Building pressurization control and makeup air field guide

Building pressurization, and why the air balance runs the building

Building pressurization is the air pressure inside a building measured against the air pressure outside, and it is set by one thing: whether more air is coming into the building than leaving it, or the other way around. Air in is the outdoor air the HVAC system supplies, the makeup air that replaces exhaust, and whatever leaks in through the envelope. Air out is the exhaust fans, the relief, and whatever leaks back out. The difference between those two sums is the pressure, and it lands the building in one of three states: positive, negative, or neutral.

That number controls more of the building than most people credit it for. It decides whether conditioned air pushes out through the envelope or unconditioned air gets pulled in through every crack. It decides whether the lobby door opens with one hand. It decides whether a gas water heater drafts up its vent or pulls flue gas back into the room. Comfort, indoor air quality, moisture in the walls, and combustion safety all trace back to the pressure, and the pressure traces back to the air balance.

The makeup-air guide and the exhaust guide each handle one side of this balance in depth. This guide is about the whole building: how the two sides add up, what each pressure state does, and how to read and set the result so the building behaves instead of fighting you.

Positive, negative, and neutral pressure

Three states, and each one does something specific. A positive building has more air coming in than going out, so the surplus pushes outward through the envelope. Open a door and air flows out. That outward flow is what keeps unconditioned, humid, dirty, or pest-laden outside air from coming in, because air only moves from higher pressure to lower.

A negative building has more air leaving than coming in, so it pulls air in through whatever gaps it can find. Open a door and air rushes in. The building is making up the deficit through cracks, the curtain wall, elevator shafts, and combustion vents, on its own terms instead of yours. Negative is the state behind most of the failures in this guide.

Neutral is the balance point, inside roughly equal to outside, with no strong flow either way. Few real buildings sit exactly neutral, and the design rarely aims for dead neutral anyway. The usual target is a slight, controlled positive overall, with the dirty rooms held negative inside that positive envelope. The states are not good or bad in the abstract. The right one depends on the space and what you are trying to keep in or keep out.

What is the building air balance?

The building air balance is the sum of all the air entering set against the sum of all the air leaving, and it is the equation that determines the pressure. On the supply side: the outdoor air the air handlers bring in, the dedicated makeup air for the exhaust systems, and the infiltration the envelope allows. On the exhaust side: every exhaust fan, the relief air that lets a positive building bleed off, and the exfiltration that leaks back out.

Written plainly, outdoor air supplied plus infiltration equals exhaust plus relief plus exfiltration, once the building settles. The piece the designer controls is the mechanical air: the supply outdoor air, the makeup, the exhaust, and the relief. Infiltration and exfiltration are the uncontrolled remainder, and the goal is to shrink them by getting the mechanical air right, so the envelope is not the thing doing the balancing.

One rule falls out of this and it is the whole game: the makeup air has to replace the exhaust. Pull air out with a fan and supply no planned path back in, and the building finds the path itself and goes negative. The makeup-air guide works this balance for a commercial kitchen, where the hood exhaust is large enough to need a dedicated unit. The same arithmetic governs the building as a whole.

Why most buildings are designed slightly positive

Slight positive is the usual design intent because it puts the building in charge of where its air comes from. A building held a touch positive pushes its conditioned air outward through the envelope, so the gaps that would otherwise be infiltration points become exfiltration points instead. The hot, humid, dusty, pollen-laden, or exhaust-tainted air outside stays outside, because the pressure gradient runs the wrong way for it to come in.

That control buys several things. Infiltration drops, so the heating and cooling loads from uncontrolled outside air shrink and get more predictable. Outdoor humidity is held off the envelope, which matters in a cooling climate where infiltrating moist air is a mold and condensation problem. Dust, pollen, and outdoor pollutants come in through the filtered outdoor-air intake rather than around the doors. And the entrance doors behave, instead of standing open to a negative building's pull.

