How Central Return Grilles Create Unintentional Comfort Problems, and How to Solve Them
Extensive supply duct systems with registers in each room can cause comfort issues
Each summer, homeowners call HVAC contractors to solve comfort problems they're sick of experiencing. Typical complaints are uncomfortable rooms, increased humidity, uncontrollable dust, and odd smells. These problems are frustrating for contractors because they are random and difficult to recreate.
Many systems where these complaints occur have central return grilles. The most common locations for these grilles are the ceiling, sidewall, or floor in an open area. These installations also have extensive supply duct systems with registers in each room. Let's look at why this typical installation often contributes to unintentional comfort problems and how you can solve them.
One CFM In = One CFM Out
Our industry measures airflow in cfm (cubic feet per minute). To visualize cfm, look into a 12 x 12 x 12 cardboard shipping box. One cfm of air is what’s inside. On a properly operating HVAC system, for every cfm that goes into the central return grille, the same amount of air should come out of the supply registers.
Let's say you have a three-ton system operating in cooling mode. If you assume 400 cfm per ton, the blower should move 1,200 cfm (400 x 3 = 1,200). A perfect installation would have 1,200 cfm of air pulled into the central return and 1,200 cfm of air blowing out of the supply registers.
If the duct system is leak-free and all interior doors are open, the system has balanced airflow into and out of the home. However, what happens when you close an interior door? How does that affect airflow to the central return grille?
Interior Doors Are Dampers
Interior doors act like manual dampers in a duct system. They control airflow movement through a home when their position changes from open to closed. Door position directly affects how air moves from supply registers back to the central return grille.
The area with the central return depressurizes (pulls a vacuum) when an interior door is closed. In the summer, depressurization causes elevated indoor humidity levels and dust as moist and dusty air pulls inside and mixes with conditioned air. The added load can overwhelm the system to a point where it cannot remove enough moisture. Any dust pulled in will often bypass the air filter and settle on furniture and flooring.
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On the supply air side, rooms with closed interior doors pressurize the space (blow out). The closed door blocks the supply airflow pathway to the central return. Supply airflow to each room will also decrease when a door is shut. If you have intermittent comfort problems, complaints, and a central return, look for this interaction.
A Real-World Example
My favorite example of this happened in a private training class a couple of years ago. There was a complaint of a problematic bedroom, so we focused on this area. As we talked to the homeowner, we discovered the comfort problem in this room only happened in the evening. We took airflow measurements using a balancing hood with the bedroom door open and then closed to see how the door position would affect supply airflow.
Supply register airflow was 171 cfm with the door open. That reading was close to the airflow amount needed based on our calculations. Next, we closed the door and took another supply airflow reading. To everyone's surprise, supply airflow dropped to 143 cfm — almost 30 cfm less. The light bulbs came on for the class because the influence of door position were visible with an air balancing hood.
Air Balance Issues
Similar issues can appear in systems with a supply and return in each room if the airflow is out of balance. This problem won't show up when the interior door is open but will once the door closes. The closed door isolates the room from the rest of the home.
When return airflow out of a room exceeds supply airflow, negative room pressure occurs. More return air is pulled from the room than supplied to it. Increased humidity and excessive dust are common complaints when this situation exists.
When supply airflow into the room exceeds return airflow, positive room pressure will occur. More supply air is delivered into the room than returned. Conditioned air is pushed outside the home when this wasteful situation happens.
Solving Pressure and Airflow Imbalances
Contrary to popular belief, interior doors undercut an inch won't solve this airflow issue. In some situations, the door undercut must be the size of a bathroom stall door to work correctly.
Another option is to use jumper ducts and transfer grilles with central returns. The jumper duct connects problem rooms to the central return area to relieve room pressure when a door is closed. Building leakage can also act as an unintentional jumper duct, so inspect the room first for can lights and attic access panels.
Ducted supplies and returns to each room are the best repair for this situation. Both ducts need dampers to control room airflow. This repair assures balanced airflow and pressure with a closed door. It isn't a viable option in some homes, so do the best you can with what you have.
In the end, it's about airflow control. Unless your solution involves airflow and pressure measurement, you're guessing whether the repair worked.
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