How to Check a Closed Room for Pressurization

Why this matters

The complaint is usually "the back bedroom is always hot" or "the kids' rooms never warm up at night," and the usual response is to balance dampers or add a supply. In a lot of houses the room already has plenty of supply and the air simply cannot get out. A bedroom with supply registers and no return turns into a small balloon when its door closes: the supply pushes the room positive, the room pushes its air out through cracks in its walls and ceiling instead of back to the return, and the supply flow drops because it is pushing against that pressure. Meanwhile the rest of the house runs negative, pulling outdoor or attic air in. You cannot see any of this. A manometer and fifteen minutes show it room by room, and the sheet you fill in is what decides which rooms need a return path and which do not. Why closed rooms behave this way belongs to Return Air Starvation and the Closed Bedroom Door; choosing the fix belongs to Transfer Grilles, Jump Ducts and Door Undercuts.

Before you start: closing every interior door with the blower running pulls the main body of the house negative. If there is an atmospheric gas or oil appliance in the main body (a water heater in a hall closet, a furnace in an open basement), wear a personal CO monitor and check its draft hood with smoke while the doors are shut, as How to Screen for Spillage After You Add an Exhaust Fan describes. If it spills, open the doors, stop the test there and route it to a qualified gas technician. If you smell gas, everyone leaves, no switches or phones are used inside, and you call the gas utility from outside.

The sheet

Fill this in as you go. The steps below explain each line.

CLOSED-ROOM SHEET       AH blower: high (cooling)
Exhaust off, ext doors and windows closed
Gauge zeroed in hall 14:05   Out 88 F  In 76 F  calm
Main body WRT outdoors, int doors open:     -0.3
Main body WRT outdoors, int doors shut:     -2.1

Room      Sup  Ret  Door gap  Room WRT hall  Flag
Bed 1      2    0    1/4 in       +4.6       YES
Bed 2      1    0    3/8 in       +2.4
Bed 3      1    0    1/4 in       +3.8       YES
Office     1    0    1/2 in       +1.9
Primary    2    1    1/2 in       +0.6
Hall bath  1    0    3/4 in       +1.1
Doors closed at night (owner): Bed 1-3, Primary

The steps

1. Ask which doors close, and when. A room whose door is never shut cannot pressurize. Write down which doors the household closes at night or while someone works inside. Skip this and you flag a guest room nobody closes and miss the nursery that is shut sixteen hours a day.

2. Close the house to the outdoors and turn every exhaust device off. Exterior doors and windows shut, bath fans, hood and dryer off, fireplace damper as found. The test is isolating what the air handler does to each room; a running bath fan pulls its room negative and masks the supply. Skip it and the bathroom line, at least, is unreadable.

3. Run the blower at its highest-airflow speed and let it settle a minute. On most systems that is the cooling speed. Call for cooling only when the outdoor temperature is inside the equipment's operating range; in cold weather use the heating call or the fan-only setting at its highest available speed, and write the speed on the sheet. The pressure a room develops rises with the airflow into it, so a low fan speed understates the worst case the room sees.

4. Zero the gauge in the hallway and read the main body with all doors open, then all closed. Put the outdoor reference hose out a door or window, sheltered from wind (the setup is in How to Read House Pressure With a Manometer). The open-door reading is the baseline. The shut-door reading shows how far the main body is pulled negative when the bedrooms trap their air, which is the number that matters for combustion appliances and for air drawn in from the attic and garage.

5. Close one door at a time and read the room against the hallway. Run the short hose under the closed door into the room, with the door resting lightly on it; the input port reads the room and the reference port stays open to the hall. Positive means the room is above the hallway. Wait 30 seconds, take a time average, write it down, open that door and move to the next. One door at a time keeps each reading about that room alone; with every door shut, each room reads a little higher because the hall itself has dropped.

6. Count supplies and returns and measure the door gap. Note each room's supply registers and whether it has any return grille, and measure the gap under the door to the finished floor (carpet, not subfloor). The gap is the room's only built-in relief, and the counts tell you whether a high reading comes from a lot of supply or from no way out.

7. Flag every room over about 3 Pa. About 3 Pa room to main body is a common field threshold in energy programs and building science practice; at that level a room is pushing a meaningful share of its supply out through its own envelope. Treat it as a starting point and tune it for your climate and your shop's experience. Skip the flag and the sheet becomes a list of numbers that nobody acts on.

8. Put the house back. Return the thermostat and fan setting to how you found them, open the doors the customer keeps open, and pull the hoses.