The hedge is that positive is not free and it is not unlimited. Push a building too positive and you drive moist indoor air into cold wall cavities in winter, force doors open against the pressure, and waste conditioning on air you are blowing straight out the gaps. The intent is slight, commonly on the order of 0.02 to 0.03 in. w.c. to outdoors, not as much positive as the fans can make. And some spaces have to run negative no matter what the building around them is doing.

Why is negative building pressure a problem?

Negative building pressure is a problem because it turns the whole envelope into an uncontrolled air intake, and the air it pulls in arrives on the worst possible terms. Four failures follow from it, and the last one is the one that hurts people.

First, infiltration. A negative building pulls outside air in through every crack, the curtain wall, the loading dock, the elevator and stair shafts. In summer that is hot, humid air landing inside the envelope. In winter it is cold air pouring in low. Either way it is unconditioned and unfiltered, and it loads the equipment with weather the design never counted on.

Second, doors. The negative pulls against the exterior doors so they are hard to open, slam shut, or whistle as air screams through the gap. A vestibule or a revolving door helps, but a door that fights you is the building telling you it is negative.

Third, moisture. The infiltrating air carries outdoor humidity into the wall assemblies and across cold surfaces, where it condenses. That is hidden water in the walls, and it shows up later as mold, stained finishes, and rot that nobody connects back to the pressure.

Fourth, and this is the one that puts someone in the hospital: combustion backdraft. A negative building can overpower the natural draft of an atmospheric water heater, furnace, or boiler and pull flue gas, including carbon monoxide, back down the vent into occupied space. A drop on the order of a few pascals is enough to spill a naturally drafted appliance. That is a fatal failure mode wearing a quiet disguise, and it is the reason negative pressure is treated as a safety issue, not a comfort one.

Every exhaust needs makeup air

Every fan that pulls air out of the building takes air that has to be replaced, and if no planned makeup path exists, the building goes negative to find it. This is the rule that ties all the exhaust in a building back to its pressure. The restroom fans, the general exhaust, the lab and equipment exhaust, and the big kitchen hood are all withdrawals from one account, and the makeup air is the deposit that keeps the balance out of the red.

Add up the exhaust before you decide it is handled. A building can carry a dozen modest exhaust fans that look harmless one at a time and total several thousand CFM together, all of it pulling against an outdoor-air supply that was sized for ventilation, not for makeup. When the exhaust total outruns the supplied outdoor air, the difference comes in through the envelope as infiltration, and the building runs negative whenever those fans are on.

The makeup can be ducted outdoor air, transfer air pulled from an adjacent space, or surplus building supply running positive enough to feed the exhausted rooms. The exhaust guide covers sizing the rates and the makeup paths for the everyday cases. The point here is the accounting: exhaust and makeup are one system, and a building that sizes the exhaust and forgets the makeup has designed half a system that pulls its other half through the cracks.

The kitchen hood is the big withdrawal

A commercial kitchen hood is usually the largest single exhaust in a building, often several thousand CFM from one cookline, and it is the exhaust most likely to drag the whole building negative on its own. A Type I grease hood over a charbroiler can move more air than the rest of the building's exhaust combined, and every cubic foot of it leaves through the roof fan.

Because the flow is so large, a kitchen hood almost always needs dedicated makeup air rather than leaning on transfer air or the general building supply. Starve the hood of makeup and two things fail together: the hood loses capture and spills smoke and grease into the kitchen, and the building goes negative enough to fight doors and backdraft appliances. The makeup-air guide handles the kitchen case in full, including how much makeup, how to temper it, and the interlock that keeps the makeup running with the hood. For the building-pressure picture, treat the hood as the dominant term in the exhaust side of the balance and size its makeup first.

