Return Air Starvation and the Closed Bedroom Door

Why this matters

The most common comfort complaint in a ducted house is the bedroom that is fine with the door open and miserable with it shut. Shops chase it with a bigger supply register, a booster fan or a damper adjustment, and none of it holds, because the room is not short of supply. It is short of a way out. Air that cannot leave a room stops coming in, and while it tries it pushes house air through the bedroom walls and pulls outdoor air into the rest of the house. That turns a comfort call into an energy, moisture and sometimes a combustion question.

The mechanism in one paragraph

A ducted system is a loop. The blower pushes air out through supply registers and pulls the same amount back through return grilles. Many houses have one or two central returns in a hallway and supplies in every room, which works only while every interior door is open and the house is one big room. Close a bedroom door and that room has a supply blowing in and no return. The air piles up until the room's pressure rises enough to force the same amount out through whatever cracks exist: the door undercut, the gaps around the door, and the outlets, top plates and window frames that connect the bedroom to the attic and outdoors. Meanwhile the central return is still pulling its full share out of a hallway that is now missing that room's air. The house pressures that result are owned by The House as One Pressure System; this card owns what the closed door does to them.

What the door actually does, drawn

 PLAN, blower running, bedroom door shut
 +----------------+----------------------------+
 | BEDROOM        |  HALLWAY / MAIN BODY       |
 |  SUPPLY in     |                            |
 |   -> air piles |   central RETURN pulls     |
 |   up, room     |   air OUT of main body     |
 |   goes (+)     |                            |
 |                |  main body goes (-)        |
 | room air leaks |  outdoor air leaks IN      |
 | OUT to attic   |  at walls, can, flue       |
 | and outdoors   |                            |
 +-------++-------+                            |
 |  DOOR || 1/2 in undercut, mostly blocked    |
 |  SHUT || by new carpet                      |
 +-------++------------------------------------+
 Room (+) to hall. Hall (-) to outdoors.

Two separate pressure differences come out of one closed door. The bedroom sits above the hallway. The hallway, and the rest of the main body, sits below outdoors, because air is being pulled out of it faster than the closed rooms return it. The first one is a comfort and duct problem. The second one is the one that pulls attic dust in at recessed cans, pulls humid outdoor air into wall cavities in summer, and can work against a natural-draft water heater sitting in the same main body.

The three symptoms, and why each one happens

The room is too hot or too cold with the door shut. As room pressure rises, the pressure difference across that supply register falls, and the register delivers less air. A register that delivered its design airflow with the door open may deliver noticeably less with it shut. The customer feels it as "the room never gets there".

The door whistles, slams or pushes open. The pressure across the door is pushing air through the gap at speed. A door that drifts open on its own when the blower starts is a strong field sign of a pressurized room.

The bills and the dust go up, and nobody connects them. Air forced out of the bedroom through the ceiling carries conditioned air into the attic. In winter that air carries house moisture into the cold attic, where it can condense on the roof deck. In summer the negative main body pulls hot attic air and outdoor humidity in at every leak. The comfort complaint is the visible part of a whole-house leak driver.

The field target, and where it comes from

A common field target is about 3 pascals or less between a closed room and the main body of the house, measured with the blower running and every other interior door in its normal position. A pascal is a very small pressure, roughly the push of a light breeze on a wall; see What a Pascal Is and Why House Pressures Are Tiny for the scale. The target is a working convention used by many weatherization and new-construction programs rather than a single model-code number, so if a program or specification governs the job, its number governs instead. On a new system the return path is a design item under ACCA Manual D; this card is about the house that already has the problem. It is set low because the house's own natural driving pressures are themselves only a few pascals, and a fan-driven difference larger than them dominates how air moves through the shell.

Worked example: measuring the opening the room needs

This method avoids sizing rules entirely. You let the room tell you how much opening it needs to hold the target, then you compare that to what the door already provides.

The house: a two-story with a central return at the top of the stairs. Bedroom 3 has one supply register. The door is 32 inches wide. The original 3/4 inch undercut was reduced when new carpet went in, and the gap now measures about 1/2 inch from the carpet face to the door bottom.

Step 1, read the closed room. Blower on high cooling speed, all other interior doors as the family keeps them, bedroom 3 shut. With the manometer's reference tube run under the door into the hallway, the room reads plus 7 pascals with respect to the hallway.