Reading this sheet

Two rooms are flagged, and both are bedrooms that close at night. Bed 1 reads +4.6 Pa with two supplies and no return under a 1/4 in door gap. Bed 3 reads +3.8 Pa with one supply, no return and the same 1/4 in gap. Both close at night, so both run at that pressure for hours.

Bed 2 is under the flag, but not by much. It reads +2.4 Pa with one supply and a 3/8 in gap. The larger gap is doing some of the relief; if new flooring raises that floor or a thicker carpet goes in, it will cross 3 Pa. Note it and tell the customer.

The primary bedroom is the control. Two supplies, one return, +0.6 Pa. That is what a room with a way out looks like, and it is the proof that the air handler is not simply oversized for every room.

The office and the hall bath read under the flag for different reasons. The office has a 1/2 in gap and one supply, so its gap is relieving it. The bathroom's 3/4 in gap is large enough to relieve one supply. Neither is closed at night, so neither needs work regardless.

The main body line is the other finding. With every door shut the main body went from -0.3 Pa to -2.1 Pa, a 1.8 Pa drop (-2.1 minus -0.3). On this summer afternoon that pulls hot attic air in through ceiling leaks. If this house had an atmospheric water heater in a hall closet, the same drop would add to whatever the exhaust fans do, and you would record both on the job and hand them to the gas tech rather than judging the combination yourself.

What the sheet does not settle. It shows which rooms need relief, not how big the relief must be. A grille or jump duct sized to bring a room under the threshold depends on the room's supply airflow, which is a flow measurement at the registers, and on the path's free area. That sizing is in Transfer Grilles, Jump Ducts and Door Undercuts, and the cutting is in How to Add a Transfer Grille to a Closed Bedroom.

What a skipped step looks like later

• The test was run with the blower on low (heating speed on a two-speed blower in summer). Bed 3 read +2.2 Pa and went unflagged. At cooling speed it is +3.8 Pa, and the hot-room complaint comes back in July.
• The doors were all closed at once and only the rooms were read. Every room read higher by about the hall's own drop, and the Office crossed 3 Pa on paper. A transfer grille went into a room that never needed one, cutting a sound path between two bedrooms for nothing.
• The bathroom was read with its fan on. It read negative, was written as "fine", and the next tech assumed it had a return.

What changes the call

• A cold climate in heating season. A positive room pushes warm, moist indoor air out through its exterior walls and ceiling, where it meets cold sheathing and the underside of the roof deck. The comfort complaint is the same, but the moisture is a second reason to fix a flagged room, and a room just under the flag in a cold climate deserves a closer look than the same reading in a mild one. See related: The Cold Surfaces Where a House Condenses First.
• A zoned system. When one zone's damper closes, more air goes to the open zones, so a room's pressure depends on which zones are calling. Run the sheet with only the zone serving the bedrooms calling, which is the worst case for those rooms, and note the zone on the sheet.
• A variable-speed blower that ramps with demand. The pressure follows the airflow, so read the rooms at the highest speed the system actually reaches in service, not at a light-load speed it happened to be running when you arrived.
• A ductless head in the room instead of a ducted supply. A wall-mounted ductless unit recirculates room air and adds none from the central system, so it does not pressurize the room. A closed room with only ductless conditioning is not a candidate for a return path, however warm it runs.
• A supply-only or balanced ventilation system with an outlet in the room. A ventilation supply adds a small, continuous flow into the room around the clock. It rarely reaches the flag alone, but it adds to the air handler's push, so run the test with the ventilation system on as it normally runs and note it.

How to confirm a fix worked

After a return path goes in, repeat steps 2 through 5 for the treated rooms at the same blower speed and write the new readings on the same sheet, beside the old ones. A successful fix brings each treated room under about 3 Pa with its door shut, or, if it does not, the sheet shows by how much it missed, which tells the installer whether to enlarge the path or add a second one. Repeat the all-doors-shut main body reading too; it should rise back toward the open-door baseline, because the bedrooms are now returning their air instead of trapping it.

References

• See related: Return Air Starvation and the Closed Bedroom Door; Transfer Grilles, Jump Ducts and Door Undercuts
• See related: How to Add a Transfer Grille to a Closed Bedroom; How to Read House Pressure With a Manometer
• See related: The House as One Pressure System; How to Screen for Spillage After You Add an Exhaust Fan
• Equipment manufacturer's installation manual for blower speed taps and the outdoor operating range for cooling