with any guidance on makeup air. Several years ago, I looked into the problem and reported what I found in an article published in the August 2006 issue of Energy Design Update.

The genesis of my research was a homeowner query posted on Breaktime, the Web forum maintained by Fine Homebuilding magazine. Cheryl Morris described problems with her expensive new 1,200-cfm GE Monogram range hood. “It will pull a negative in the house,” she wrote. Reached by phone, Morris told me, “It pulled the ashes out of the fireplace, halfway across the room, right up to my husband’s Lazy Boy chair.”

According to Morris’s Breaktime posting, the range hood manufacturer could not propose a solution. “I have been contacting GE, with absolutely no help,” she wrote. “Their comment is that they have never heard of this problem.”

Since most residential kitchens are adequately served by a 150-cfm or 250-cfm range hood, it comes as no surprise that a 1,200-cfm range hood can cause depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. and backdrafting problems. However, the homeowner’s claim that GE had never heard of such problems needed to be verified, so I set out to discover whether GE’s experts are really as clueless as Morris alleged.

“Lends elegance to any kitchen”

General Electric was marketing its 1,200-cfm range hoods to residential customers. In fact, a GE Monogram press release bragged that the appliance could be installed anywhere: “The ventilation system operates at maximum venting capacity of 1,200 cubic feet per minute to keep the kitchen free of fumes and odors. … Beautifully finished on all sides, the new Monogram island hood lends elegance to any kitchen.”

Morris had purchased model ZV48SSFSS; its powerful fan draws 12.5 amps and requires a 10-inch-diameter exhaust duct.

Silent about makeup air

I got a copy of the GE Monogram installation instructions, and discovered that they were remarkably unhelpful. Astonishingly, the instructions make no mention of the range hood’s requirement for makeup air. The issue was deflected by a confusing reminder that combustion appliances require adequate combustion air: “Sufficient air is needed for proper combustion and exhausting of gases through the flue (chimney) of fuelburning equipment to prevent backdrafting. Follow the heating equipment manufacturer’s guideline and safety standards such as those published by the National Fire Protection Association (NFPA), and the American Society for Heating, Refrigeration and Air Conditioning Engineers (ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. ), and the local code authorities.”

Although it is true that furnaces and water heaters require adequate combustion air, the GE appliance in question is a range hood, not a combustion appliance.

Range hoods have at least one thing in common with combustion appliances: they also require makeup air, although the GE instructions fail to note that fact. So why do the instructions refer to the need for providing makeup air for furnaces and water heaters — but fail to inform installers that range-hood fans also need a source of makeup air?

Moreover, in a house with an atmospherically vented water heater, a 1,200-cfm GE Monogram range hood may cause backdrafting of flue gases, even if the water heater was originally installed in compliance with NFPA and ASHRAE standards. So complying with these standards for water-heater installation won’t keep you out of trouble.

Tracking down the standards

The reference to ASHRAE and NFPA standards in the GE installation instructions was particularly unhelpful, since the numbers of the intended standards were not specified.

GE went out of its way to advise range-hood installers of the best way to install a water heater or a furnace — that is, by following ASHRAE standards. Why didn't GE bother to tell range-hood installers that ASHRAE also has something to say about the best way to install a range hood?

ASHRAE's residential ventilation standard, ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant., limits exhaust fans to a maximum of 15 cfm per 100 square feet of occupiable space, unless a backdrafting test is performed. According to this standard, the smallest home in which a 1,200-cfm range hood could be installed without verifying makeup air needs would be one measuring 8,000 square feet — a very large McMansion indeed.

Wouldn't it behoove GE to let the installers of its monster fan know how to perform a backdrafting test?

Concerning the reference to NFPA standards, I tried to pin down what GE was hinting at. I contacted Allan Fraser, a senior building specialist with NFPA. After hearing me read the relevant paragraph from the GE instruction booklet, Fraser said, “That is a bad reference. As far as makeup air for range hoods is concerned, NFPA doesn’t cover it. Frankly, it is not an issue on our radar screen.”

Answer Center ignorance

Armed with the limited recommendations provided by GE’s installation instructions, I sought more information by placing two calls to the GE Answer Center (800-626-2000), asking, “Does GE have any recommendations on providing makeup air for a 1,200-cfm GE range hood installed in a residential kitchen?” The first GE expert responded, “What is makeup air?” An explanation was provided. She responded, “Do you mean you want to know the cfm of the fan?” After further discussion, I was put on hold. A few minutes later, the expert returned to the phone to report, “That information is not something we would have here.”

Later, a second call to the same number reached a GE expert with more confidence. “What you need is a range-hood damper, part number JXDA22,” the expert explained. “It’s a damper for a 7-inch round duct. We can also sell you an adapter if you need it.” But where does this damper go? “It goes in the ductwork, from what I understand.” Is it possible to speak to a technician familiar with the installation of this damper? “We don’t have technicians you can speak to.” (As it turns out, part number JXDA22 is a backdraft damper designed to prevent exterior air from entering the exhaust duct when the fan is not in use; the damper has nothing to do with the provision of makeup air.)

