The Pennsylvania Housing Research Center Kitchen Ventilation Systems: Part 2 Providing Adequate Makeup Air Builder Brief: April 2012 Anthony C. Jellen, PE & Brian M. Wolfgang, EIT, Michael A. Turns, MS INTRODUCTION and require larger exhaust systems leading to higher The first Builder Brief of this two-part series operation costs, and increased risk of house discussed the relationship between kitchen exhaust depressurization and associated hazards. In addition, rates, house tightness, and house depressurization. every cubic foot of exhausted air is a cubic foot of That brief also provided an overview of the health makeup air that must be heated or cooled at the and safety hazards associated with house homeowner’s expense. depressurization and the presence of combustion appliances. The required exhaust rate for standard, low-powered, residential kitchen ranges is typically specified per In this brief, we provide design guidance for linear foot of range. Table 1 shows recommended introducing makeup air for a residential kitchen exhaust rate per linear foot (LF), according to the exhaust system using three common techniques: (1) Home Ventilating Institute (HVI). engineered openings, (2) HVAC-integrated air Table 1. HVI recommended and minimum ventilation systems, and (3) dedicated makeup air units. We will rates for kitchen range hoods. also discuss common design practices for meeting the interlocking and closure requirements of the LOCATION Rec. Vent. Rate Min. Vent. Rate 2009 International Residential Code (IRC) Section per LF of Range per LF of Range M1503.4. Wall 100 CFM 40 CFM Island 150 CFM 50 CFM PROPER RANGE EXHAUST SELECTION Kitchens that contain high-powered, commercial- The objective of a kitchen range exhaust system is to style cooking equipment will require a higher capture moisture and airborne contaminants created exhaust rate. The installer should always check the during cooking, and vent them to the outside. The equipment specifications for exhaust system selection of an exhaust system is largely dependent recommendations. If no information is available, on the size and type of range, and its intended use. equipment should be classified by type, application Wall mounted hoods are the most common exhaust and power consumption, and sized in accordance system used in the residential kitchen; however, with the ASHRAE Handbook (HVAC Applications island canopy hoods and downdraft systems have section), or other reputable source. risen in popularity in recent years. The goal of an exhaust system specifier should be to MAKEUP AIR REQUIREMENTS provide an adequate, but not excessive, amount of exhaust without jeopardizing occupant safety or Once the exhaust rate is specified, the designer can comfort. First, a homeowner or homebuyer should determine the need for makeup air. As discussed in assess their cooking habits and select appliances Part 1 of this Builder Brief series, Section M1503.4 based on need rather than a “bigger is better” of the 2009 IRC requires makeup air at a rate mentality. Larger appliances demand more power roughly equal to the exhaust rate for systems capable of exceeding 400 CFM. That brief demonstrated that 1 the 400 CFM threshold, although seemingly interior pressure becomes increasingly negative. The arbitrary, is appropriate for tight to moderately tight effects are hard to predict and highly dependent on residential enclosures. Figure 4 in Part 1, shows that the location of the opening and connectivity of the potential for depressurization exists at almost any rooms within the house. Mechanical systems exhaust rate; therefore, it is prudent for a designer to introduce makeup air into the house by a fan, where conduct some type of risk assessment for all it can be routed directly to the kitchen, or drawn into installations. See Part 1 of this Builder Brief series, the central HVAC system and distributed throughout or ASTM E1998, Standard Guide for Assessing the house. When properly installed, a fan-assisted Depressurization Induced Backdrafting and Spillage solution should eliminate any adverse pressure from Vented Combustion Appliances. effects created by the exhaust system. Figure 1 illustrates the effects of both methods on building pressurization and the neutral pressure plane. INTRODUCING MAKEUP AIR #3 #4 Properly introducing makeup air is essential to ensuring a safe, comfortable, and efficient home. Improperly specified makeup air systems can lead to NPP uncomfortable drafts, overstressed HVAC NPP equipment, mold, and house depressurization. An ill- F F “X” “X” “X” conceived exhaust and makeup air system design can “Y” CFM CFM CFM be expensive as well as non-functional. CFM F To maintain occupant comfort, most design guidance recommends providing a makeup air supply Figure 1. Comparison of air movement in a house with an temperature within 10 degrees F of the room engineered opening (Scenario #3) versus a house with