Makeup air units and dedicated outdoor air systems

A makeup air unit (MAU) brings in outdoor air, conditions it, and delivers it to replace exhaust and hold the building's pressure where the design wants it. A dedicated outdoor air system (DOAS) does a related job for ventilation, supplying tempered and often dehumidified outdoor air to the spaces separately from the recirculating heating and cooling. Both are how a building gets controlled outdoor air in on purpose, instead of pulling it through the envelope.

The MAU is sized to the exhaust it replaces. The DOAS is sized to the ventilation the spaces need under ASHRAE 62.1. On many jobs the same outdoor air also provides the slight positive surplus that pressurizes the building, so one stream does triple duty: ventilation, makeup, and pressurization. The makeup-air guide covers MAU selection, tempering, and interlock in depth. The piece to carry into the pressure conversation is that the conditioned outdoor air a MAU or DOAS supplies is the controllable deposit on the supply side of the balance, the lever you set to put the building where you want it.

The outdoor-air damper, the economizer, and where the air leaves

An air handler controls how much outdoor air it brings in through its outdoor-air damper, and on an economizer cycle that damper can swing wide open to use cool outdoor air for free cooling. The more outdoor air the unit pulls in, the more air has to leave the building, or the space climbs positive past its target and the doors start to fight back. So the outdoor-air side and the relief side have to move together.

This is where relief earns its place. As the economizer opens the outdoor-air damper, a relief or exhaust path has to open in step to let the matching volume out, or the building over-pressurizes. Many systems do this with a return-air relief damper or a powered relief fan that tracks the outdoor-air damper, dumping the surplus so the building holds its slight positive instead of ballooning. An economizer that brings in 100 percent outdoor air with no matching relief is a pressure problem the first mild day it runs.

The balance is the whole point: outdoor air in, relief out, and the small surplus between them is the pressurization. Get the relief sizing wrong and the economizer that was supposed to save energy instead blows the building positive and props the doors open.

Relief air: bleeding off the positive without over-pressurizing

Relief air is the planned path for a positive building to let its surplus out, the counterpart to makeup air on the supply side. Hold a building positive and that positive has to go somewhere as the supply outdoor air comes in. Relief is where it goes on purpose, instead of being forced out through doors and gaps.

Relief comes in two flavors. A barometric or gravity relief damper is a counterweighted flap that opens when the building pressure rises above a small setpoint and closes when it drops, passive and self-regulating. A powered relief or exhaust fan does the same job actively, modulated off a building-pressure sensor, and it is the choice on larger systems where a gravity damper cannot pass the volume or hold the pressure tightly enough. Either way the relief is sized to pass the surplus at the target pressure.

The failure to look for is a positive building with no relief, or with a relief damper stuck shut. The supply keeps pushing air in, the building climbs past its target, and the pressure that was supposed to be a slight, useful positive becomes enough to bow the doors and waste conditioning out the cracks. Relief is what keeps a positive building slightly positive instead of grossly positive.

How do you measure building pressure?

Measure building pressure with a sensitive manometer reading the difference between inside and outside, in inches of water column, with one tube referenced to the space and the other run to the outdoors. The numbers are small. A useful building positive lives around 0.02 to 0.03 in. w.c., a few pascals, so the gauge has to resolve hundredths of an inch and the reference tube has to be placed where wind is not slamming it around.

Read it with the building running the way it normally runs, all the fans on, the economizer where it sits, the doors closed. A reading taken with a loading dock door open or a big exhaust off tells you nothing about how the building behaves in service. For a space-to-space relationship, like a restroom to a corridor or a kitchen to a dining room, run the reference tube to the adjacent space instead of outdoors.

The smoke pencil is the field sanity check that costs nothing. Crack an exterior door a half inch and release a little smoke at the gap. Smoke pulled inward means the building is negative; smoke pushed outward means positive. It will not give you a number, but it tells you the sign and the rough strength in seconds, which is often all you need before you set up the manometer. The door test is the same idea with your hand. The target is a slight positive overall, but the right number depends on the design and the space, so confirm it against the project intent rather than a rule of thumb carried in from the last job.

