How to Provide Makeup Air for Range Hoods

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How to Provide Makeup Air for Range Hoods

If your kitchen has a powerful exhaust fan, it may be making your indoor air worse

Posted on Jun 8 2015 by Martin Holladay
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When Cheryl Morris moved into her new home, she realized that the kitchen exhaust fan was probably too powerful. Whenever she turned on the 1,200-cfm fan, strange things happened. “It pulled the ashes out of the fireplace, halfway across the room, right up to my husband’s chair,” she says. Those dancing ashes demonstrate an important principle: Large exhaust fans need makeup air.

The air that fans remove has to come from somewhere

Most homes have several exhaust appliances. They can include a bathroom fan (40 cfm to 200 cfm), a clothes dryer (100 cfm to 225 cfm), a power-vented water heater (50 cfm), a woodstove (30 cfm to 50 cfm), and a central vacuum-cleaning system (100 cfm to 200 cfm). The most powerful exhaust appliance in most homes, however, is the kitchen range-hood fan (160 cfm to 1200 cfm).

Although tightening up homes is a good way to make them more energy-efficient, builders need to remember that plugging air leaks makes it harder for air to enter a home. Every time an exhaust fan removes air from your house, an equal volume of air must enter. If a house doesn’t have enough random air leaks around windows, doors, and mudsills, makeup air can be pulled 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.. Because the flue gases of combustion appliances can include carbon monoxide, backdrafting can be dangerous.

One important way to limit backdrafting problems is to avoid installing a wood-burning fireplace or any atmospherically vented combustion appliance. Appliances in this category include gas-fired or oil-fired water heaters, furnaces, and boilers connected to old-fashioned vertical chimneys or flues. Instead, install sealed-combustion appliances with fresh-air ducts that bring combustion air directly to the burner. Most sealed-combustion appliances are practically immune to backdrafting problems.

Small exhaust fans—those rated at 300 cfm or less—usually don’t cause backdrafting problems. Anyone planning to install a big fan (400 cfm or more), though, needs to provide a dedicated source of makeup air. As a stopgap measure, it’s possible to open a window every time the range-hood fan is turned on, but this solution won’t satisfy most homeowners. Moreover, builders who suggest this remedy may still be legally liable for future backdrafting problems.

A ventilation system isn’t a makeup-air system

Most U.S. homes include a forced-air distribution system for heating or cooling. Just because your house has ducts, though, doesn’t mean that you have a makeup-air system. Some new houses (especially tight ones) also have a mechanical ventilation system to bring fresh air into the house—for example, a heat-recovery ventilator (HRV), an energy-recovery ventilator (ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.), or an outdoor-air duct connected to the furnace’s return-air plenum. If you’re wondering whether your home’s ventilation system can provide makeup air for your range hood, the answer is no.

Why not? First, most ventilation systems include dampers and controls that open according to the anticipated needs of the occupants; this ventilation schedule has nothing to do with the operation of a range-hood fan. Second, most mechanical ventilation systems provide only small quantities of fresh air—generally between 50 cfm and 100 cfm. That’s much less than the amount of makeup air needed for a 600-cfm or 800-cfm range hood.

Some houses also include outdoor-air ducts that supply combustion air to appliances such as furnaces and boilers. In most cases, these outdoor-air ducts have been sized to meet the needs of only one appliance, so they can’t be depended on to supply the makeup air for a range hood.

If you decide to install a makeup-air system for your range hood, you’ll need a dedicated duct that introduces outdoor air into your house. (In most cases, this duct will be larger than a ventilation duct.) This makeup-air duct can be connected to your home’s existing forced-air ductwork if you want, but it needs separate controls from those used to regulate your home’s ventilation system.

Follow the code, not the manufacturer’s advice

When I contacted four range-hood manufacturers recently to ask about makeup-air requirements, the quality of the answers ranged widely. The technical advisers at General Electric were the least informed, and four phone calls all elicited the same response: “What is makeup air?” A call to Broan was more fruitful, and a helpful representative advised, “If you start going above 300 cfm, then you might start considering makeup air, especially in a brand-new house that might be fairly airtight.” Other manufacturers had answers somewhere in between: “We don’t have any recommendations other than advising you to do whatever the code says.”

According to section M1507.3 of the 2009 International Residential Code, the minimum rating of a kitchen range hood is 100 cfm. Section M1503.4 of the code notes that you don’t need a makeup-air system for a range hood rated at 400 cfm or less.