fan-assisted a makeup air system (Scenario #4). In temperature. For example, if a house is maintained Scenario #3, the neutral pressure plane rises, and the level at 70 degrees F in the wintertime, the makeup air of house depressurization is less predictable. In Scenario should be provided at a supply temperature of no #4, pressures in the house mirror those when no exhaust less than 60 degrees F. or makeup air systems are installed (natural conditions). Design considerations for makeup air systems that exceed supply rates of 400 CFM are similar to that ENGINEERED OPENINGS of a commercial kitchen or industrial exhaust system. Thus, it is highly recommended that a An engineered opening is an intentional opening HVAC expert be involved in the design process. placed in the enclosure for the purposes of Significant quantities of makeup air will increase transferring air between the interior and exterior of a loads placed on the central HVAC system. The building. An opening could be as simple as a hole HVAC system or building enclosure will likely placed through an exterior wall, or may include require modification to function efficiently with the ductwork to a remote space such as the kitchen. newly introduced makeup air. Figure 1 shows a typical configuration for an engineered opening that supplies makeup air directly Makeup air can be introduced in many ways with into the kitchen. varying degrees of effectiveness. The two most common methods are engineered openings and Providing one or more engineered openings in the mechanical systems. Engineered openings in the enclosure is a simple and inexpensive technique to enclosure allow air to be drawn into the house as introduce makeup air. The effects are similar to that 2 Motorized damper - Closed except during periods of exhaust Makeup air distributed to kitchen Intake hood with insect screen Toe-kick diffuser or Fresh air into house grille Duct – Properly sized, R-8 insulation preferred Figure 2. Typical engineered opening. The engineered opening in this diagram allows makeup air for a range hood to enter the house into the same room that the exhaust air leaves. This will help to alleviate negative pressure that may result from range hood operation. of opening a door or window. Typically a damper is Part 1 of this series (wind, stack effect, and HVAC incorporated into the opening design to minimize equipment). Table 2, which is derived from Equation heating and cooling costs and to ensure airflow only 36 of Chapter 16 in the 2009 ASHRAE Handbook of during times of depressurization or exhaust Fundamentals, shows estimates of airflow through operation. typical opening sizes (diameter) at various pressure differentials. Note that engineered openings only Airflow through engineered openings depends make sense for small amounts of makeup air. primarily on the area of the opening and pressure Pressures of 3 Pa or greater create a risk of difference across the opening. The pressure backdrafting fireplaces and certain combustion differential depends on all the factors discussed in appliances. Table 2. Airflow through an engineered opening. For example, say a homeowner wants to install an 800 CFM Kitchen exhaust fan in a house that Pressure Airflow (CFM) Based on Opening Size contains a masonry fireplace with no outdoor intake. (Pa) 4 in 6 in 8 in 10 in 12 in The HVAC professional calls for a blower door test, 1 14 30 54 84 122 which results in an air tightness level of 2,500 CFM 2 19 43 76 119 172 at 50 Pa. The HVAC professional specifies a single 3 23 53 94 146 211 10” makeup air duct routed directly to the kitchen 4 27 61 108 169 243 area. Would this installation be adequate? 5 30 68 121 189 272 6 33 74 132 207 298 Part 1 of this series demonstrated the pressure 7 36 80 143 223 322 differential between inside and outside of the house 8 38 86 153 239 344 should be kept below 3 Pa to avoid dangerous 9 41 91 162 253 365 backdrafting of combustion appliances. According to 10 43 96 171 267 384 Table 2, a 10-inch opening can only supply 119 3 CFM of makeup air at a pressure of 2 Pa. Clearly the distributes makeup air throughout the house along opening is inadequate to provide the required 800 with the return air. CFM of makeup air while maintaining safe pressure levels. The makeup air is mechanically drawn in by the air handler through an opening provided in the Even if air infiltration through the building enclosure enclosure. An HVAC technician balances the flow is included in the analysis, this installation falls rate of the incoming fresh air to the desired rate. short. Interpreting Figure 4 from Part 1 of this series When the range hood is not in operation, the opening in a slightly different way, it can be determined that is typically closed automatically by a motorized infiltration could provide about 300 CFM of makeup damper that is interlocked with the range hood air while maintaining a safe pressure of 2 Pa.