Intentionally vague

I sought better answers by appealing to General Electric’s press office, where I eventually reached Paula Cecere, GE’s product specialist for the GE Monogram product line. When Cecere was informed that the GE Answer Center experts seemed to have no understanding of makeup air requirements, she didn’t disagree. “I don’t doubt you,” said Cecere. “You have uncovered a communications gap. This subject is something that I am trying to learn about myself in my role working with these appliances. I’m not a representative of the technology team. As far as answering your specific questions, this is something I will look into.” She further noted, “Our statement in the installation instructions referring to the need to follow ASHRAE and NFPA guidelines is intentionally vague, because different homes are set up different ways.”

Cecere suggested submitting any technical questions by e-mail. However, my e-mailed questions were never answered. Instead, a GE press spokesperson, Allison Eckelkamp, responded, “We have decided that we’d like to answer your questions with a basic statement.”

The e-mailed statement was vague. It noted, “GE makes it clear in product documentation for pro range hoods that installers should refer to ASHRAE guidelines to determine the airflow needs of an entire home. It is not possible for GE to determine the airflow needs of a home based solely on the cooking ventilation system, as there are other ventilation systems in the home that contribute to airflow.”

Eckelkamp’s statement put a remarkably favorable spin on GE’s product documentation. The installation instructions certainly aren’t “clear” about the need for makeup air; nor do the instructions even mention the need to determine the airflow needs of an entire home. In short, Eckelkamp's statement was misleading.

An expensive proposition

GE never informs its customers that designing a makeup air system for a 1,200-cfm range hood is no simple task. I know of several commercially available powered makeup air units suitable for residential use (all manufactured by Electro Industries of Monticello, Minnesota). The largest of these units is a $1,800 appliance that is rated at only 632 cfm — not enough to solve Cheryl Morris’s problem. In heating mode, the makeup air appliance draws a whopping 10,000 watts.

Lacking any guidance from GE, neither Cheryl Morris nor her HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor were sure how to proceed. The contractor’s $1,000 solution was to install two 12"x12" exterior air makeup air grilles on the wall between the stovetop and the range hood. Unfortunately, this source of makeup air is so close to the hood that an airflow short-circuit was created, allowing cooking smoke to drift into the kitchen.

“They put an ugly damper system on the outside of my house, and now when it is 30 degrees outside, you can feel the cold air blowing into the kitchen all the time,” Morris told me back in 2006. “I’m at a standstill now. I’ve spent an extra $1,000, and it still doesn’t work. The air-conditioning contractor says he doesn’t know anything else to do, and I’m no better off than before. What irritates me most is that every magazine advertises these big range hood units. I think it’s unfair that the companies that make them can’t find a way for them to work in the houses where they are installed.”

How’s GE doing, four years later?

Four years later, GE’s installation instructions for its range-hood fans are basically unchanged. GE is still selling high-cfm monster fans to homeowners. Their instructions still make no mention of makeup air requirements. Their instructions still include the misleading paragraph referring to ASHRAE and NFPA and requirements for venting combustion appliances, with no identification of which ASHRAE standard or NFPA provision applies.

I decided to call the GE Answer Center to see if GE has managed to educate their phone operators about makeup air requirements during the four years that have elapsed since I called them last.

I explained that I was planning to install a kitchen range hood, and I wanted more information on providing makeup air for the exhaust fan. Predictably, the GE representative came back with a question: “What do you mean by makeup air?” So I explained the concept to her. She then said, “Let me look into that,” and put me on hold.

When she returned to the phone, she explained, “The makeup air has to go through a special pipe into your house, and GE doesn’t have anything in the instructions about that because GE doesn’t have any information on how much airflow your house has. So you have to call up a company — I wrote it down, it’s a company called ‘ASHRAE’ — and follow their guidelines.” Innocently, I asked, “Do you have a phone number for ASHRAE?” She answered, “No, I’m sorry, you’ll have to look that up.”

Code changes on the horizon

Until recently, residential codes had little to say about providing makeup air for range-hood fans. But the 2009 version of the International Residential Code (IRC) will, at long last, address the issue.

Here’s the new IRC provision, which is found in section M1503.4: “Exhaust hood systems capable of exhausting in excess of 400 cfm shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system.”

Avoiding problems

So, here’s my advice:

  • Select a modest, 30-inch-wide residential range. It will perform better than a 48-in.-wide Viking or Wolf range, since these larger commercial-style ranges require very powerful (and problematic) exhaust fans.
  • Buy a range hood with a small exhaust fan; for most homes, 150 cfm to 250 cfm is plenty. In a tight house, a stronger exhaust fan can cause problems with backdrafting. Most building codes (for example, Section M1507 of the 2006 IRC) require that kitchen range hoods have a minimum rating of 100 cfm. Broan makes a simple range hood (the 40000 series) rated at 160 cfm; you can find it in stainless steel for only $80 on the Web.
  • If you have a huge family, and you indulge in commercial-scale cooking that requires a powerful exhaust fan, you’ll need to install a powered makeup air unit like those manufactured by Electro Industries of Monticello, Minnesota or Fantech. These units move a lot of air; in winter, the incoming air is cold, but it can be heated with 10,000-watt electric resistance elements if you want. Be prepared to suffer a huge energy penalty. The model EM-WH1025K from Electro Industries will supply 632 cfm at a 50 F° temperature rise. If you want a 1,200-cfm range hood, you'll need two of these makeup air units; they cost about $1,800 each.
  • If you prefer to install a passive makeup air duct rather than a powered makeup air unit, the duct must include a motorized damper that is wired to come on simultaneously with the range-hood fan. Broan sells motorized dampers for this purpose, and also provides detailed and useful installation instructions. Broan recommends installing an outdoor air duct connected to a grille mounted on the kitchen wall or connected directly to the return plenum of your furnace; the motorized damper is installed in the duct. One 8-inch-diameter duct works for a range hood rated up to 1,000 cfm. If your range hood is bigger than that, you'll need two ducts.
  • Unless you are willing to install and commission a carefully designed powered makeup air system, a large kitchen exhaust fan is incompatible with a wood-burning fireplace.
  • As a stop-gap measure, a homeowner with backdrafting problems can open a window near the kitchen every time the range-hood fan is turned on. Although this solution works, it won’t satisfy most homeowners, and builders who suggest this remedy may still be legally liable for future backdrafting problems.
  • Some Passivhaus builders are experimenting with range-hood fans that don’t exhaust air to the exterior. Instead, they pull air from above the stove and pass it through a charcoal filter before returning the air to the kitchen. On the ceiling of the kitchen, in a location as far from the stove as possible, they also install a grille connected to the exhaust system of the home’s HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. .
  • If you're about to buy a new water heater or furnace, remember that sealed-combustion appliances are virtually immune to backdrafting problems, and are therefore always a safer bet than atmospherically vented appliances.

