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Undercut Doors

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    Door undercuts are commonly used to provide a return air pathway from rooms with closed doors
    Scope

    Provide for pressure balancing between bedrooms and the rest of the house.

    • Install ducted returns or a combination of ducted returns, transfer grilles, jump ducts, and/or door undercuts in bedrooms to allow pressure balancing between bedrooms and the rest of the house in homes with ducted heating and cooling systems by providing a path for stale air to return to the return side of a central air handler.
    • ENERGY STAR Single-Family New Homes requires that the dedicated return ducts, transfer grilles, jump ducts, and/or door undercuts together achieve a rater-measured pressure differential of ≥ -3 Pascals and ≤ +3 Pascals (0.012 inch water column) with respect to the main body of the house when bedroom doors are closed and the air handler is operating on the highest design fan speed. A rater-measured pressure differential of ≥ -5 Pascals and ≤ +5 Pascals (0.020 inch water column) is acceptable for rooms with a design airflow ≥150 cfm.
    • If dedicated return ducts are installed in each bedroom, contractors must seal all seams, gaps, and holes of the return duct system with mastic and seal the return box to the floor, wall, or ceiling with mastic, caulk, and/or foam.
    • If transfer grilles or jump ducts are used, refer to the balancing report provided by the HVAC contractor for the bedroom air flows to size the grilles or ducts. Ensure that both openings have the required free area. Seal all seams, gaps, and holes in the ducts and connections.
    • If door undercuts are used, pay attention to the door material. For hollow doors, you may not want to undercut or you might need to provide a bottom support after making the undercut, as the typical bottom supports are roughly 1 inch tall.
    • Test the pressure differential with the bedroom doors closed.

    See the Compliance Tab for related codes and standards, and criteria to meet national programs such as ENERGY STAR Certified Homes, DOE’s Zero Energy Ready Home programENERGY STAR Single-Family New Homes, and EPA Indoor airPLUS.

    Description
    Description

    Door undercuts are a commonly used method for pressure balancing between rooms (Figure 1) but they are not a recommended approach as they do always provide an adequate return air pathway for HVAC systems. Seasonal changes in wood can result in variations in the height of the door undercut opening. Changes to the flooring material, such as adding carpet, can obstruct the return pathway. The ENERGY STAR pressure-balancing thresholds are difficult to achieve with just a door undercut for most applications, except for rooms with minimal supply airflow (<20 cfm). In most applications, the door undercut would need to be 1.5 inches or greater to allow enough air to pass. There are some through-the-door alternatives that can be effective. Consider installing a transfer grille in the door or employing another return air pathway method, such as transfer grilles or jump ducts. The video on the Training tab shows one of these options, transfer grilles.

    Door undercuts are commonly used to provide a return air pathway from rooms with closed doors.
    Figure 1. Door undercuts are commonly used to provide a return air pathway from rooms with closed doors. (Source: Steven Winter Associates.)

     

    Success
    Ensuring Success

    To determine if an adequate pathway exists for air to return to centrally located returns, the following room-to-room pressure measurement can be used:

    1. Turn on the air handler to high.
    2. Close all interior doors.
    3. Using a manometer, connect tubing to the input port. The reference port for the differential pressure measurement can remain open.
    4. While standing in the center of the house or hallway, place the tubing from the manometer under each door and record the pressure difference from each room with respect to the main body of the house (note the presence of a negative or positive sign). The bedroom will typically be pressurized (positive) when the doors are closed.
    5. ENERGY STAR requires that rooms should not be pressurized or depressurized by more than 3 Pascals for any room being supplied with less than 150 cfm of conditioned air. If the supplied airflow to a room exceeds 150 cfm, a threshold of ±5 Pascals is required. These are good metrics to strive for regardless of whether or not pursuing ENERGY STAR certification. 
    Climate
    Climate

    No climate specific information applies.

    Training
    Right and Wrong Images
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    Door is not undercut therefore not contributing to pressure balancing
    Door is not undercut therefore not contributing to pressure balancing
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    Door has been undercut to allow for specified amount of air flow therefore contributing to pressure balancing
    Door has been undercut to allow for specified amount of air flow therefore contributing to pressure balancing
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    Right – Transfer grilles, Jump ducts, and wall grilles provide passive returns for air returning from bedrooms to the central HVAC system
    Right – Transfer grilles, Jump ducts, and wall grilles provide passive returns for air returning from bedrooms to the central HVAC system
    Videos
    Compliance

    Compliance

    The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

     

    ENERGY STAR Single-Family New Homes, Version 3/3.1 (Rev. 11)

    National Rater Field Checklist

    HVAC System.
    6. Duct Quality Installation - Applies to Heating, Cooling, Ventilation, Exhaust, & Pressure Balancing Ducts, Unless Noted in Footnote.
    6.2 Bedrooms pressure-balanced (e.g., using transfer grilles, jump ducts, dedicated return ducts, undercut doors) to achieve a Rater-measured pressure differential ≥ -3 Pa and ≤ +3 Pa with respect to the main body of the house when all air handlers are operating. Test configuration and an alternative compliance option in Footnote 37.37

    Footnote 37) Item 6.2 does not apply to ventilation ducts, exhaust ducts, or non-ducted systems. For an HVAC system with a multi-speed fan, the highest design fan speed shall be used when verifying this requirement. For an HVAC system with multiple zones, this requirement shall be verified with all zones calling for heating or cooling simultaneously; additional testing of individual zones is not required. When verifying this requirement, doors separating bedrooms from the main body of the house (e.g., a door between a bedroom and a hallway) shall be closed and doors to rooms that can only be entered from the bedroom (e.g., a closet, a bathroom) shall be open. As an alternative to the ± 3 Pa limit, a Rater-measured pressure differential ≥ -5 Pa and ≤ +5 Pa is permitted to be used for bedrooms with a design airflow ≥ 150 CFM. The Rater-measured pressure shall be rounded to the nearest whole number to assess compliance.

    Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable in in your state.

     

    Associated Air Barrier Council (AABC) 

    AABC National Standards for Total System Balance 2002. The manual details the minimum standards for total system balance.

     

    National Environmental Balancing Bureau (NEBB)

    NEBB Section 15990 – Testing, Adjusting, and Balancing. NEBB is a certification association whose members perform testing, adjusting and balancing (TAB) of heating, ventilating, and air-conditioning systems and commission and retro-commission building systems. This document is the TAB procedural standards.

    Retrofit
    Existing Homes

    Door undercuts are not a recommended approach to provide an adequate return air pathway for HVAC systems. An alternative is to install a transfer grille in the door or consider other return air pathways methods, such as installing transfer grilles through the wall or jump ducts through the ceiling. 

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    More Info.

    Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

    References and Resources*
    Author(s)
    Building Science Corporation
    Organization(s)
    BSC
    Publication Date
    Description
    Article discussing the risks that can occur from undercutting bedroom doors to provide return airflow from bedrooms, as well as corrective measures that can be taken to address these risks.
    Author(s)
    Guertin Mike
    Organization(s)
    Fine Homebuilding Magazine
    Publication Date
    Description
    Central return designs require an open pathway between the room and the heart of the house where the return grill is located.
    *For non-dated media, such as websites, the date listed is the date accessed.
    Contributors to this Guide

    The following authors and organizations contributed to the content in this Guide.

     Steven Winter Associates, Inc., lead for the Consortium for Advanced Residential Buildings (CARB), a DOE Building America Research Team. And the Pacific Northwest National Laboratory.

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