The stack effect in tall buildings

Stack effect is the pressure a tall building makes on itself from temperature alone. Warm air is less dense than cold air, so in winter the heated air inside a tall building rises and wants to escape near the top, which pulls cold outdoor air in near the bottom. The taller the building and the colder the day, the stronger the draw, and a high-rise can generate more pressure from stack effect than its fans ever do.

The neutral pressure plane is the height where inside and outside pressure are equal. Below it, the building is negative to outdoors and air infiltrates: cold air pouring into the lobby, doors that will not stay shut, elevator doors that whistle. Above it, the building is positive and air exfiltrates, pushing warm, moist indoor air out through the upper envelope where it can condense in cold assemblies. The lobby complaint and the top-floor moisture problem are the same stack effect showing up at opposite ends of the building.

Winter is the worse case because the indoor-to-outdoor temperature difference is larger, which is what drives the effect. Vertical shafts, stairwells, and elevator hoistways are the express lanes that connect the floors and strengthen it. You manage stack effect with vestibules and revolving doors at the base, tight shaft and stairwell separation, and a ventilation system that can hold the lower floors positive against the draw rather than letting the stack run the building. On a tall building, stack effect is a pressure load the design plans around, not an afterthought.

Wind and building pressure

Wind pushes the building pressure around from the outside, and it does it unevenly. The windward face sees a positive pressure as the wind piles against it, while the leeward and side faces see a negative as the wind pulls away. One side of the building can be infiltrating while the other is exfiltrating, all from the same gust, and the pattern shifts with wind direction and speed.

For the mechanical balance, wind is the variable that makes a borderline pressure reading swing. A building set right at neutral can read positive on the windward side and negative on the leeward side on a windy day, which is why a pressure measurement taken in a gale is suspect and why reference tube placement matters. Wind also stacks with the stack effect, so the worst infiltration on a tall building is a cold, windy day with the wind driving the lower windward floors even more negative. Design for a slight positive with enough margin that ordinary wind does not flip the lower floors negative.

Combustion safety and negative pressure

This is the failure that kills, so it gets stated plainly: a building pulled negative can backdraft a naturally drafted combustion appliance and pull carbon monoxide into occupied space. An atmospheric water heater, furnace, or boiler relies on the buoyancy of its hot flue gas to draft up the vent. That draft is weak, on the order of a few pascals. Put the appliance in a room the building is holding negative, and the negative can overpower the draft, reverse the flow, and spill flue gas, including carbon monoxide, back down the vent into the space.

The chain is ordinary, and that is what makes it dangerous. A big exhaust comes on, the makeup was never provided, the mechanical room goes negative, and the water heater that drafted fine yesterday spills today. Nobody sees it. Carbon monoxide has no smell, and the first sign is people getting sick.

The fixes are known. Provide combustion air to the appliance so its local pressure stays near neutral, the way the mechanical and fuel-gas codes require. Better, use sealed-combustion or direct-vent appliances that draw combustion air from outdoors through a dedicated pipe and never see the room pressure at all, which takes the building's negative out of the equation. And size the makeup air so the building does not go hard negative in the first place. Where atmospheric appliances share a space with large exhaust, a worst-case depressurization test, fans on and doors closed, checking that the vents still draft, is the check that proves it before someone gets hurt. Do not assume an appliance that drafts on a calm commissioning day will draft when the building is loaded up and pulling negative.

Pressure, moisture, and the dew point in the walls

Pressure decides which way moisture moves through the envelope, and the wrong direction puts water where it condenses. The driving idea is the dew point: when humid air reaches a surface colder than its dew point, the water comes out of the air and onto the surface. Pressure controls whether that humid air is being pushed into the wall or held out of it.

In a cooling climate, the danger is a negative building pulling hot, humid outdoor air inward through the envelope. That air hits the cool back side of the wall and the cooled interior surfaces, drops below its dew point, and condenses inside the assembly. Hidden water, then mold, then rot. Holding the building slightly positive keeps that humid air from being drawn in, which is one of the main reasons a slight positive is the cooling-climate intent.