Advice from Building Science Corporation

Design advice for kitchen makeup air systems is provided in a Building Science Corporation document called First Deal with the Manure and Then Don't Suck.

Last week’s blog: “To Install Stucco Right, Include an Air Gap.”

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Image Credits:

  1. GE
  2. Energy Design Update
  3. Bill Smith,

Nov 19, 2010 7:10 AM ET

sealed combustion fallacy
by The dadaist

Just a minor note to a good post. Sealed combustion appliances aren't always sealed 100%

For example... Triangle Tube Prestige boiler. When you take off the front panel it is breathing from the room. And if one looks at the cabinet it is not fully sealed. Yes the exhaust is sealed so no CO is going to be emitted, but no the intake is not fully sealed. Air pressure changes do effect the start up of some appliances, tighter homes may cause more problems then just moisture, rot and mold.

Nov 19, 2010 8:57 AM ET

by Steve El

Wow, wonderful timing! Thank you very much Martin, adding a kitchen vent is on the list for early winter, and I was worried about the time I'd spend spinning my wheels trying to find this very info.


Nov 19, 2010 9:22 AM ET

Great Stuff!!
by Jamie Kaye

Thanks Martin. What an important item to be discussing and exposing! I live in a retirement/second home community where a Viking stove and range hood seems essential for quality home status. The ducting of these are more times than not, afterthoughts, and not to your surprise, not one of them have ever thought of make-up air. The IRC's new code is a great addition, but getting code officials to enforce the issue and compliance is another battle all unto itself.

Nov 19, 2010 9:38 AM ET

by Interested Onlooker

"•Some Passivhaus builders are experimenting with range-hood fans that don’t exhaust air to the exterior"

Are these the ones that didn't fit a commercially-available recirculating range hood?

Nov 19, 2010 9:54 AM ET

Response to Interested Onlooker
by Martin Holladay

Interested Onlooker,
Here are three links for further discussion of kitchen exhaust options for Passivhaus buildings:

Nov 19, 2010 10:14 AM ET

Like a glass works hood?
by Michael Chandler

We recently did a teaching studio for a client who works lamp-work glass jewelry and glass art work and needed an efficient studio with localized ventilation for open flame working of colored glass involving associated heavy metals and toxic fumes. We basically created a hood within a hood that was in someways similar to a heat recovery ventilator and would work for a person needing a large volume hood in an air tight house for cooking as well, though the version we use had 200 CFM out in a 7" duct with and 200 CFM in in a 10" duct wrapped around the out-going duct and a hood that distributed the incoming air around the edge of the hood. the key was that we ran the air concentrically to a point very near the exterior of the house to maximize heat exchange and then split the streams by using a 10" wye with an end cap and a 7" A-collar to pull the outflow up through the roof through an inline fan and the inflow just below the soffit so the roof overhang isolated the air streams. I do remember that my hands and wrists were quite bloody by the end of the assembly process, concentric duct work is a bear to assemble and seal, We used galvanized stock for this one and painted it with a metallic oil rubbed bronze finish after completion (it also had wiring for lights that was very specific due to the need for good color rendering on the flames associated with the glass work)

We make a lot of copper hoods for island kitchens and also do the big tile, stucco and even timber hoods (See p 88 of this falls Fine HomeBuilding Kitchen and Bath special) for wall applications that would be adaptable to this concentric venting system but for wall applications it's easier to use a cape hood damper behind the range and talk the folks into using a smaller fan. (We've even pulled the blower out of one unit and replaced it with a smaller blower)

Fantech makes in-line duct fans and flex duct that is rated for kitchen exhaust. (check out the 980 CFM FKD10) and i find that giving the client the option to use a fan speed controller helps to keep the CFM at a more reasonable level. Once you get over four amps aggregate for the two fans you are locked into using the fairly industrial looking Varifan MVS-1. Below 4 amps the Lutron speed controller works well.

Nov 19, 2010 10:22 AM ET

PS jump two duct sizes on concentric venting
by Michael Chandler

If your client is fixed on a 1,000 CFM exhaust you need a 10" out (78 sq in) and 14" in (154 sq in) to get the right cross-sectional area and you will still get more volume going out in the ten than in through the 14 cut down to a 2" ring w/ 76 sq in NFA you could make a strong arguement for jumping the outer duct to a 16" but aesthetically that's sort of glommy-looking for even a 48x36 island hood.

