Controlling Pollutants Caused by Cooking
by Paul Eldrenkamp
One of Byggmeister’s goals with each project is to improve indoor air quality. Cooking, it turns out, is a major source of indoor air contamination. A recent Lawrence Berkeley Laboratory study found that “a significant portion of residences exceed outdoor air quality standards for several pollutants on a weekly basis as a result of cooking with gas burners.” Cooking with electric burners can also generate contaminants, but not to the same levels as gas burners.
As with other indoor pollutants, we implement a twofold strategy: source control and dilution. Source control means doing what we can to avoid introducing the contaminants to begin with; dilution means providing adequate ventilation to minimize the concentrations of any pollutants that are generated despite our attempts at source control.
Ventilation – bringing in fresh outdoor air and exhausting stale indoor air – is essential, but it should be done judiciously. There’s an energy price to be paid for ventilation. You’ve paid to heat (or cool) that stale indoor air that you’re exhausting; you also have to pay to heat (or cool) that fresh outdoor air that you’re bringing in. So you want a Goldilocks approach to ventilation — not too much, not too little, but just enough (click here to learn about our overall approach to ventilation).
Best practice and good building science requires us to install a range exhaust hood – a point-source ventilation system adequately sized to remove indoor contaminants. Range hood exhaust capacities are typically expressed in “cfm” (cubic feet per minute).
Furthermore, the building code states that “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.”
This means that if a homeowner selects a kitchen range for which the manufacturer calls for an exhaust hood with a capacity of more than 400 cfm, we need to install a system that brings in an equal amount of outdoor air while the range hood is operating. This is like opening a window on a cold winter day and installing a box fan in the window, blowing in. Our opinion is that we should work closely with our clients during the design phase to avoid triggering this requirement.
A 400 cfm exhaust fan does not require a dedicated make-up air system because, in general, homes are leaky enough that an exhaust flow of 400 cfm can be compensated for by means of air infiltration through inevitable air leaks in the building enclosure. This assumption can be risky in certain circumstances, especially in very tight homes or homes with wood burning stoves or fireplaces, but the Byggmeister team can help manage those risks.
Unfortunately, a range hood’s cfm rating does not necessarily tell you much about its actual effectiveness at removing water vapor, gas combustion byproducts, and particulates generated by cooking. Poorly designed or installed ductwork can constrain the exhaust; we once tested a range hood that was rated at 1200 cfm and found that it was actually exhausting 21 cfm. The geometry of the range hood also affects its ability to remove contaminants. This is known in the industry as “capture effectiveness.” A range hood that is only half as as deep as the range itself, for instance, will allow a large percentage of cooking byproducts to escape into the room and will have poor capture effectiveness. A downdraft typically has poor capture effectiveness, which it tries to overcome with a more powerful fan.
In practice, here’s what this all means:
We will evaluate the existing air quality and ventilation in a house, determine what air quality improvements the homeowner would like to make, and place all our subsequent recommendations regarding cooktops and range hoods in that context.
We will hope that our clients will select an induction cooktop rather than a gas cooktop. This will help minimize the quantity of pollutants we need to manage. Induction cooktops require cooking pots and pans that are ferrous (a magnet will stick to the bottom) but otherwise there are no special techniques or requirements. Several members of our team cook on induction cooktops, and all are well fed.
Regardless of the fuel source (gas or electric), we will hope that our clients select a cooktop that does not require a range hood with a capacity greater than 400 cubic feet a minute (cfm) of exhaust. This will yield the most economic, efficient, and comfortable outcome.
We will also work with our clients to select a range hood with good enough “capture effectiveness” that it can actually do the work it’s supposed to, regardless of cfm rating.
We will install the range hood according to code and to manufacturer’s specifications.
Finally, we will test the installation to make sure we’ve done everything correctly and that the fan functions as intended.