Winter flips the risk. A building pushed too positive drives warm, moist indoor air outward through the envelope, and that air condenses when it reaches the cold outer layers of the wall. So positive is protective in summer and a moisture risk in winter if it is overdone, which is the real reason the target is a slight positive and not as much as you can make. The pressure, the climate, and the dew point have to be read together, and the safe positive in a humid summer is a smaller positive on a hard winter day.

Pressurization, ventilation, and indoor air quality

Pressurization and ventilation are tied together because the outdoor air that ventilates the building is the same air that pressurizes it. ASHRAE 62.1 sets the minimum outdoor air a building needs for acceptable indoor air quality, calculated by space type and occupancy, and that outdoor air is the supply-side deposit in the pressure balance. Bring in the ventilation air the standard calls for, keep a small surplus over the exhaust, and the same air does two jobs: it dilutes indoor contaminants and it holds the building positive.

The two goals usually point the same way, but not always. A building starved of outdoor air to save energy is both under-ventilated and prone to going negative as its exhaust outruns its supply, so the indoor air quality problem and the pressure problem arrive together. A slight positive supports good indoor air quality because the outdoor air comes in filtered through the intake rather than dirty through the cracks.

ASHRAE 62.1 also lets makeup air for exhaust be outdoor air, recirculated air, or transfer air, which is the standard acknowledging that the pressure balance and the ventilation calculation are the same air accounting viewed two ways. Size the outdoor air for the ventilation requirement first, then check that it covers the exhaust with a surplus left for pressurization. If it does not, the makeup is short, and the building will tell you by going negative.

What pressure does each special space need?

Some spaces have to run a specific pressure regardless of the building around them, set by what has to be kept in or kept out. The split is simple: spaces that make something you want contained run negative, and spaces you want protected from contamination run positive.

Negative spaces hold their air in. A restroom is held negative to the corridor so odor stays out of occupied space. A commercial kitchen runs slightly negative to the dining room so grease and smoke stay on the cook's side. A laboratory or a chemical storage room runs negative so fumes do not migrate. An airborne-infection isolation room in a hospital runs negative so pathogens stay in the room, commonly at a differential on the order of 0.01 in. w.c. or more to the corridor, with the exact value set by the governing standard.

Positive spaces keep contamination out. A cleanroom runs positive so unfiltered air and particles cannot leak in past the gowning line. An operating room and a protective-isolation room for an immune-compromised patient run positive to hold ambient air and airborne contaminants out. Healthcare pressure relationships in particular are governed in detail by ASHRAE 170, which assigns a required relationship to each room type, and the design and the adopted standard set the value, not a rule of thumb. The building-pressure job is to hold the overall envelope where it belongs while these spaces sit at their own required pressures inside it, each one balanced against the spaces it adjoins.

Data center and containment pressurization

A data center manages pressure at two scales: the room and the containment. At the room scale, the data hall is usually held positive to the surrounding spaces so dust and unconditioned air cannot leak in onto the equipment, the same protective logic as a cleanroom. At the containment scale, the cold aisle is held positive to the hot aisle so the cooling air is pushed forward through the servers instead of letting hot exhaust air recirculate back to the inlets.

That cold-aisle positive is the one operators watch with differential-pressure sensors. If the cold aisle goes negative relative to the hot aisle, hot exhaust gets drawn back through the racks and the inlet temperatures climb, which is the recirculation failure that overheats equipment even when the cooling capacity is adequate. A common target keeps the cold aisle slightly positive to the hot aisle, on the order of a few pascals up to roughly 20 Pa depending on the containment, with the exact setpoint per the design.