Nov 19, 2010 11:32 AM ET

Make-up Air
by Armando Cobo

I’m glad to see a more dedicated article on this topic. When I raised the issue back in February, there wasn’t much discussion about it. To be fair, it’s no just GE, it applies from Amana to Zero (or Subzero). Back then I called every major appliance and hood manufacturer, several distributors and a couple of engineers, and to my surprise, no a single entity had a clue about it, and more scary than that is that no one entity had any plans in the near future to address this code change.
Here is another site you may consider looking at:

Nov 19, 2010 11:48 AM ET

Response to Armando Cobo
by Martin Holladay

Thanks for the link to the page about units from Electro Industries.

These units are electric resistance heaters. If you buy the unit, it doesn't include a blower or a motorized damper -- you've got to buy those separately.

One model to consider is model EM-WM2035L. It draws a whopping 20,000 watts.
It can supply 790 cfm of makeup air at an 80-degree temperature rise, or 1,580 cfm at a 40-degree temperature rise. The list price is $870. But remember, you still have to buy all the other necessary components.

Nov 19, 2010 11:53 AM ET

Great work, thanks
by John Linck


Very good discussion. One point. As a woodworker I have spent years working with dust collection. A very important part of effective dust collection is to get your "source of suck" very close to the "source of dust" (sorry for the technical jargon). This principle applies to kitchens too. When I see range fan inlets several feet away from the burner or frying pan I shudder. They are moving about 90% clean air and only a bit of fumes with all the consequent energy loss. A much lower CFM fan can work much better with close collection than a huge fan further away. Cheers and thanks for your good work. john the toymaker

Nov 19, 2010 11:58 AM ET

Lowering the "source of suck"
by Martin Holladay

Thanks for your comment. You're right, of course.

However, there's no easy way to lower the "source of suck." Ideally, you want a range hood that doesn't obstruct lines of sight for the cook, and doesn't get in the way of a tall stockpot, and doesn't interfere with lighting. A tall order.

Maybe range manufacturers will come up with a new design -- perhaps a range hood with dangly octopus arms that are adjustable, so you can move a flexible stainless-steel octopus arm near the steak you are grilling.

Nov 19, 2010 12:17 PM ET

MA Supply Location
by Armando Cobo

A couple of ways we’ve done it lately was to place 3 supply vents 10’ away in the ceiling, and a second way 2 supply vents under the cabinets by the toe-kick. Installing 2 or 3 supply vents minimizes noise pollution, plus you could insulate the equipment or cavity as well. Also, you need distance between the supply vents and the hood to create an air loop close to the range, but far enough to not draw from the house make-up air. The equipment can be installed in the attic, floor trusses above, pantry or cabinets.

Nov 19, 2010 12:30 PM ET

Ducted Articulated Arm Fume Hood Extractor
by Jesse Thompson


Here's a fun way to lower the source of suck from a scientific supply house:

Nov 19, 2010 12:36 PM ET

My octopus arm!
by Martin Holladay

My octopus arm! Somebody already invented it!

Darn -- I was already spending the anticipated proceeds from my patent license agreements. There goes another six-digit idea...

Nov 19, 2010 12:57 PM ET

balanced fans
by 5C8rvfuWev

I notice that FanTech has inline fans that are rated for vent or supply -- Could these be wired to balance one another (no, the supply wouldn't be conditiioned, but ...)

Just a thought.

Joe W

Nov 19, 2010 1:05 PM ET

Response to Joe Wilson
by Martin Holladay

You've got the right idea: to supply makeup air, you need to wire a supply fan to come on simultaneously with the range hood. In a post on this page, Armando Cobo has provided good suggestions for locating the grilles for delivering this supply air.

Whether or not you want to condition the supply air is up to you. If you are a builder in Minnesota, and a client likes to cook with the 1,200 cfm fan running for four hours on Thanksgiving, I imagine that 1,200 cfm of cold makeup air will lead to complaints. On Christmas day, when the makeup air is 20 degrees below zero, you've got real problems.

Nov 19, 2010 4:39 PM ET

Balanced venting hood
by Michael Chandler

Joe that is what we were doing in the hood I discussed above. by using concentric duct work and a fairly modest pair of 200 CFM blowers on a single fan speed controller we were able to gain some of the benefit of an HRV as well. But mainly the benefit was that the make-up air was integral and concentric in the hood. (even at 200 CFM X 2 it's strong enough to blow out a propane cigarette lighter)

We haven't had any condensation issues with that one but I can imagine that in certain situations that would be a possible problem.

Nov 19, 2010 5:25 PM ET

Plan B
by James Morgan

The problem is partly the arms race of bigger commercial-style stoves and bigger more powerful hoods to go with 'em, but it's also in the layout. By virtually universal request almost all the kitchens we design these days are integral with the main living area. This is fine for heating up a ready meal, but you can't saute a pan full of garlic shrimp on a stove top in the living room and expect not to smell it, no matter how many cfm you're pulling.

A lot of folks around here just cook the smelly stuff outside. If this doesn't work for you and your cooking style is too stinky to live with, just accept you'll need to plan a traditional separated kitchen area where you can close the doors to the rest of the house and open a window. Good gracious, why do we insist on making our lives so complicated?

