Building Pressure Air Systems
Two pages of this chapter have now made the same promise: the air the dampers bring in has to leave the building somewhere. This page is where it leaves. Building pressure is the quietest point in the whole system — a signal so small most gauges can't see it — and it announces its failures at the front door, literally: doors that stand open on their own, doors that pull like they're sprung, a whistle at the weatherstripping. One question, start to finish: why does a building go positive or negative, and how do relief, return, and power-exhaust fans keep it near neutral while the dampers move?
This page leans on the mixing box from Air Handlers and the modulating damper assembly from Economizers. If "the relief damper opens in step with the OA damper" doesn't ring a bell, read those first — this page is where that promise gets kept.
The Air Ledger
A building is a slightly leaky box with fans pushing air across its envelope in both directions, and its pressure is nothing more mysterious than the bookkeeping. Write it as a ledger. Deposits: outside air brought in by the air handlers — on most buildings, the only deliberate entry. Withdrawals: relief air leaving back through the unit, the dedicated exhaust fans (restrooms, kitchen hoods, janitor closets) that run whether or not the AHU cares, and exfiltration — air pushed out through every crack, joint, and door gap the envelope owns. Supply and return air never appear on the ledger at all: they circulate inside the envelope, moving air from one room to another without ever crossing it. The building's pressure is simply the residual — whatever imbalance is left after the fans have done their pushing, expressed as the tiny pressure it takes to force that leftover air through the envelope's leaks.
And the target isn't zero. Designers hold buildings slightly positive — a setpoint around +0.02 to +0.05 in. w.c. relative to outdoors — so the leakage always flows outward: conditioned air seeping out through the cracks, instead of unfiltered, unconditioned, humid air seeping in. Get it wrong in either direction and the building tells you at the doors. Too positive, and exterior doors drift open against their closers and push back when you pull them shut. Too negative, and doors go heavy, slam behind people, and whistle at the gaps — and in humid weather the infiltrating air drags moisture into wall cavities where nobody wants it. Elevator lobbies and stair doors act up for the same reason: a shaft is just a very tall duct connecting floors that disagree about pressure.
One asymmetry worth naming before the picture, because it bites hardest on large units: the return duct never brings back everything the supply duct delivered. An air handler serving whole floors loses a steady bite of its supply air to the dedicated exhausts drawing from the same spaces — gang restroom fans are the classic — air that leaves the loop mid-lap and exits the roof without ever seeing the mixing box again. That stolen share isn't optional ventilation; it's a standing withdrawal the unit must cover. Size the outside-air minimum below it and the ledger opens every morning already negative, with the dampers sitting innocently at their commanded position. The working rule: minimum outside air ≥ everything the exhaust fans take from the unit's territory, plus the surplus that holds the building positive.
Worked example · one line of arithmetic
A constant-volume unit supplies 10,000 CFM with its dampers at a minimum admitting 20 % outside air, so it deposits 2,000 CFM. (Damper position and flow fraction aren't actually the same thing in a real mixing box — position-to-flow is nonlinear and drifts — but this page treats them as equal to keep the ledger arithmetic readable.) The restroom exhaust fans withdraw 800 CFM. The remaining 1,200 CFM has nowhere to go but out through the envelope — and the modest pressure it takes to push that surplus through the cracks, a few hundredths of an inch, is the building pressure. Every scenario on this page is this same subtraction with different numbers. (Dedicated kitchen and lab exhaust, and life-safety smoke control, play this same ledger with stricter rules — both are their own topics.)
The Relief Lineup
The relief damper you met in the mixing box is an opening, not a mover — and that distinction is the whole story of this section. At minimum outside air there's so little surplus that a passive opening handles it fine. But open the dampers up for economizing and the arithmetic turns brutal: at 100 % outside air, everything the supply fan moves has to leave the building, and a passive opening asked to pass that much air needs a pressure difference to push it — which is exactly the pressure you're trying not to build. So buildings grow a lineup of relief devices, from dumbest to smartest, and knowing which one you're looking at tells you how it should be controlled.
Barometric relief is the dumbest and the most honest: hinged blades with adjustable weights, no motor, no wires, no point on the BMS. When inside pressure wins against the weights, the blades swing open and air leaves; when it stops winning, gravity swings them shut. Two things follow from the physics. It needs the building already positive before it does anything — the pressure error is its actuator — so it rides high at big airflows. And it is strictly a one-way valve: it can bleed off a positive building, but against a negative building the blades just seal harder. Barometric relief can never fix a negative building. Power exhaust is what you install when barometric can't keep up: a fan in the relief opening that pushes air out instead of waiting for pressure to build. It gets the next section to itself, because it's the one the field gets wrong. The return / relief fan is the built-up-unit answer: a second fan in the return duct itself, sized near the supply fan, helping air home from the space and giving the relief opening real pressure behind it. Its control is tracking — run at an offset below the supply fan, in measured flow, so the difference between the two is the ledger's balance, continuously, no stages, no waiting for an error to develop. (Flow, not speed: a fixed speed offset holds no particular CFM difference — the two fans ride different curves against independently drifting resistances, and speed-offset tracking is the classic way this scheme gets botched.) And that offset is not a fudge factor: it's the ledger again — the exhaust the building's dedicated fans steal from the unit's territory, plus the surplus that keeps the building positive. Commission it lazily and the tracking fan balances a ledger that was never written down.
Power Exhaust: the Fan That Follows the Damper
Here's the sentence this section exists for: power exhaust is not a return fan. They can look alike on a drawing — a fan somewhere near the return side of the unit — and they obey opposite rules. A return fan lives in the airstream: every cubic foot going back to the mixing box passes through it, so it runs whenever the supply fan runs, full stop. Power exhaust lives in the relief opening, a side door off the return path — and it only has a job when there's surplus air to throw away. At minimum outside air there is almost no surplus; a power exhaust fan running there isn't relieving the building, it's evacuating it, pulling the pressure negative against dampers that were never asked for more air. The two fans share a neighborhood and nothing else.
