14/08/2026Air curtains

How to size an air curtain correctly

What is the right heating capacity for a doorway? That is the core question of air curtain sizing, and answering it starts with a few variables:

• outdoor design temperature
• room temperature
• the base heating load of the space
• doorway geometry

The need is also affected by the door's usage profile, meaning how often the door is open and for how long, a possible vestibule, the building's pressure conditions and the prevailing wind.

Start with the instantaneous heat loss

A good starting point is to calculate the instantaneous heat loss of the doorway in a situation where the door is open and air flows driven only by the indoor-outdoor temperature difference. The calculation is based on the two-way buoyancy-driven flow through the opening:

V̇ = (Cd / 3) · W · H^(3/2) · √(g · ΔT / Ti)
Q  = ρ · cp · V̇ · ΔT

where V̇ is the volume flow, W and H the width and height of the doorway, ΔT the indoor-outdoor temperature difference, Ti the indoor temperature in kelvin, Cd the discharge coefficient, g the gravitational acceleration, ρ the density of air, cp the specific heat capacity of air and Q the instantaneous heat loss.

Constants used in the calculation: Cd = 0.65, g = 9.81 m/s², ρ = 1.2 kg/m³, cp = 1.005 kJ/(kg·K).

An example in design conditions

The door is 3.2 metres wide and 3.0 metres high. Indoor temperature +20 °C, outdoor temperature −26 °C, the design temperature of Finnish climate zone I.

About 16,100 m³/h of air flows through the open door, and the instantaneous heat loss is about 250 kW.

The number is large, and it should be. It tells you what an open door does in freezing weather, not what size of unit to choose for the door.

For comparison, the same door in milder conditions, +9 °C outdoors: the flow is about 7,900 m³/h and the instantaneous loss about 29 kW. The temperature difference grew fourfold, the loss almost ninefold. The relationship is not linear, because ΔT affects both the flow and the amount of heat transferred.

Why the instantaneous loss is not the design capacity

Two things separate the instantaneous loss from the design capacity.

The door is not open all the time. The instantaneous figure applies only at the moment the opening is fully open. The real average load depends on the usage profile: a ten-second opening once a minute is a different thing from a loading door that stands open constantly. The total energy is calculated from the instantaneous power and the open time, not from the instantaneous power alone.

The air curtain is not the only heat source. Part of the residual loss belongs to the space's base heating. The air curtain's job is to handle the additional load arising at the doorway, not to heat the whole building.

Climate separation effectiveness reduces the loss

According to the Eurovent 16/5 recommendation, the design value for the climate separation effectiveness (CSE) of a correctly selected, installed and controlled air curtain is typically 0.60–0.80.

With this assumption, the residual loss for the example door is:

These are calculated values. They tell you the size class, not a finished design capacity.

The calculation is a starting point, not a finished answer

The example calculates only the static-case load. The other factors are taken into account with safety factors, and they depend on the site.

If the door is located in a sheltered entrance, the building's pressure conditions are under control and the door has a vestibule, a moderate safety factor can be used. In that case the calculated heating demand already gives a fairly good picture of the size class needed.

If, on the other hand, the door opens directly outdoors, is open frequently, sits on a windy facade or the building is under negative pressure, a calculation based on temperature difference alone is not enough. The actual airflow through the doorway can be considerably larger than in the static calculation case, and the required air volume, jet velocity and heating capacity grow with it.

Sizing looks at three things together

Doorway geometry. Width and height set the baseline. The higher and wider the opening, the larger the air volume and jet reach needed. Height affects the flow to the power of 3/2: of the opening's dimensions, it is the one that matters most.

Conditions at the door. Wind, negative pressure, ventilation balance, a vestibule and how often the door is used can change the actual need significantly. Separation efficiency is sensitive to pressure difference: a pressure difference across the opening lowers the separation efficiency below its design value, and in a building under negative pressure this should be checked separately.

Air curtain unit performance. Heating capacity alone is not enough; the unit must be able to form a sufficiently even air jet that carries across the full height of the doorway. If the jet does not reach the floor, heating capacity alone will not solve the problem. Too strong a jet, in turn, hits the floor so hard that the resulting turbulence increases the mixing of indoor and outdoor air.

For this reason an air curtain unit should not be sized on kilowatts alone. The right choice comes from the combination of air volume, discharge velocity, installation height, heating capacity and control method.

Rule of thumb: the calculation tells you the size class, the operating conditions determine the safety margin, and the quality of the air jet decides whether it works.

The right size of unit for the real operating situation

The goal is not the largest possible unit. Too small an air curtain does not protect the doorway sufficiently, and too large a unit causes unnecessary noise, a feeling of draught and energy consumption.

Good sizing answers three questions:

1. Does the air jet carry across the full height of the doorway?
2. Is the heating capacity enough to cover the residual loss?
3. Does the solution work in the real operating situation, not only on the calculation sheet?

When these are met, the air curtain does its job: it reduces the inflow of cold air, improves indoor conditions and cuts the energy loss through the door.

Windis Select supports the sizing

Windis Select (select.windis.fi) combines the site's input data with unit-specific performance values. The doorway heat-loss calculation gives a calculated basis for the heating demand, and for the selected unit the tool shows performance values in real time and produces a PDF sizing report, a site-specific functional description, a functional diagram and MagiCAD product codes as appendices to the design.

The tool does not replace the designer's judgement. If the site is exceptionally windy or under negative pressure, or the doors are constantly open, the sizing should be checked case by case. Select does not choose the unit for the user, and help with choosing the size class is available directly from Windis. The tool runs in the browser without installations or licences, in Finnish, Swedish and English.

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The calculation method and the separation-efficiency design values are based on the recommendation Eurovent 16/5 – 2025, Air Curtain Units – Technical Overview (Eurovent, Brussels).

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