The control links the cooling units to the pressure. The cooling fans modulate to hold the cold-aisle pressure as the IT load swings, so the supply tracks demand the way makeup tracks exhaust elsewhere in this guide. ASHRAE TC 9.9 gives the thermal guidelines the room is run to, and the project design sets the pressure setpoints. The principle is the familiar one: match supply to demand and hold the pressure so the air goes the right way.

The door as a pressure gauge

A door is the cheapest building-pressure instrument you own, and it reads the result before you set up a manometer. An exterior door that is hard to pull open, slams behind you, or whistles at the gap is reporting a negative building pulling air in past it. A door that drifts open on its own, resists closing, or lets air rush out when cracked is reporting a positive. A door that swings normally and latches with light effort is the building near its target.

This is the first thing a good service tech checks walking in, because it costs nothing and it points straight at the balance. A complaint about doors, an entrance that needs two hands, a stairwell door that bangs, a vestibule that never seals, is almost always a pressure complaint in disguise. Read the doors first, then bring out the gauge to put a number on what they already told you.

Commissioning and balancing the building pressure

Balancing a building is not done until the pressure is proven, not just the airflow at each diffuser. Test, adjust, and balance work usually centers on the airflows, the supply, return, exhaust, and outdoor air set to design, but a building can hit every design airflow and still sit negative if the outdoor air, exhaust, and relief were never reconciled against each other. The pressure is the sum of all of it, and it is the reading that proves the balance is real.

Set it in order. Confirm the exhaust totals, set the supply outdoor air to cover the ventilation and the makeup with a surplus, then set the relief to bleed off only the surplus so the building lands at its target positive. Read the building-to-outdoor pressure with everything running, check the space-to-space relationships for the rooms that have to hold a specific pressure, and walk the doors. The air balancing report is where these numbers live, and the pressure reading belongs in it alongside the airflows.

The reading people skip is the worst case, not the calm afternoon. Stage the big exhaust on, swing the economizer to full outdoor air, and check that the building holds and the combustion vents still draft under each condition. A building balanced only at one benign operating point will go wrong at the operating point nobody tested, and the time to find it is at commissioning, with a gauge in hand, not on the first cold windy day the building is occupied.

Controlling building pressure with the BAS

On any building past the simplest, the pressure is held by a control loop, not by a one-time balance. A building-pressure sensor referenced to outdoors feeds the building automation system, which modulates the relief or exhaust, and sometimes the outdoor-air damper, to hold the pressure at setpoint as conditions change. The exhaust ramps up and the relief or makeup tracks it. The economizer opens and the relief opens with it. The loop does continuously what the balancer did once.

Demand-control ventilation sits inside this. As occupancy drops and the system pulls back the outdoor air to save energy, the control has to keep the supply, exhaust, and relief tracking together so the pressure does not wander negative at part load. A scheme that ramps exhaust without ramping makeup just relocates the negative-pressure failure to the hours nobody is watching.

The failures here are control failures: a pressure sensor with its reference tube in a bad spot reading wind instead of building pressure, a relief damper that an override left shut, a sequence that controls airflow but never actually closes the loop on pressure. Trend the building pressure on the BAS the way you trend a space temperature, because a building that drifts negative does it slowly and quietly, and the trend log catches it before the doors and the combustion vents do.

What to document

A building-pressure record is what lets the next person tell whether the building is behaving or drifting. The balance set at commissioning walks over time as filters load, dampers stick, and exhaust gets added, and without a baseline number there is nothing to measure the drift against.

Capture the building-to-outdoor pressure at the design operating condition and at the worst case, the space-to-space relationships for any room that has to hold a specific pressure, the outdoor-air, exhaust, and relief airflows that produced them, the combustion-appliance draft check where atmospheric appliances are present, and the door-test result. Record the target each reading was held to and who took it, so a reviewer can reproduce the balance instead of guessing at it.