If your fan is larger than 400 cfm, however, you’ll need a makeup-air system with a motorized damper and interlock controls. The code states, “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.”

If you are building a superinsulated house and, therefore, want to limit the number of penetrations in your building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials., consider using a recirculating range hood rather than an exhaust fan. Combined with a ventilation appliance such as an HRV, this solution works well for some families. Those who do a lot of roasting and frying, however, may not find this solution satisfactory.

The bottom line is that powerful range hoods are great for getting rid of smoke from blackened redfish, but they are difficult to integrate into a tight house. It is best to avoid exhaust fans altogether, but if you can’t live without one, be sure to provide plenty of makeup air. Backdrafting from combustion appliances can be deadly. □

Three ways to provide the makeup air you need
The IRC requires a dedicated makeup-air system for range hoods that are larger than 400 cfm. The necessary outside-air duct is usually connected to a wall or ceiling grille in the kitchen or directly connected to the return-air plenum of your furnace. As long as the duct dumps the makeup air into your home, however, it can dump the air almost anywhere.


Although an outdoor-air duct connected to a motorized damper solves the makeup-air problem, it can still lead to comfort complaints if the air isn’t conditioned. This problem can be reduced by hooking the outdoor-air duct to your furnace plenum. If your house doesn’t have a forced-air heating system, however, you’ll need to investigate other solutions.

(click illustration to enlarge)

One solution is a powered makeup-air unit that tempers incoming air during the winter with electric-resistance heating coils. Such units include a supply-air fan wired to turn on when the kitchen exhaust fan is activated. Electro Industries has one such system (model EM-WH1025K; about $1800), which includes a 10,000w heater and a blower, and can supply 632 cfm of makeup air while maintaining a 50°F temperature rise.

A 1200-cfm range hood would need two of these units. In addition to their high upfront cost, these systems carry a severe energy penalty whenever they are turned on.

Small ducts squeeze big fans
In theory, a makeup-air duct should be at least as large as the duct connected to the range-hood fan. The sizing calculation is complicated, however, for several reasons. As an exhaust fan begins to depressurize a house, some of the necessary makeup air is inevitably drawn through cracks that exist in the building’s envelope. This means that the air flowing through a makeup-air duct is only a fraction of the makeup air needed.

Also, there is a dynamic relationship between the airflow through an exhaust duct and the airflow entering a makeup-air duct. As the motorized damper in the makeup-air duct opens, some (but not all) of the necessary makeup air will begin flowing into the house through the duct. If the home’s envelope is tight and the makeup-air duct is undersize, the exhaust fan won’t be able to perform at its rated specification. In other words, a 1200-cfm exhaust fan may be exhausting only 900 cfm. This happens because the volume of air leaving the house and the volume of air entering the house are always balanced. If the makeup-air duct in this example were larger, the exhaust fan would experience less resistance and might be able to ramp up to 1100 cfm. These dynamic effects complicate duct-sizing calculations.

Broan recommends one 6-in. duct for a range hood up to 500 cfm, one 8-in. duct for a range hood rated at 501 cfm to 1000 cfm, and two 8-in. ducts for a range hood rated at 1001 cfm to 1500 cfm.

Tight-house solution: A recirculating hood and an HRV
Code requirements for kitchen ventilation vary widely from jurisdiction to jurisdiction. If your local building official is willing to accept the installation of a recirculating range hood that doesn’t exhaust any air to the exterior, you can sidestep the makeup-air dilemma entirely. This approach has been pioneered by Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. builders, who usually try to avoid unnecessary exhaust fans or makeup-air supply ducts when they are building.

(click illustration to enlarge)

Passive House designers often specify a recirculating range hood connected to a replaceable charcoal filter. They also install an exhaust grille on the kitchen ceiling as far away from the stove as possible. The grille is connected to the exhaust duct of a heat-recovery ventilation(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. (HRV) system. The exhaust grille is located away from the stove to limit the amount of filter-clogging grease reaching the HRV.

Why can’t range-hood exhaust ducts be designed to recover heat?
Many mechanical ventilation systems are designed with a pipe-within-a-pipe system that allows heat to be exchanged between the outgoing and incoming airstreams. Range hoods can’t do that for two reasons. First, the pipe diameters get big fast. If your range hood requires a 10-in.-dia. exhaust duct, then the duct-within-a-duct solution would require a 14-in. or 16-in. duct. It’s hard to find that much room for ductwork in a house. Elbows compound the awkwardness.

