The Essential Ingredients
By ROBERT BEANEngineering Houses Indoor Air Quality Residential Buildings
I am smiling as I write this article because I think we might actually be making progress in the world of kitchen exhaust and make-up air systems. Let me explain. While en route to a recent project meeting I was fully expecting to educate another architect, interior designer, appliance salesperson and owner about the perils and pitfalls of commercial gas fired ranges in residential buildings.
You know the conversation. It is the one where an appliance’s sexiness induces a brain fog that causes some creative types to head butt the basic laws of physics and chemistry. But to my elation this team had been doing their homework, having studied the most recent indoor air quality (IAQ) research involving kitchen cooktops and range hoods from a scientist out of Lawrence Berkley National Laboratory (LBNL).
Let’s take a step back (and have some fun with food puns) and ask why would LBNL get funding to look at something that seems to be as routine as poutine. I mean fixing up dinner is pretty much “well done” – right?
Well, it turns out (again) that the things we do on autopilot in our homes are coming back to bite us in the rump roast. It is now apparent to researchers that the smorgasbord of pollutants we sense as aromas, heat and moisture from indoor cooking are reaching concentration levels, which if measured outdoors would have environmental protection agencies shutting down kitchens and issuing fines.
Since there are no environmental protection regulations governing indoor residential kitchens, your lungs, skin and digestive systems have become the de facto filter for a soufflé of carbon monoxide, nitrogen dioxide, formaldehydes, volatile organic compounds, polycyclic aromatic hydrocarbons, fine and ultrafine particles and other pollutants associated with meal preparation. Toss in the exposed interior design features and what is left behind is an accumulation of contaminants in the form of chemical films, soot and odours on surfaces, similar in affect to what one finds in the homes of smokers.
To be clear, like many IAQ issues, the concern is not so much the exposure from a single meal preparation but rather the accumulation of hundreds and thousands of exposures to irritants known to be responsible for respiratory problems and cardiovascular disease. And once again those highest at risk are women and children.
This does not matter if it is not a surprise to you but it does matter that it is a big surprise to most consumers. This explains why residential kitchen ventilation system must be seen as part of an aggregate solution to IAQ problems. It also draws attention to the importance of incorporating cooktop exhaust at the early stages of design and why it should not be placed in the hands of appliance dealers.
HOW MUCH IS ENOUGH?
What do various codes and standards have to say about exhaust air? In Canada (subject to depressurization limits) you will find text along these lines, “…the NBC requires that make-up air systems be provided for any exhaust device (or group of devices operated from a single control) which exhausts more than 75 L/s (150 cfm)” and, “Clothes dryers, central vacs, range hoods, downdraft cooktops, supplemental exhaust fans and HRVs with unbalanced defrost cycles are examples of appliances that may require make-up air.”
In the U.S., the Department of Energy advises, based on industry standards, to use a minimum five ACH which in a 20′ x 10′ x 9′ kitchen works out to 75 L/s (150 cfm).
These statements lead us to ask, “How much exhaust air should be needed?” It is no surprise that it depends on the characteristics of the home and its ventilation system (moisture, infiltration and back drafting concerns), meal preparation (culture, style, habits and hygiene) and the characteristics of the appliances. For example, there is a big difference between stir frying a chicken versus boiling its eggs, whether that is done on a gas or electric range, and whether it is done in a leaky or tight home. For this reason, designers are cautioned to not use recommend minimum values as maximum in practice.
The people who write codes, standards and guidelines can hardly be expected to know how your client’s house will perform or be operated after constructed, nor can they anticipate the meal culture of that specific family. Rates have been developed based on appliance characteristics –so let’s talk about cooktops.
Gas ranges are commonly available with various combinations of 9000, 12000 and 18000 Btuh per burner. A conservative four burner range could easily reach 45000 Btuh and for the uber chef as much as 108000 Btuh. To put this in perspective, with that latter amount, you could heat an 11000 ft2 Type 4 high performance home in the coldest climates on earth.
Electric ranges are commonly available in a resistive coil or sealed surface type typical for halogen and induction models with element capacity’s ranging from 700 W to 3700 W and in cooktop widths of 15″, 30″, 36″, 48″ and 60″. Table 1 provides general guidance for minimum exhaust air flow rates from either electric or gas ranges with various hood types. The operative word in the preceding sentence is “general” since the additional factors of home characteristics and cooking culture must be considered in selecting this value. Note there is a strong voice against the perceived high rates published in Table 1 but the arguments against such high rates are based on concerns of back drafting of combustion devices and not the generation and repeated exposure to cooktop associated pollutants. The latter being the argument I support.
First things first, the hood for collecting or trapping the cooking plumes should be seen as a capture device and not the suction end of a vacuum. In other words, hoods strictly do not suck but the duct connected to the hood does. This is not semantics. It is much like the comparison between supply air diffusers and placement of return air grills; or swimming pool outlets and placement of inlet. You have to motivate the air or water with positive pressure into the vicinity of the suction for it to be pulled out of the space. That fluid stream in the vicinity of the exhaust hood can be disturbed or even interrupted with competing air velocities rendering the exhausting of pollutants ineffective (see Figure 1).
