Ventilation Guidance for Residential Kitchen with Gas Stove

Recently the residential gas stoves were in the News due to the generation of pollutants during their operation. Cooking activities as well as the combustion of natural gas through gas stove burners produce several indoor pollutants. Exhausting these pollutants from the kitchen using mechanical ventilation with range hoods or over-the-range microwave hoods is essential for reducing occupant exposure to cooking-related indoor pollutants. However, several field studies indicate that the installed flow rates from these devices are lower than the flow rates specified in the product literature. Field observations also show that the actual usage of the kitchen ventilation during cooking is low. Occupants often do not use their range hood due to the lack of awareness of the benefits of kitchen ventilation. About 47.10 million households or roughly 38% of all households in the US have a gas stove. The inadequate use of ventilation during cooking necessitates proper education and guidance in using kitchen ventilation to reduce the exposures.

We performed systematic analyses using a simple mass balance approach and Computational Fluid Dynamics (CFD) simulations to evaluate the impact of several parameters on the concertation levels of Nitrogen dioxide (NO₂) and resulting occupant exposure. This first blog presents the mass balance results and guides in operating the kitchen exhaust based on these results. A separate blog is developed to explain the CFD analysis results.

Analysis with the Mass Balance Approach

The mass balance approach considers the kitchen space as a large well-mixed control volume without consideration for the spatial variations in the contaminant concentration within the space. It predicts the transient variation of the “volume-averaged” contaminant concentration by balancing the rate of contaminant generation with the rate of contaminant removal by the ventilation air. The parameters evaluated were the exhaust air flow rate, delay in starting the exhaust fan after starting the burner, the duration for which the exhaust fan is left running after the burner is turned off, and the size of a kitchen.

Figure 1: Variation of NO₂ concentration and resulting occupant exposure with time for various exhaust flow rates showing the exhaust flow rate should be at least 100 cfm. It also shows that during the first few minutes of cooking the exposure level of NO₂ remains below the EPA acceptable level of 100 ppb-hr.

Figure 2: Variation of NO₂ concentration and resulting occupant exposure with time for various delay times in starting the exhaust fan indicating that the exhaust fan should be started within 10 minutes of starting the burner to keep the exposure below the acceptable level.

Figure 3: Variation of NO₂ concentration and resulting occupant exposure with time for various durations for which the exhaust fan is kept ON after the burner is off shows that running the exhaust fan for at least five minutes would keep the exposure below the acceptable level.

Guidance for Operating the Kitchen Exhaust Fan

Based on the above mass balance analyses the following guidelines are developed:

• Kitchens should operate with an exhaust fan flow rate of at least 100 cfm.
• The exhaust fan should be started within ten minutes of starting the burners (cooking).
• The exhaust fan should be kept on running for at least five minutes after the cooking.

These analyses further indicate that homes with open floor plans (larger kitchens) would experience relatively lower exposure than the small enclosed kitchens. In general, the occupant exposure levels within the first few minutes of cooking remain below the acceptable level.

The limitations of this study are that the mass balance approach assumes instantly well-mixed conditions and cannot predict the spatial variations of pollutants within the space. To predict the concentration in the breathing zone of occupants a detailed three-dimensional transient Computational Fluid Dynamics (CFD) analysis is required.