Second (and the biggest problem with the pipe-within-a-pipe idea), the incoming cold air would cool the exhaust duct, encouraging the suspended grease in the exhaust air to congeal and moisture in the exhaust airstream to condense. The cooling effect of the incoming air makes an exhaust duct become dirtier than if it had stayed warm. This process is similar to what happens when woodstoves are vented to outdoor chimneys; cold flues become clogged with creosote much faster than warm flues.

Small range hoods don’t need dedicated makeup air
Because the sizing guidelines provided by range-hood manufacturers or kitchen-equipment dealers often result in the installation of oversize fans, some experts advise ignoring such guidelines. If you choose a residential-size (30 in. wide) range, a fan rated at 160 cfm to 200 cfm will keep you out of trouble. One model you might consider is the Broan 40000 series fan (available in stainless steel for $80); it’s rated at 160 cfm.

The simple way to avoid backdrafting: choose a range hood with a small fan. Low-cfm fans won’t depressurize your house and are quiet.

Sources: http://www.Broan.com; http://www.GEAppliances.com; http://www.Whirlpool.com. Photos: top left, courtesy of Broan; bottom left, courtesy of Whirlpool; right, courtesy of Kaplan Thompson Architects.

Martin Holladay is a senior editor. Drawings: John Hartman. Photos: Charle Miller, except where noted.

From Fine Homebuilding No. 232 — Download a PDF of this article.


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Image Credits:

  1. Charles Miller

1.
Jun 8, 2015 1:07 PM ET

Location of make-up air
by Malcolm Taylor

If you are going to supply un-tempered make-up air, I wonder what difference there is in where it is located? A motorized damper and grill in the backsplash of the range might mitigate comfort and heat-loss concerns. Does anyone know if there has been any studies showing whether the proportion of air exhausted that comes from the vent, not the room, varies significantly based on it's location?


2.
Jun 8, 2015 1:54 PM ET

Edited Jun 8, 2015 2:45 PM ET.

Response to Malcolm Taylor
by Martin Holladay

Malcolm,
Ideally, the range hood exhaust fan will pull away the smoke and odors emanating from the stove. The problem with your backsplash idea, which has been tried, is short-circuiting: the exhaust fan often ends up pulling most of its air from the makeup air grille, allow the smoke from the roasting meat to drift into the kitchen.


3.
Jun 8, 2015 2:39 PM ET

Good Point
by Malcolm Taylor

I still can't but think that a directed supply air could somehow be more efficient. Years ago I worked on a couple of high-tech manufacturing facility designs. Their air supply and exhaust was situated to take the contaminants produced directly out of the rooms without affecting the products being produced. The same is true of hospital operating rooms with their vertical air flows.
Someone with more ingenuity than me might come up with a solution.


4.
Jun 8, 2015 3:27 PM ET

Makeup air design
by Carl Seville

Martin - Although this is about commercial equipment, http://www.fishnick.com/ventilation/ventilationlab/ has some really good info, including some downloadable PDFs on this link on how to design effective makeup air systems. It addresses the short circuiting you mention as well. It would be nice to see this level of research on residential systems.


5.
Jun 8, 2015 3:41 PM ET

Providing Makeup Air (CEC and BSC Resources)
by Kohta Ueno

As a general warning on providing makeup air--in general, you want to let the warm plume of air rising off the cooking surface rise **into the hood** to be exhausted. Introducing lots of makeup air too close to this "plume" will disrupt it, and often result in worse capture efficiency. Even concepts that seem to make sense--like a "curtain" of air at the front of the hood--can result in disrupted airflow and worse capture efficiency. The less you disrupt the plume, the better it works.

There's a great report from the California Energy Commission using schlieren (flow visualization) images to show what's going on. It looks like it's the same (or similar) material that's in ASHRAE HVAC Applications.

http://www.energy.ca.gov/reports/2003-06-13_500-03-034F.PDF

Also, for those who want the entertaining version of kitchen exhaust a la Joe:

BSI-070: First Deal with the Manure and Then Don't Suck
http://www.buildingscience.com/documents/insights/bsi-070-first-deal-wit...