Common mistakes

• Exhausting air with no makeup path, so the building goes negative and pulls its makeup through the cracks.
• Letting a negative building backdraft an atmospheric combustion appliance and spill carbon monoxide into occupied space.
• Ignoring the kitchen hood makeup air, so the largest exhaust in the building drags the whole building negative.
• Running a positive building with no relief, or with a relief damper stuck shut, so it over-pressurizes and props the doors.
• Running an economizer to full outdoor air with no matching relief, blowing the building positive on the first mild day.
• Putting the wrong pressure on a special space: a positive restroom that smells in the corridor, or a negative cleanroom that leaks in dust.
• Ignoring the stack effect on a tall building, so the lower floors infiltrate and the upper floors exfiltrate and condense.
• Balancing the airflows and never measuring the building pressure, so a negative building passes commissioning unnoticed.

Field checklist

Standards and references

The framework is split across the ventilation, mechanical, energy, fuel-gas, and balancing standards, and each owns a piece of the pressure question. ASHRAE 62.1 is the ventilation standard, setting the minimum outdoor air by space type that both supplies indoor air quality and forms the supply side of the pressure balance, and it allows exhaust makeup to be outdoor, recirculated, or transfer air. The mechanical code, commonly the IMC, adopts those rates and adds the installation rules for exhaust, makeup, and outdoor-air systems.

Combustion safety is governed hard, and it is the part to get right. The mechanical code and the fuel-gas code, commonly the IFGC or NFPA 54, require combustion air for fuel-fired appliances and address the depressurization and backdraft hazard. Where atmospheric appliances and large exhaust share a building, the worst-case depressurization check belongs to this requirement. ASHRAE 90.1 is the energy standard that drives demand-control ventilation and energy recovery, which change how much outdoor air moves and so how the pressure holds at part load. Healthcare space pressurization is governed by ASHRAE 170, which assigns a required relationship to each room type.

Test, adjust, and balance work follows the procedures of the balancing bodies, AABC and NEBB, whose methods cover measuring the airflows and proving the building pressure relationships. The exact section numbers, rates, and pressure values shift between code cycles and standard revisions and are amended by jurisdiction, so confirm each against the adopted edition and the project specification before citing it on a submittal. Hedge the pressure values to the design. Never hedge the combustion-safety check.

Units, terms, and conversions

Building pressure borrows its units from duct static and combustion both, so the same small numbers show up written several ways across a drawing set and a controls submittal.

Building pressure is in inches of water column, written in. w.c. or in. wg, and the metric form is pascals, where 1 in. w.c. is about 249 Pa. The numbers are small: a slight building positive of 0.02 to 0.03 in. w.c. is roughly 5 to 7.5 Pa, and the few pascals that backdraft a naturally drafted appliance is well under 0.05 in. w.c. Airflow is in CFM, cubic feet per minute, or liters per second and cubic meters per hour in metric. Pressurization is the surplus of supply airflow over exhaust, so it gets read in both pressure and airflow depending on which side of the balance you are setting.

Building pressurization

The air pressure inside a building relative to outdoors, set by the air balance

Air balance

Supply outdoor air plus infiltration set against exhaust plus relief plus exfiltration

Makeup air

Air supplied to replace what is exhausted, by outdoor, transfer, or surplus supply air

Relief air

The planned path that lets a positive building bleed its surplus out, gravity or powered

in. w.c. / in. wg

Inches of water column, the unit for building and duct pressure; about 249 Pa per inch

Stack effect

Buoyancy-driven pressure in a tall building, infiltration low and exfiltration high in winter

Neutral pressure plane

The height in a building where inside and outside pressure are equal

Backdraft

Reverse flow that pulls combustion flue gas back down a vent into the space

Infiltration / exfiltration

Uncontrolled air leaking in through, or out through, the building envelope

DOAS

Dedicated outdoor air system supplying tempered ventilation air separate from recirculated air

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• What is a makeup air unit?
• How much exhaust does a restroom need?
• How does a gas furnace work?
• What is a UL 508A panel?
• What is fault detection and diagnostics?