Dealing with a High-Capacity Range Hood
Big fans need lots of makeup air, but where does it come from?
A range hood that whisks away cooking odors, moisture, and grease is almost always recommended as a way of keeping indoor air healthy. But what happens to the equation when the range hood is a behemoth, capable of sucking up 900 cubic feet of air per minute?
That’s the dilemma that Sean McLoughlin is facing as he designs a 3,500-sq. ft. house in southern California. The kitchen will be outfitted with a professional-size range and barbecue grill.
“We’ve had one before — we used it 4 or 5 nights a week — and they produce huge amounts of heat and smoke,” McLoughlin writes in a Q&A post at GreenBuildingAdvisor.
California requires that houses with high-power exhaust fans include equipment to supply makeup air, to ensure that the exhaust fan doesn’t depressurize the house, and McLoughlin is trying to sort out the details of how to accomplish that.
“The HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. system we're planning will include an outside air exchanger (not an energy-recovery ventilator(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 heat-recovery ventilator(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. , just a controlled damper) ducted into the return before the filter stage on each of the two forced air units/systems in the house,” McLouglin says.
“Is it possible that those two sources of outside air can be tied in to the exhaust hood to provide sufficient makeup air for the kitchen exhaust hood? Or should we be planning to add non-centrally tied in make up air ducts direct from outside using automatic dampers?”
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McLoughlin’s search for a practical solution to this problem is the starting point for this week’s Q&A Spotlight.
How about a change in plans?
Anticipating an obvious reaction, McLoughlin says he’d rather not hear suggestions to install a smaller kitchen range or simply open a window when the hood is running. But rethinking his kitchen plans is an option he’s urged to consider.
GBAGreenBuildingAdvisor.com senior editor Martin Holladay notes that McLoughlin's question has come up before. Holladay recommends a previous article, Makeup Air for Range Hoods. “In general, ‘huge amounts of heat and smoke’ aren’t very compatible with indoor air quality, especially in an energy-efficient house,” writes Holladay. “That’s why in centuries past, especially in rural homes of the well-to-do, cooking was often done in a separate outbuilding behind the house.”
These “summer kitchens” were especially popular in the South, where the combination of suffocating summer temperatures and a hot stove could make cooking indoors a hellishly uncomfortable chore. Getting the heat out of the house was the answer.
“If it were me, I might consider adjusting the architectural program to put this virtual commercial kitchen in a separate building outside the dwelling,” adds Kevin Dickson. “A breezeway connection to the house might give you the result you want.”
John Klingel wonders whether there’s an “aesthetically pleasing “ way of temporarily enclosing the kitchen with large double-doors or an accordion wall that would, in effect, create a kitchen outbuilding. In any event, he adds, “call me when the burgers are on.”
Keep the HVAC system separate
McLoughlin’s initial thinking would somehow use the HVAC system to provide makeup air for the range. But Armando Cobo argues the two should never be tied together.
“You can possibly double the size of your HVAC system with that amount of [makeup air],” Cobo writes. “1,200 cfm requires about a 12"-14" pipe coming from the outside; that is a huge amount of uncontrolled and unfiltered air coming into your house.”
As far as Cobo knows, Broan is the only manufacturer that makes an automatic damper to provide makeup air. It automatically opens up when the exhaust fan is turned on.
Broan, in fact, provided the solution for John Clarke when he faced a similar problem. Although Clarke didn’t install an indoor barbecue, the range hood was still robust and the house was supertight (blower-door tested at 0.5 air changes per hour at 50 pascals of 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., exceeding PassivhausA 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. requirements for airtightness). “My discussions with appliance retailers yielded a lot of blank stares and questions about what makeup air was,” Clarke says.
The solution was to install a 6-in. duct connected to a Broan MD6T motorized damper assembly controlled by a current-sensing relay.
“Broan advertises that the damper is only for their specially equipped hoods but it is simply a 24v damper that has rubber sealing rings on the damper assembly,” Clark adds. “The 'tricky' part is that the motor needs to be activated only when the fan is operating, hence the current sensing relay. In our case, the lights did not draw enough current to trigger the relay but the blower motor, even on low speed, triggers the relay and the damper opens.”
Clarke bought all the components for the system (not including the ductwork or external vents) for less than $200.
Our expert’s opinion
Here's what GBA technical director Peter Yost had to say:
It’s hard to add anything on this topic beyond Martin’s excellent coverage in his blog, Makeup Air for Range Hoods. But maybe it will help to think of it this way:
A 900 cfm kitchen range hood is essentially an air handler directly connected to the outdoors. With the proper makeup air, when this exhaust fan is running, the kitchen will be mechanically rendered outside the house. This is imposing a mechanical solution to a design problem. The design problem is locating a commercial-scale cooking station inside the home. That’s OK, so long as the unavoidable and staggering energy penalty is factored into the design decision.
To me, this is the same as locating the master bedroom suite 125 lineal feet from the tank water heater located in the garage (yes, this can and does happen). You can get hot water to the master suite’s bathroom by installing an on-demand (high-speed) recirculation pump and insulating all 125 feet of hot water piping, but that is a mechanical solution to a design problem.
This may sound crazy, but here is what we do at our home in the summer: we set up our Coleman cook stove outside right next to our gas grille and do all of our cooking outdoors to avoid dumping all that heat into the house on those blazing hot summer days. But frankly, on the craziness scale, it’s probably right up there with the indoor barbecue grille and 900 cfm kitchen range hood.
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