Step 2, find the opening that holds 3 pascals. Open the door slowly while watching the gauge. The reading falls as the gap widens. At a gap of 1-1/4 inches along the latch edge, the room reads plus 3 pascals.

Step 3, compute what that gap represents. The door is 80 inches tall, so the latch-side gap is 80 times 1.25, which is 100 square inches. That gap was working alongside the existing undercut, 32 times 0.5, which is 16 square inches. So the total opening that holds 3 pascals is 100 plus 16, or 116 square inches.

Step 4, subtract what is already there. The undercut already supplies 16 square inches, so the room needs roughly 100 square inches of additional free path to the hallway. Notice the pressure only fell from 7 to 3 while the path to the hall grew from 16 to 116 square inches, about sevenfold. Had the undercut been the room's only exit, that much opening would have dropped the reading far more. The gentle fall says a real share of the room's air was leaving through the ceiling and walls, which is the leak this test exists to expose, and the fix closes both problems at once.

Step 5, read the answer. A 100 square inch addition cannot come from the undercut: on a 32 inch door that would be another 100 divided by 32, a little over 3 inches of gap, which nobody accepts. So the realistic options are a transfer grille, a jump duct or a dedicated return, and the choice among them belongs to Transfer Grilles, Jump Ducts and Door Undercuts. The cutting steps belong to How to Add a Transfer Grille to a Closed Bedroom.

What to watch in this method. The gap area you measure is gross area: an open crack with nothing in it. A grille's free area is smaller than its face, often much smaller, so the grille is chosen by its published free area, not its nominal size. And read the room at the blower speed it runs most. A room that passes at low fan speed can fail on high cooling speed.

What changes the answer

The room has its own return. Then this card does not apply to that room. Check that the return actually draws, since a return grille on a disconnected or crushed duct is decoration.

The system runs a low fan speed most of the time. Pressures scale with airflow, so a variable-speed blower running at part speed for most hours sees smaller differences than the high-speed test shows. Test at high speed anyway, because that is when the closed room hurts most and when the main body goes most negative.

A ductless or room-by-room system. A ductless head or a through-wall unit recirculates within the room and has no central return to starve. The closed-door pressure problem is a ducted-system problem.

A combustion appliance shares the main body. If a natural-draft water heater or furnace sits in a space that goes negative when bedroom doors close, the closed-door test is also a combustion question and goes to a qualified gas technician for a worst-case test. See related: How a House Pulls Flue Gas Back Down a Vent.

The failure mode in the field

The classic wrong fix is adding supply. A tech sees a warm bedroom, opens the damper to that room or swaps a bigger register, and the room gets slightly better with the door open, where it was already fine, and no better shut, because the limit was never the supply. Worse, the extra supply raises the closed-room pressure, pushes more air out through the ceiling, and pulls the main body further negative. The second wrong fix is a booster fan in the supply boot, which does the same thing harder.

The other failure is the remodel that lowers the undercut without anyone noticing. New carpet, a thicker pad, a door replaced with one cut flush: each one quietly removes a return path the house was depending on. The flooring installer and the finish carpenter are the trades that create this problem most often, and neither of them has a reason to think about air.

How to tell the read was right

You have read the room correctly if the closed-door pressure is written down with the fan speed it was taken at, the additional opening is computed from that measurement rather than guessed, and you have checked that the main body's pressure with respect to outdoors also moved in the right direction once a path was added. Recheck it after the fix: the room should read about 3 pascals or less with the door shut and the blower on high, or if the fix fell short, the new reading tells you how much more path is needed using the same gap method. Cite the job to the homeowner by both numbers, before and after. See related: Return Air Deficiency Assessment for when the problem is the whole return system rather than one room.

References

• See related: Return Air Deficiency Assessment
• See related: The House as One Pressure System; What a Pascal Is and Why House Pressures Are Tiny
• See related: How to Check a Closed Room for Pressurization; How to Add a Transfer Grille to a Closed Bedroom
• See related: Transfer Grilles, Jump Ducts and Door Undercuts; The Bedroom That Was Only Cold With the Door Shut
• ACCA Manual D residential duct design practice, for return path design on new systems