A unit is mounted above a ground-floor entrance, sized correctly for the opening width, and set to full output. Outside air still moves through the doorway. Staff near the entrance report the same draft they did before installation, and the heating load on that zone does not fall.
The airflow across the opening was adequate. What defeated it was pressure difference across the building envelope, which was pushing air through that doorway continuously regardless of what the doorway equipment was doing.
An air barrier at an opening works against the pressure across that opening. Where the pressure is significant, the barrier has to be specified for it, and where it is not addressed the installation underperforms in ways that appear as equipment failure.
Buildings Are Not Pressure Neutral
Air moves through a building envelope wherever a pressure difference exists across it.
Temperature difference produces one. Warm air inside a building is less dense than cold air outside, so it rises and exits through upper openings while cold air enters through lower ones. The taller the building, the greater the effect.
Mechanical systems produce another. Exhaust fans, kitchen hoods, and combustion appliances remove air, and that air is replaced through whatever openings exist. A building with more exhaust than supply runs at negative pressure and pulls air inward through every door.
Wind produces a third, positive on the windward face and negative on the leeward, varying continuously.
At any given moment a doorway sits at some net pressure, and the direction and magnitude change through the day and the season.
Stack Effect Concentrates at Ground Level
In multi-story buildings, stack effect creates the strongest inward pressure at the lowest openings during heating season.
Warm interior air exits at the top, and the replacement air enters at the bottom. Ground-floor entrances are the largest low-level openings, so they carry the bulk of that infiltration.
The pressure difference increases with building height and with the temperature differential. A tall building on a cold day generates enough pressure at its ground-floor doors to make them noticeably harder to pull open.
Airflow through the opening under those conditions can exceed what a doorway barrier sized on opening dimensions alone can hold.
Exhaust Imbalance Is Often the Larger Factor
Where a building runs at net negative pressure from exhaust, the effect is continuous and independent of weather.
Commercial kitchens are the common case, with hood exhaust rates that substantially exceed dedicated makeup air where makeup air was undersized or has failed.
Restrooms, laboratories, and process exhaust contribute. Rooftop units set to over-exhaust contribute.
The replacement air enters through the path of least resistance, which is usually the main entrance. Staff at that entrance experience it as a permanent draft, and doorway equipment installed to address it fails because the driving pressure remains.
Correcting the imbalance addresses the cause. Doorway equipment addresses the symptom, and it has to be sized for the pressure rather than for the opening.
Velocity and Volume Both Matter
A doorway air barrier works by projecting a stream of air across the opening with enough momentum to deflect infiltrating air.
Momentum depends on both velocity and volume. A high-velocity stream with low volume is thin and easily disrupted. A high-volume stream at low velocity lacks the force to reach the floor and deflect cross-flow.
Reaching the floor matters, because an incomplete stream leaves a gap at the bottom of the opening where infiltration passes through. Manufacturers state a maximum mounting height for this reason, and installing above it means the stream dissipates before reaching floor level.
Specification for Global Industrial air curtains and equivalent equipment therefore starts with mounting height and opening dimensions, then accounts for the pressure the stream has to work against.
Mounting Position Affects Performance
The unit’s position relative to the opening determines where the stream lands.
Mounted directly above and inside the opening, the stream projects downward across the full width. Offset from the opening, part of the stream misses it.
Nozzle angle is usually adjustable, and it is set to angle the stream slightly outward at the bottom, so that infiltrating air is deflected before it enters rather than after.
Obstructions within the stream path disrupt it. Signage, door hardware, awnings, and stored goods near the entrance all create gaps.
Width coverage has to match or exceed the opening. Two units side by side on a wide opening need to abut without a gap between their streams.
Door Behavior Changes the Duty Cycle
How often and how long a door stands open determines how much of the time the barrier is working.
High-traffic entrances with doors opening every few seconds effectively hold the barrier in continuous operation, and the equipment should be specified for continuous duty.
Loading doors standing open for extended periods present a sustained condition, and pressure across the opening has time to establish fully.
Automatic doors with short open cycles reduce the exposure. Vestibules reduce it further, since the two door sets are rarely open simultaneously and the vestibule buffers the pressure difference.
Where a vestibule exists, doorway equipment addresses the residual rather than the full infiltration load.
Control Integration Determines Actual Savings
A unit running continuously while doors are closed consumes energy without providing benefit.
Door switches tie operation to door position, running the unit only when the opening is actually open. On high-traffic doors the difference is small; on occasional-use doors it is substantial.
Two-speed and variable operation allows lower output during mild conditions and full output when the pressure difference is greatest.
Where units are installed without controls, they typically run whenever the building is occupied, and the energy consumption offsets some of the infiltration saving.
What Specification Requires
The information needed goes beyond the door dimensions.
Opening width and height. Available mounting height above the opening, and whether it falls within the unit’s rated range. Building height and number of stories, for stack effect. Exhaust and makeup air balance for the building or zone. Door duty cycle and whether a vestibule exists. Obstructions in the stream path.
Opening size determines the unit’s width and nominal output. The pressure across the opening determines whether that output is sufficient, and it is the input most often left out.
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