Return air ventilation. Often overlooked, always needed.

For a while we had three children in the same bedroom. Upon entering the room for the first time each morning it felt “stuffy” like there wasn’t enough air in the room. After adding a ceiling fan and continually adjusting the dampers for the supply vents, i still could not fix the issue.

A lack of proper flow will cause a room to constantly stay hot or cold. It can cause odors, mold, and all sorts of general nastiness. So besides being uncomfortable, this issue just had to get fixed.

Getting air into a room is fairly straightforward. Run a branch line off an air conditioning handler supply trunk to the room. This is always checked when a builder is building a house. Does the room have a vent? Yes! A room must always have a vent to be considered livable space….building inspector checks the box and moves on.

Less apparent and one of the most overlooked problems with HVAC flow (especially in bedrooms) is the air finding a return path back out of the room to your air conditioning system. In order for air quality to be good (right temperature, humidity, and quality) it needs to have a proper flow or exchange rate. So the air has to cycle continually into the room and then back out.

One of the most popular ways air gets out of rooms is the gap under the doorway. This is commonly called the “undercut” at the bottom of the door. Without this essential pathway the room builds up “back pressure” and air just sits in the room until the door is opened again. In some houses, enough traffic comes and goes or rooms are used infrequently enough that this lack of flow is barely noticed. However, back pressure also causes a strain on the overall efficiency of your HVAC system.

A survey of my home revealed a wide set of variances in doors with no undercut to large undercuts leading me to believe there was no real concerted effort to ensure any level of proper airflow.

Door 1. Example of door with little to no return air flow.
Door 2. Example of an “undercut” door with good airflow.

There is an easy fix for this. Measure from finished floor to preferred undercut height. Mark with pencil on the door, remove door, draw a horizontal line, cut a strip off the bottom with a circular saw. DEWALT 20V MAX 7-1/4-Inch Cordless Circular Saw with Brake Kit (DCS570P1)

Put door back on hinges. Viola! Air flow.

Notes:

Some hollow doors may be problematic to cut. Pay attention to the construction of the door or you may need to add back in bottom support for the door making this a much more painful and involved project.

You may want to consider a through door vent. Tamarack Technologies has some decent looking return air solutions. https://www.tamtech.com/product-category/transfer-grilles-rap/ Some include a baffle to reduce noise transfer while maintaining airflow. Here is one on Amazon. Tamarack Technologies Return Air Pathways (14X8 Retrofit)

A 1” undercut provides 47 cfm (cubic feet per minute) of return air from a room on a 30” wide door.

Transfer grilles or jumper ducts should be used when even more return air is required

https://www.buildingscience.com/documents/information-sheets/information-sheet-wrong-undercutting-doors

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