Nov 19, 2010 7:20 PM ET

I don't get it either
by John Brooks

I live in a simple home with a simple range and a simple not-so-sucky venthood.
When you guys start talking about these big honkin venthoods..
might as well be talking about your Polo Ponies
This is not my world

Nov 19, 2010 8:07 PM ET

by Garth Sproule

The duct free recirculating model in this link
looks like it has some potential for regular sized cooktops. They have a video explaining how it works. Anyone here tried this one yet?

Nov 19, 2010 11:36 PM ET

Hot air rises, so why downdraft?
by Steve El

Re comment by James Morgan.... my inlaws have one of those arrangements with the stove top in an island looking over the breakfast bar into the entertaining area. To keep the cook from losing sight line to family and guests, the stove has a built in downdraft exhaust.... and WOW does it suck. All other things being equal, do downdrafts have to pull more air than an identical kitchen setup using a hood over the stovetop?

Nov 20, 2010 5:48 AM ET

Response to Steve El
by Martin Holladay

Q. "Do downdrafts have to pull more air than an identical kitchen setup using a hood over the stovetop?"

A. Yes. You're not going to find any 150 cfm or 200 cfm downdraft fans. Most such fans pull between 500 and 600 cfm -- enough to cause backdrafting problems.

The reason is simple: the heat output of the stove burners and the high temperature of the cooking pots heat the air above the stove, causing a column of hot air to rise from the stovetop toward the ceiling, and pulling cooler air toward the burners from either side of the stove. A downdraft fan has to fight this convection current, while a range hood fan can take advantage of it.

Nov 20, 2010 9:24 AM ET

Energy Star downdrafts..... oxymoron?
by Steve El

Thanks Martin,

I'm assuming some downdraft models can get Energy Star labels, and I'm finding it rather hard to get my brain around that, since fighting against physics seems to make them inherently wasteful no matter how efficient the motor might be. Can you make sense of that?

Nov 20, 2010 9:42 AM ET

Second response to Steve El
by Martin Holladay

Are you sure that Energy Star gives labels to downdraft fans? I just did a quick Google search and didn't find any -- so I'm skeptical.

Nov 20, 2010 3:07 PM ET

Sex and Advertisements and Profits
by Daniel Ernst

Are people interested in performance, or image? Are they interested in cooking gourmet food, or in owning a "gormet looking" kitchen?

Viking was the first company to start producing a commercial style range for the residential market. They started the trend toward larger and more powerful ranges and hoods.

Now, regardless of the company---Wolf, Monogram, Viking---the product development managers are simply following that market trend, and the business managers are looking at the bottom line. Premium products, and especially BRANDED premium products, usually have a significantly higher profit margin. So these upscale 1,200 CFM hoods are a combination of sexy advertisements and increased profit margins.

I'm not sure it's the demand or supply side of the market that got us here---but here we are . . .

James noted that kitchens used to be kept separate from the main living areas. They were too smelly to be part of the public place. Working exhaust fans were part of the reason kitchens moved inside the house (well, that, and a lot of other technology like refrigeration and running water).

As Martin noted, competing trends---tighter houses and more-powerful hoods---are causing a new problem. I think that, if we're going to build tighter houses, then what we need are BIGGER hoods with SMALLER fans.

Heated air rises vertically, but the much of an exhaust hood's fan energy is working to pull the air laterally (following the path of least resistance - from around the open edges of the hood). And even the big CFM hoods don't necessarily overcome the vertical lift and volumetric expansion from gas stove tops and steaming pots.

Most hoods are only 20"- 24" deep, but ranges and stovetops are 36" deep. Therein lies the problem---and bigger fans are not the solution.

Custom hoods, set above head height, and built with a surface area LARGER than the stovetop, can exhaust cooking fumes very effectively---and they can do it with a small fan.

Maybe the kitchen exhaust hood manufacturers need to follow a modified version of ASHRAE 110-95??? Ha!

"Specifies a quantitative test procedure for evaluation of a laboratory fume hood. A tracer gas is released at prescribed rates and positions in the hood and monitored in the breathing zone of a mannequin at the face of the hood. Based on the release rate of the tracer gas and average exposure rate to the mannequin, a performance rating is acheived."

Nov 20, 2010 3:44 PM ET

Martin, My bad, I was assuming
by Steve El

No Martin, I have no such knowledge.... just assuming, since so many other products that to me seem rather whacko do get the labels. But..... I was wrong. I apologize for sending you on a wild goose chase.

Nov 21, 2010 1:53 AM ET

Island Range Hood
by Rahul Baijal

I had previously posted about Icynene and the Ventilation choices/requirements with Iycnene. We now have an island range hood on our house plan ( 715 CFM ). Reading the postings and blog above, I am in complete violation with these two competing goals. Or is it possilble to achieve the tight envelope along with the ventilation options that Martin had discussed with me. Woud I still need a make up air unit. Or do I simply need to pick one goal.

Nov 21, 2010 5:29 AM ET

Response to Rahul Baijal
by Martin Holladay

Rahul Baijal,
I strongly urge you not to abandon your goal of achieving a tight envelope. That's always a good idea.

My advice concerning your range hood is the same advice as I gave in the list of bullet points at the end of my article. Here are your choices:

1. Choose a range hood with a low-flow fan.

2. If you want a 715 cfm fan, provide a powered makeup air unit.

3. If you want to imitate Passivhaus builders, use a range hood with a recirculating charcoal filter, and pull your exhaust air from the kitchen ceiling, far from the stove, using a low-cfm fan.