Which raises the control question: what should turn a power exhaust fan on? Not the supply fan — that's the return-fan rule. The classic scheme stages it off outside-air damper position: as the economizer drives the dampers open past a threshold, stage one starts; further open, stage two joins; and back down the same ladder as the dampers close. It's open-loop — the fan never actually measures the building pressure, it infers the surplus from where the dampers are — and it's coarse, stepping in chunks while the damper sweeps smoothly, so the pressure sawtooths gently between stages. Smarter systems close the loop: a building static-pressure sensor drives a VFD on the exhaust fan (a drive, same as any other) and modulates it to hold setpoint directly. Either way, the enable logic answers to the dampers — or to the pressure they create — never to the supply fan's status point as the trigger. (As a run-permissive the fan status is fine, even wise — relief hardware shouldn't run on a dead unit; it just must never be the reason the exhaust starts.)
The chart is the whole discipline in one picture: more damper, more relief; less damper, less relief; and at minimum position, no power exhaust at all — the passive relief path handles the trickle. Notice the family resemblance to the economizer staging chart one page back. That's not a coincidence, it's a rhyme: the economizer stages cooling against load, and the power exhaust stages relief against the economizer. When the sequences are married correctly the whole thing breathes — dampers open, relief follows, pressure holds — and nobody in the building ever thinks about it. The widget below lets you break that marriage the way it actually gets broken.
When the Dampers Move, the Building Feels It
Now pay off the promise the Economizers page filed: every extra cubic foot the OA damper admits has to leave the building somewhere. There are two classic ways that accounting fails, and they bracket everything you'll see in the field. Full economizing with no working relief path: the dampers drive to 100 %, the unit force-feeds the building its entire supply airflow as outside air, and the relief device — seized damper, dead exhaust fan, undersized barometric blades — can't pass it. The building inflates until its own leakage carries the flow: exterior doors stand open against their closers, ceiling tiles lift, and the lobby whooshes at everyone who walks in. A big exhaust load against minimum outside air: the kitchen hood starts at full draw while the dampers sit at their ventilation floor, and the ledger goes negative — the hood pulls its makeup air backwards through every crack in the envelope, doors go heavy, and pilot flames and elevator doors start misbehaving. Note the asymmetry: relief devices can only take air out. A negative building needs more air in — a higher OA minimum, a dedicated makeup-air unit — and no amount of relief hardware can supply it.
One wrinkle worth filing for later: on a VAV system the supply fan itself speeds up and slows down, so the ledger's deposit column moves with fan speed as well as damper position — that interaction belongs to the VAV pages of this chapter. The scenarios below hold the supply fan constant so the dampers and the relief lineup stay the whole story.
Balance the Ledger Yourself
One building, live bookkeeping. The unit supplies a constant 10,000 CFM; you set the outside-air fraction, choose what's exhausting, and pick which relief device the building owns. The gauge shows where the ledger settles. Sweep the damper slowly with power exhaust selected and watch the sawtooth — then try the last preset, which reproduces a mistake this page's author has personally made.
What the widget holds still: wind and stack effect (zero here, never zero in the field), a single envelope leakage curve, and a constant-volume supply fan. The relief openings get no passive capacity of their own — a real linked relief damper bleeds some air even without a fan behind it — so the "none" case runs a little more dramatic than life. The relief fans are also pressure-independent here — a real exhaust fan's CFM droops as the building fights it. The directions and the moral hold: relief can only subtract, staging is coarse but honest when it follows the damper, and tracking balances exactly the ledger it was commissioned for — flip the kitchen hood on under return-fan tracking and watch a stale offset meet a ledger that moved.
Measuring a Whisper
Everything above assumed you can actually see the building pressure, and that deserves its own paragraph, because the signal is a whisper. A setpoint of +0.03 in. w.c. is fifty times smaller than the 1.5 in. w.c. a supply duct might run — a breath of wind on the wrong wall face is bigger than the whole setpoint. So placement is most of the craft. The indoor probe goes in a large, representative space — a corridor or open office mid-building — away from exterior doors, elevator lobbies, and anything that swings or gusts; every door transit is a pressure spike the loop must not chase. The outdoor reference is harder: a bare tube on a windward wall reads wind, not pressure, so it wants a static-averaging tip, shielded, ideally sampling more than one face — and even then, wind and winter stack effect ride the signal as noise you filter, not information you act on. Control accordingly: heavy filtering, a wide deadband, slow loop action, and patient alarms. A building-pressure point that updates crisply and swings confidently is usually telling you about its own placement, not the building.
Not the same pressure · building static ≠ duct static
The classic conflation, worth killing on sight: duct static pressure is the supply fan's control variable, measured inside the ductwork at whole inches of water column; building static pressure is the envelope's residual, measured space-to-outdoors at a few hundredths. Different magnitude, different sensor, different loop, different failure symptoms — a healthy duct static says nothing about the ledger, and a neutral building says nothing about the fan. How the supply fan holds its duct static — and why that loop closes this chapter — is a page of its own, still to come.
That's the ledger, kept: outside air is the deposit, exhaust and relief and leakage are the withdrawals, and pressure is the residual the envelope feels. Barometric relief bleeds a positive building, power exhaust follows the dampers — never the supply fan — and a return fan tracks continuously. The doors testify when any of it goes wrong. Next in this chapter: standing in front of a unit you've never met and working out what it actually is — naming the box — before the chapter turns to VAV systems and the fan's own speed.