6.
Jun 8, 2015 3:44 PM ET

Why can’t range-hood exhaust ducts be designed to recover heat?
by Kohta Ueno

Just for folks' reference--in **commercial** kitchens, range hood heat recovery does exist--per the Environmental Building News cover story from February 2010. But I've never heard of it for residential. The story emphasizes just how hard of a problem it is to deal with cooking grease, per Martin's column.

Capturing the heat from CKV seems like common sense, and the technology has been around for years, but major manufacturers no longer sell systems, mainly because of one big problem: grease.

Grease in cooking effluent accumulates on hoods, ducts, fans, and even building roofs, and it poses a serious fire hazard. Heat exchangers provide an ideal spot for this grease to accumulate. The typical method for removing the grease from these systems is through an automatic washdown system, but these can be expensive to buy and maintain, and they consume a lot of water. Despite these challenges, heat-exchange technology is poised for a comeback. Innovent (www.innoventair.com) is selling a flat-plate heat exchanger for CKV that includes automatic washdown. The company does not recommend the unit for use with broilers or other heavy-duty equipment, but it could be cost-effective in an institutional setting or school cafeteria.


7.
Jun 8, 2015 4:47 PM ET

Response to Kohta Ueno
by Martin Holladay

Kohta,
Thanks for the useful links. And thanks for backing up my analyses of the problems associated with exhaust fan short-circuiting and gummed-up heat-exchange systems.


8.
Jun 9, 2015 8:52 AM ET

"Dynamic Relationships" = depressurization
by Brian Just

Martin, your "dynamic relationships" seem to be a euphemism for, or maybe a distraction from, the depressurization that accompanies large exhaust and inadequate make up air. The code requires make up air for large exhaust to minimize depressurization. Designing for reduced exhaust due to small make up air openings seems to embrace depressurization as a solution to its own problem.
I like Broan's approach: give us really big (passive) openings. Once a designer contemplates the opening required to match the range exhaust, perhaps they'll consider reducing the size of the range fan.


9.
Jun 9, 2015 9:23 AM ET

Response to Brian Just
by Martin Holladay

Brian,
My use of the phrase "dynamic relationship" was neither a euphemism nor a distraction. I am well aware of the problem of depressurization. Although the magazine editors at Taunton chose to omit most references to depressurization in this article, referring instead to backdrafting problems, there is no doubt that the entire focus of the article is on the problem of depressurization.

For a more extensive discussion of these issues, you can read the online article on which this magazine article was based: Makeup Air for Range Hoods. In that article, 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."

I certainly don't recommend that readers design insufficient makeup air systems to "embrace depressurization," as you put it.

The dynamic relationship I discussed is a fact. It is based on physics; it is not a description of desirable outcome. As an exhaust fan ramps up, infiltration through cracks in the envelope increases. The change in the infiltration rate happens as a direct result of the depressurization caused by the exhaust fan -- not because a designer wants it to happen.


10.
Jun 9, 2015 12:27 PM ET

Edited Jun 9, 2015 12:38 PM ET.

Gas cook tops vs. induction
by Peter Whitman

It seems like one of the contributing factors in the make-up air issue is the choice of cook top. The commercial gas ranges that are all the rage produce huge amounts of heat and off-gas, and also have high ventilation requirements.
In the highly insulated home we are building, our HRV contractor recommends an induction cook top with small hood. When we need to turn on the ventilation system, we switch the boost on the HRV.
Any comments?
As usual, thank you in advance.


11.
Jun 9, 2015 1:00 PM ET

Edited Jun 9, 2015 1:12 PM ET.

Response to Peter Whitman
by Martin Holladay

Peter,
Like you, I prefer range hood fans to be small. As I wrote in the article, a range hood fan rated at 160 cfm to 200 cfm is a good way to go.


12.
Jun 10, 2015 7:19 PM ET

Peter
by Malcolm Taylor

Another GBA article you may find interesting:
http://www.greenbuildingadvisor.com/blogs/dept/green-building-news/hazar...


13.
Jun 16, 2015 11:27 AM ET

Gas vs. induction
by Charlie Sullivan

Peter, yes, it is a good idea from an air-quality perspective to run a hood at least on low whenever you have a gas burner on. That probably negates the efficiency advantage of gas vs. electric, and now that we have the induction option, the combination of induction with a small (200 cfm or less) hood seems good. Then you only run the hood if the cooking is generating stuff you want to vent.


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