What actually sets a building's static pressure?

The residual of the air ledger: whatever imbalance is left between the air brought in and the air taken out, pushed through the envelope's leaks.. Building pressure is bookkeeping: outside air in (the deposit), exhaust plus relief plus exfiltration out (the withdrawals), and pressure is the tiny residual it takes to force the leftover air through the envelope's cracks. Supply and return never appear on the ledger — they circulate inside the envelope without crossing it — so a bigger supply fan doesn't pressurize the building directly; it only moves the ledger by dragging more outside air in through whatever opening the dampers allow.

Building-pressure setpoints sit slightly positive — around +0.02 to +0.05 in. w.c. (+5 to +12 Pa) — rather than at zero. Why?

So the envelope leakage always flows outward — conditioned air seeping out instead of unfiltered, unconditioned, humid air seeping in.. A neutral building has no margin: any gust, stack effect, or exhaust fan tips the leakage inward, dragging in dust, pollen, and — worst in humid weather — moisture that condenses inside wall cavities. Held slightly positive, every crack leaks conditioned air outward instead. The number is deliberately small; much past +0.05 in. w.c. (+12 Pa) and the doors start fighting their closers.

A large AHU's floors are served by dedicated exhaust fans — gang restrooms plus a janitor closet — pulling a total of 2,200 CFM (3,740 m³/h) that never returns to the unit. The designer wants an 800 CFM (1,360 m³/h) surplus to hold the building positive. What is the smallest outside-air flow the minimum position must deliver? Enter the answer in CFM.

3000 CFM. The ledger, run in the design direction: minimum outside air must cover everything the exhaust fans steal from the unit's territory plus the pressurization surplus — 2,200 + 800 = 3,000 CFM (3,740 + 1,360 = 5,100 m³/h). Size the minimum below that and the building opens every morning already negative, with the dampers sitting innocently at their commanded position. The return duct never brings back everything the supply duct delivered.

A barometric relief damper can pull an over-negative building back toward neutral.

False. False — barometric relief is strictly a one-way valve. Its weighted blades are opened by inside pressure winning against gravity; a negative building pulls them tighter shut. It can bleed off a positive building (slowly, riding a standing pressure error), but against a negative one it does exactly nothing. A negative building needs more air in — a higher OA minimum or makeup air — and no relief device of any kind can supply that.

A power-exhaust fan and a return fan can look alike on a drawing — a fan near the return side of the unit. What actually separates them?

A return fan lives in the airstream and runs whenever the unit runs; power exhaust lives in the relief opening and only has a job when there's surplus air to throw away.. Opposite rules for neighbors. Every cubic foot returning to the mixing box passes through a return fan, so it's interlocked to the unit — on with the supply fan, always. Power exhaust sits in the side door off the return path, and at minimum outside air there is almost no surplus to relieve: run it there and it isn't relieving the building, it's evacuating it. Confusing the two is how a correct-looking program drags a building negative all winter.

On the classic staged scheme, what tells a power-exhaust fan to start?

Outside-air damper position crossing a threshold — the fan infers the surplus from where the dampers are.. The classic scheme is open-loop: as the economizer drives the dampers past a threshold, stage one starts; further open, stage two joins; back down the same ladder as they close. The fan never measures building pressure — it follows the dampers, which is coarse but honest. Smarter systems close the loop with a building static sensor modulating the exhaust fan on a VFD. Either way the enable answers to the dampers or the pressure they create — the supply fan's status is at most a run-permissive, never the trigger.

A January morning. The front doors are heavy, and the vestibule whistles. The BMS shows the readings below. What's the story? OA DAMPER CMD 20 % — minimum SF-1 STATUS ON EF-1 (PWR EXH) ON BLDG STATIC −0.08 in. w.c. (−20 Pa) OA-T 18.0 °F (−7.8 °C)

The power exhaust is running against minimum outside air — its enable is interlocked to the supply fan instead of following the damper.. Read the witnesses. The doors and the whistle corroborate the sensor — the building really is negative, so it isn't (b). Weatherstripping (a) changes how loudly a negative building complains, not whether it's negative — leakage is where the pressure shows up, not why. And a slightly-low minimum (d) can't produce −0.08 in. w.c. (−20 Pa) on its own. The smoking gun is the pairing: EF-1 running while the dampers sit at 20 %. Power exhaust has no job at minimum position — there's no surplus to relieve — so if it's on whenever SF-1 is on, someone applied the return-fan rule to the wrong fan. Fix the enable: call the fan with the damper, not with the unit.

An economizer drives to 100 % outside air on a mild morning — but the relief damper's actuator seized shut last month and nobody noticed. What does the building do?

It goes strongly positive: the unit force-feeds its whole supply airflow in as outside air, and the building inflates until doors stand open and its own leakage carries the flow.. At 100 % outside air, everything the supply fan moves is a deposit — and with the relief path seized, the ledger has no working withdrawal except the envelope itself. Pressure climbs until the leakage (and the standing-open doors, which are just very large leaks) carries the whole surplus. This is the promise the Economizers lesson filed: the relief damper opens in step with the OA damper because every extra cubic foot admitted has to leave. Nothing reverts automatically — no controller is watching a point that would reveal the failure unless someone trends building static.

A kitchen hood at full draw against an AHU sitting at minimum outside air has pulled the building negative. What category of fix actually works?

More air in — raise the OA minimum during hood operation, or provide dedicated makeup air.. Note the asymmetry the lesson keeps returning to: relief devices can only take air out. A negative building is a ledger short on deposits, and every relief-side move fails its own way: more power exhaust makes it worse, barometric blades just seal harder, and a relief damper driven open becomes a backwards infiltration path — unfiltered outside air sneaking in through a bigger hole, which is not a fix. The fix is on the intake side: interlock a higher OA minimum with the hood, or give the hood its own makeup-air unit. Door hardware treats the symptom while the building keeps sucking its makeup air backwards through every crack in the envelope.

Where does the indoor probe of a building-pressure sensor belong?

In a large, representative space mid-building — a corridor or open office — away from exterior doors, elevator lobbies, and anything that swings or gusts.. The signal is a whisper — a setpoint of +0.03 in. w.c. (+7.5 Pa) is fifty times smaller than a supply duct's static — so placement is most of the craft. Every door transit is a pressure spike the loop must not chase, which rules out the vestibule even though that's where the symptoms live; you want the average building, not its noisiest corner. The supply duct is a different pressure entirely (that's duct static), and the roof is the outdoor reference's side of the measurement, not the indoor's.

Practice · Building Pressure

Building Pressure Quiz Air Systems

Read the Building Pressure lesson first, then keep the ledger from memory — ten questions down the lesson walk: what actually sets a building's pressure, why the setpoint sits slightly positive, sizing the OA minimum against the exhaust it must cover, what barometric relief can and can't do, the power-exhaust-vs-return-fan distinction, and the heavy-door diagnosis that convicts a wrong interlock. Each reveal links back to the section it came from.

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