Nov 21, 2010 10:44 AM ET

Question for Martin
by Garth Sproule

In point #3 above, when you refer to "exhaust air from the kitchen" are you talking about this as part of the whole house ventilation system?

Nov 21, 2010 10:58 AM ET

Response to Garth Sproule
by Martin Holladay

Yes. Most Passivhaus buildings have an HRV with dedicated ventilation ductwork. Typically, exhaust air is pulled from bathrooms and the laundry room, and fresh air is delivered to bedrooms and the living room. In some cases -- the cases I'm referring to -- Passivhaus builders put a ceiling grille in the kitchen, far from the stove, and connect the grille to the exhaust ductwork for their HRV.

HRV and ERV manufacturers warn installers that a range hood should never be connected directly to an HRV.

Nov 21, 2010 8:51 PM ET

by Andy Ault, CLC

One of the other recommendations in the PHIUS strategy is to consider using a humidistat switch in the Kitchen close to the cooking appliance so that the HRV (ERV) is automatically kicked into high speed once cooking begins. This may not work in the case of say "stinky garlic" which doesn't necessarily generate enough steam to trigger the humidistat, so a manual switch would also be needed. Another suggestion I got from one of the more prominent HRV manufacturers was to hard wire a relay switch into the non-vented hood so that the HRV was automatically kicked into high-speed as soon as the hood was turned on.

I asked them if both of these options didn't create a double energy penalty since you were running two fans to perform the work that's usually accomplished by one. They responded that this strategy allowed for one less direct penetration in the building envelope and thus one less source of uncontrolled and unconditioned air entry. So then I countered with the idea of using a gasketed controlled damper on a dedicated range hood exhaust duct. They still felt that since the range hood might be used for an hour per day at most and then the duct would sit there the other 23 hours per day, that the passive loss of the duct was worse than the "momentary" double penalty of the dual-fan power draw.

This was for a LEED Platinum Habitat for Humanity project so we had a LEED Rater available to run the modeling for us as well as a PHIUS Architect who could run it through their complex spreadsheet. My understanding is that they both ran it through their respective software programs and both came up with the same results. Extra duct and penetration bad. Short-term dual-fan power draw more energy friendly (in a mixed humid mid-Atlantic climate).

The project's still under construction, so we don't have any final results yet to compare against the modeling. But I'm very interested to see the final results and get feedback from the future homeowners after periods of use (say 6 mos and 1 yr post). I remain skeptical how well the system will perform, but the "modeling" says it's the way to go. Stay tuned ...

Nov 21, 2010 10:50 PM ET

Passivhaus, HRV, makeup air
by squibt

So I'm wondering if you go the way of a Passivhaus ventilation as described by Martin could you add a 4" vent or two behind the fridge or stove for makeup air...might be good for the fridge (draw off some heat) and keep the draft off your feet (under counter venting). Would you still need dampers?


Nov 22, 2010 12:16 AM ET

More island range hood questions
by Rahul Baijal


We are a family who cooks daily with strong South Asian cuisine, so we need a reasonably fuctioning kitchen exhaust system. Our current home has a recirculating down-draft that fails miserably, so given our desire for an island range hood. After reading this discussion and the discussion on the internet, what is an ideal location for a make up air unit. My only concern is that if i punch a hole into the building envelope, have i not defeated what i am trying to accomplish. Being in Houston, let's assume, I cool this hot, humid air, then I am not defeating the goal of energy efficiency. Where I am more confused is, let's say I achieve a tight-buiding envelope and then appropriately ventilate the house with the following suggestions you had recommended: 1. fresh air intake with damper plus air cycler 2. ERV, how does this independent system affect that ventilation system. Should I be worried about an independent system bringing in this hot , humid houston air into my home. How would that impact the rest of my system?



Nov 22, 2010 12:43 AM ET

Response to Daniel Ernst:
by James Morgan

"Most hoods are only 20"- 24" deep, but ranges and stovetops are 36" deep. Therein lies the problem---and bigger fans are not the solution."
I agree with the general point, (though most ranges and stovetops are 24 - 26" deep, not 36"): to exhaust even moderately effectively a hood should be not only at least as deep and wide as the stovetop but also held down close to the heat/smell/vapor source. Unfortunately this arrangement would make the processes of cooking difficult if not impossible, which is why it is never set up this way, or only temporarily until the homeowner gets tired of hitting their head against the hood. Not that a conscientious inspector would sign off on an installation lower than the manufacturer's recommended minimum height, which is generally 24" - 30" above the cooking surface and far too high for comprehensive vapor capture without a fan large enough to power a hovercraft. The problem is fundamentally one of multiple incompatible goals: to be able to cook as if in a commercial kitchen; do this in the main living room and not expose it to odor and vapor; and to be to be energy-efficient. Where are we headed: a completely enclosed lab-style fume cupboard? That would sure impress the neighbors.

Nov 22, 2010 7:04 AM ET

Response to SquibT
by Martin Holladay

You asked, "If you go the way of a Passivhausi ventilation as described by Martin could you add a 4" vent or two behind the fridge or stove for makeup air...might be good for the fridge (draw off some heat) and keep the draft off your feet (under counter venting). Would you still need dampers?"

I don't recommend this method of providing makeup air, because you end up with a wall penetration that leaks air, even when not in use. It's possible to include a motorized damper with such an installation; that would be better than a backdraft damper, but it would still leak.