Hood and grease filter design is an art as much as a science. Its effectiveness is based on the physics of energy and mass flow, the necessary geometry for capture and proximity to the cooktop (see Figure 2). At a minimum, wall hoods must be at least as wide as the cooking surface. The better design is to allow for an overhang of at least two inches on either side and over the front of the range.
Island hoods should have at least a three-inch overhang on all sides meaning a 36″ cook top should have a 42″ hood width. Downdraft “hoods” must be as wide as the cooktop but unless recent technological improvements have been made to increase their effectiveness, I personally do not recommend them. Likewise I do not recommend flat or shallow sumps but do favour the deeper sumps.
The distance from the cooktop to the leading edge of the hood (a.k.a the clearance height) affects capture effectiveness. This is in part dictated by building codes, which in Canada generally state the minimum clearance required directly above the cooktop burner or element to a combustible surface is 750mm (30″) or 600mm (24″) to non-combustibles. In contrast, the U.S.-based International Code Council states, “A clearance of at least 24 inches (610 mm) shall be maintained between the cooking surface and the combustible material or cabinet.” What is the explanation for the difference in clearance to combustibles (750mm (30″) in Canada versus 610mm (24″) in the U.S.)?
Though the closer the hood is to the cooktop the better, there comes a “distance of pract
icality” for accessibility for cooking, cleaning and maintenance. For distances greater than 30″ and with wider hoods, a higher capacity fan is required. Designers need to consult with the appliance manufacturer for additional clearances, as well as any special requirements from the local authorities having jurisdiction (AHJ).
In Canada, required clearances around the cooktop and hood are found in the NBC 2010 Section 9.10.22, 9.32.2, 9.33.3, 9.33.5 and Appendix items A-9.10.22, Figure A-9.10.22 and A-126.96.36.199.(10).
THE EXHAUST DUCT AND FAN
The exhaust duct from the range hood to the wall or roof hood should be as short as possible but not at the single expense of noise, otherwise occupants will refrain from using the system. Where possible, have at least 24” to 36” before elbows and transitions and install silencers as directed by manufacturers. Do not install ribbed flex connectors or any conduit that could capture and accumulate grease. The duct must be constructed with a galvanized sheet metal or stainless steel (no flex duct), and be completely sealed and terminated with a screened hood with a back draft damper. Pollutants must be exhausted directly outdoors and be independent of any other exhaust system (i.e. not connected to an ERV/HRV or other exhaust system or ducts used in the ventilation of the building). It must be insulated and horizontal runs sloped, and if condensation is a risk, provisions must be made to capture the condensate.
In my practice I select kitchen exhaust duct diameters and fans to maintain between 1100 and 1500 fpm to ensure heavier pollutants remain airborne but know this is an area of contention. Some designers use more aggressive values and some more conservative. Because of these higher air velocities and potential for noise I am not a proponent of integrated fan/hoods. I prefer to place inline type blowers more towards the exterior rather than closer to the cooktop. If the entire system can be designed for three to five sones measured at the remote inline fan you will be in the ballpark for acceptable noise (normal conversation is about four sones).
Make-up air systems should be designed according to the requirements of CSA F326 or ASHRAE 62.2 or where permitted, GM-1501. In the U.S. the International Code Council states, “Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with make-up air at a rate approximately equal to the exhaust air rate. Such make-up air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system.”
This is considerably higher than the 150 cfm required in Canada (again subject to depressurization limits); and the damper controlled make-up air interlocked with the exhaust fan to control pressure might be a simple solution in moderate climates but in extreme climates this could create serious thermal comfort and building moisture issues. For this reason you can find statements like this, “The NBC requires that make-up air be heated to at least 12 °C if it is introduced directly to a living area in the house.” In my opinion this is much too low when standalone high capacity ventilation systems are used and in practice we discharge to temperatures within a few degrees of room temperature.
For noise consideration we select fans and duct diameters for 600 to 800 fpm and select registers for low noise and placement so the supply air velocity does not disturb the cooking plumes. Preheat or reheat coils should be protected with MERV 8 filtration and a minimum MERV 11 downstream of the coils in the primary supply air stream. Coils, if hydronic, should have freeze protection. I prefer constant circulation with a glycol mix to outdoor design conditions (see Figure 3) but we have also used variable flow using three way
The biggest challenge with make-up air systems is the lack of capacity coordination between the exhaust hood and the make-up air fan. More often than not we come across designs where the exhaust fan is variable speed and the make-up air is multi speed or vice versa. Sometimes this cannot be avoided so it makes it tricky to create balanced ventilation flow. My advice to manufacturers of residential kitchen exhaust and make-up systems is to provide identical motor and control configurations.
Obviously this is not the whole meat and potatoes. There is more to the discussion but you do get a taste for what should be considered for a safe and functional residential cooktop exhaust and make-up air system. All puns intended.
Robert Bean, who is president of Indoor Climate Consultants Inc., is a Registered Engineering Technologist in building construction through the Association of Science and Engineering Technology Professionals of Alberta and a Professional Licensee in mechanical engineering through the Association of Professional Engineers, Geologists and Geophysicists of Alberta. He has served two terms as an ASHRAE distinguished lecturer, serves on ASHRAE committees TC 6.1 (Hydronics), TC 6.5 (Radiant), TC 7.4 (Exergy) and SSPC 55 (Thermal Comfort) and is a recipient of ASHRAE’s Lou Flagg Award.