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 backdrafting. 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 depressurization 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 , 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 (ASHRAE), 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 62.2, 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.)
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 ). 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 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.”
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 () 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 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 for this purpose, and also provides detailed and useful 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.
- 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 .
Last week’s blog: “To Install Stucco Right, Include an Air Gap.”