I've posted an illustration above (as Figure 2); the drawing comes from the April 2003 issue of Energy Design Update. The article accompanying the illustration noted, "Glenn Mitchell, a designer in Comox, British Columbia, has come up with a clever solution: he ducts exterior makeup air from an intake in the rim joist to a grille on the wall behind the refrigerator. That way, the incoming cold air improves the efficiency of the refrigerator, while the refrigerator coils preheat the air. Mitchell, who posted the tip on a Journal of Light Construction Web forum, calls his system 'a poor man’s HRVi.' ”

As I noted, this solution increases air leakage, and is not recommended. Moreover, it should be noted that a 4-inch duct cannot balance the exhaust of a 1,200-cfm fan, since the fan requires a 10-inch duct.

Nov 22, 2010 7:39 AM ET

why 1200 cfm?
by Travis T

Why would they select a range hood that is probably 3 times the capacity required?

Nov 22, 2010 9:12 AM ET

Second response to Rahul Baijal
by Martin Holladay

Rahul Baijal,
First of all, when your house is complete, I hope you invite me over for a home-cooked South Asian meal. I don't know who does the cooking in your family, but it sounds like you're lucky and you eat very well.

There is no need to have two types of mechanical ventilation systems in your home. You need to choose either a central-fan-integrated supply ventilation system (that's the one you describe as "fresh air intake with damper plus Air Cycler") or an ERV with dedicated ventilation ductwork. You need one or the other -- not both.

If you choose a supply ventilation system, it can't be used to help exhaust your kitchen.

If you choose a balanced ventilation system with an ERV, it can be used to help exhaust your kitchen, as long as the exhaust grille isn't too close to the stove. (You don't want to introduce grease into an ERV.)

You are correct that a 700-cfm fan will do a better job removing grease, smoke, and cooking odors from your home than a 200-cfm fan. But there is no getting around the energy penalty. If you operate a 700-cfm fan during the summer in Houston, you will inevitably be pulling in 700 cfm of hot, humid outdoor air into your house. One way or another, that outdoor air will need to be cooled by your air conditioner every time you operate your exhaust fan.

Nov 22, 2010 9:18 AM ET

Response to Travis T
by Martin Holladay

Travis T,
You asked, "Why would they select a range hood that is probably 3 times the capacity required?"

There are several possible reasons:

1. To impress the neighbors, or

2. Because they saw the range hood in a catalog and it looked slick, or

3. Because they can afford it and they always want to buy the most powerful available option, or

4. Because they do commercial-style cooking on a huge stove and are convinced they need a lot of exhaust.

Travis, I agree with your point. That's why I wrote, "Since most residential kitchens are adequately served by a 150-cfm or 250-cfm range hood, it comes as no surprise that a 1,200-cfm range hood can cause depressurization and backdrafting problems."

Nov 22, 2010 12:31 PM ET

Response to Andy Ault
by John Semmelhack


I would be very surprised if the folks at PHIUS were recommending a dehumidistat located in the kitchen and wired to an ERV/HRV. They know very well that this setup is not a good one for most climates. If the moisture content of the outdoor air is greater than the indoor air (a good part of the year in MD + VA), you'll wind up bringing in extra moisture INTO the house. Perhaps it was a recommendation from an independent Certified Passive House Consultant? I'm not sure what a PHIUS Architect is.

Nov 22, 2010 5:03 PM ET

What about TWO humidistats and a chip?
by Steve El

John, is anybody marketing a system that also has a sensor at the inlet for the replacement air? Seems like a computer chip should be able to help automate what to do with moisture or BTUs in the house (ie, add more, move some outside, or redistribute within building). Such a system would need a gizmo to sniff at the incoming air, though.

Nov 22, 2010 5:06 PM ET

ERV Range Hood?
by Matt Dirksen

So... If the high end range hood manufacturers are so into 1200cfm hoods, why can't the next generation be erv's? What's another wall penetration if it pulls in fresh air (albeit warmer)? I just brought up the 2009 code req. here at my office, and no one knew of it. I had inadvertanly stumbled accross it a couple weeks ago, and they've been enforcing 2009 IRC here since July. Thanks for the post. I only wonder when the field inspectors start enforcing it.

Nov 22, 2010 5:14 PM ET

Response to Steve El
by Martin Holladay

I'm not sure what you are suggesting. The fact is, if a homeowner turns on a 1,200 cfm exhaust fan, the homeowner is also bringing in 1,200 cfm of outdoor air into the house, whether the homeowner wants to or not. One way or another, that outdoor air has to be conditioned -- heated in winter, and, if we are talking about an air-conditioned home, cooled in summer.

That air is coming in, like it or not -- whatever your humidistat recommends. The only way it won't come in is if the fan isn't capable of exhausting 1,200 cfm.

Nov 22, 2010 5:17 PM ET

Response to Matt Dirksen
by Martin Holladay

Matt Dirksen,
Q. "If the high end range hood manufacturers are so into 1,200-cfm hoods, why can't the next generation be ERVs?"

A. In theory, it's possible to design a 1,200-cfm HRV. (I wouldn't suggest using an ERV for this application). There are several problems with your suggestion, however:
-- Grease would gum it up.
-- It would be huge and expensive. Most residential HRVs are rated at 60 to 120 cfm.

In the meantime, Michael Chandler is experimenting with his homemade HRVs. They work, but not if you have to handle grease.

Nov 22, 2010 7:50 PM ET

Strategic location of makeup air vents
by TJ Elder

It might reduce the energy penalty to locate makeup air vents below the range, as suggested above. This way some of the exhaust air takes a short trip past the range and right up the hood, never getting conditioned conditioned indoors. An air inlet above the range could compromise ventilation effectiveness, so ideally the incoming air would flow across the range toward the hood.

Nov 22, 2010 7:51 PM ET

Response to John Semmelhack
by Andy Ault, CLC

Picky Picky, Picky ... A PHIUS Architect is simply shorthand for an AIA Registered Architect who also happens to be a Certified Passive House Consultant. I was trying to keep it simple for the sake of expediency.

And no one recommended a DE-humidistat. The suggestion was for a simple humidistat relay. This is completely unrelated to any outdoor RH values. It is a simple relay which would go in the Kitchen and be pre-set by the installer / homeowner for a specific indoor RH in the Kitchen area only. Just like in a bath, when there is a user triggered event which causes the indoor RH to rise above the setting, it would kick the fan into boost mode until the RH drops back below the setting ( a common PHIUS recommendation for dealing with bath settings to satisfy local inspectors who would otherwise require a separate, dedicated, direct-ducted bath fan per IRC 2009).

Yes, in theory this could bring in overly humid outside air, but if you A) use an ERV instead of an HRV this would be partially mitigated and B) the set point would presumably be one that would be high enough that you would want the interior air to be exhausted regardless and C) The original post was about make-up air, so any make-up system would create this issue. But at least an ERV would attempt to give you a fighting chance.

Nov 22, 2010 8:07 PM ET

Guess I was going beyond just hoods, Martin
by Steve El

Sorry I wasn't more clear, Martin. And this bit of babble takes a little bigger view of ventilation than JUST range hoods....

I'm unclear about it myself, so I'll try to explain this way. First, take three variables:

X = some indoor activity
Y = some set of outdoor conditions
Z = what's happening with open or closed windows and shades, and HVAC of all stripes

In my simple house, if I'm thinking about it when I do X, I might manually fine tune Z in response to Y. If Y changes I might tweak Z, but that means being home and thinking about it.

That got me to thinking about high tech super tight homes. Could that be automated? Maybe depending on temp it makes sense to open the window instead of run the AC. But maybe depending on RH it makes more sense to close the window. Similarly, maybe there are different times where you might fine tune the range hood exhaust, or maybe not, I don't know. But that was the gist I was getting at. To give the computer brain data to make these decisions, the chip needs data on outdoor conditions.

Hope that is more clear.... I'm fuzzy about the idea myself, which is unsurprising since I'm just an interested homeowner that knows almost nothing about HVAC.

Steve El

Nov 22, 2010 9:59 PM ET

Response to James Morgan
by Daniel Ernst

Thanks for catching my mistake. I meant most ranges are 26" deep, not 36".

Also, I didn't mean to imply that we should head toward laboratory fume hoods in the kitchen. ;-)

My point was that hoods are not rated, tested, or measured in any possible way. Nobody knows how one performs against the other. So how does a consumer purchase a hood? They pick the most powerful one, the one with the highest CFM rating (logically, the best performing, right?!).

I think mfg. restrict the hood installation height to 24" - 30" because they are trying to make up for a basic design flaw. Their hoods are just too small. Larger hoods, set higher (above eye level and the head bump caution area) can do the job with less horsepower.

Nov 22, 2010 10:27 PM ET

Response to Daniel Ernst
by Martin Holladay

I'm not sure that I agree that "larger hoods, set higher, can do the job with less horsepower."

I'm most skeptical of the "set higher" part. Evidence? Data? Test results?

Nov 22, 2010 11:57 PM ET

Chasing rainbows
by James Morgan

I watch water vapor rising from pans on my range and the higher it goes the more it disperses sideways at a visible angle which seems to average at about 20° from the vertical. If a walk-under hood is set at 80" that would suggest a hood at least 16" wider all round than the cooktop. Though I'd hate to have to integrate that into a cabinetry run with any degree of elegance it might stand a chance of capturing a significant portion of the vapor load, even with a low speed fan - but only if we completely ignore turbulence and vapor dispersal effects. Taking those into account would surely take some fairly extensive aerodynamic studies and hood-makers seem more interested in cranking up the brute force than in nuanced engineering.
Odor capture is another and still more demanding issue altogether. Odor spreads through a room by completely different mechanisms which have little to do with the 'hot air rises' scenario, which is why I mention fume cupboards. Labs use them for a reason. Bigger fans, bigger hoods - all chasing a rainbow.

Nov 23, 2010 7:54 AM ET

Response to Andy Ault
by John Semmelhack


I'm going to be picky picky again...I stand by calling the switch you describe a dehumidistat. A dehumidistat is used to turn on a machine when the RH% exceeds a certain set point. The machine will then run until the RH% drops below the setpoint. A


is used to turn on a machine (such as a humidifier) when the RH% drops below a certain setpoint. The machine will run until the RH% rises above the setpoint. Yes, they're essentially the same device with different control logic.

In my opinion, a dehumidistat has no place in a home in a mixed-humid climate unless its wired to a machine that can actually dehumidify. I do agree with you that using an ERV-only strategy makes sense. I just don't agree with boosting it via a dehumidistat control.

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