A heated air curtain can use electric coils, hot water coils, or steam coils, and the choice trades heating capacity, response speed, and running cost against the available building services.
Why Heat the Curtain
At a cold-climate doorway the air curtain does double duty: it blocks infiltration and it pre-warms the incoming sheet so the doorway feels comfortable and the space near the door does not chill. The heater must add enough kilowatt-equivalent to raise the curtain air from outdoor to near-room temperature across the opening, which scales with door area and the temperature lift, so a 4 m industrial door needs far more than a 2 m shop door even at the same outdoor condition.
The three media differ sharply in how fast they respond and how much they cost to run, which is why the decision is rarely only about first cost. Electricity is instant but dear; hot water is slower but cheap if already distributed; steam is powerful where it exists but adds drainage and control complexity that some sites cannot support.
A worked estimate keeps the discussion honest. Heating an air curtain stream takes roughly 0.34 W for every cubic metre per hour of air and every degree of temperature rise, so lifting 8,000 m³/h by 25 K needs on the order of 68 kW. That figure is why heated curtains on large industrial doors move quickly into the range where the electrical supply becomes the dominant cost of the installation. Duty cycle softens the picture: a door open 10% of the working day does not need continuous full heat, and a door contact or presence sensor that stages the heater with the opening is often the single most effective energy measure available on the unit.
Electric, Hot Water, Steam Compared
Electric coils respond almost instantly and need only power, but electricity is the most expensive heat per unit and large doors can need 20โ60 kW. Hot-water coils use the building's hydronic system, respond in minutes, and run cheaper, but need a coil sized for the available temperature. Steam coils deliver high capacity and fast warm-up where steam exists, but add condensation drainage and control complexity, including a steam trap that itself needs maintenance.
| Item | Specification | Note |
|---|---|---|
| Electric capacity | 20โ60 kW | Per large doorway |
| Response | Seconds (electric) | Minutes (HW/steam) |
| HW coil | 60โ90ยฐC supply | Uses hydronic loop |
| Steam coil | Low-pressure steam | Needs drain and trap |
| Running cost | Electric highest | Steam/HW lower |
| Heating rule of thumb | ≈ 0.34 W per m³/h per K | Air-side sensible heating |
| Worked example | 8,000 m³/h at 25 K ≈ 68 kW | Shows why large doors get costly |
A hybrid is common in retail: electric for quick defrost at the sill, hot water for steady heating. The control must stage the heat with the fan so the coil never runs dry, because a live coil with no airflow overheats and shortens element or tube life, an easy fault to avoid with correct sequencing.
Each medium brings its own protection requirements. A hot-water coil at a doorway is exposed to outdoor air whenever the door opens, so it needs a frost thermostat, a circulating pump that keeps water moving at low load rather than sitting still in the coil, and glycol where exposure is severe. Electric elements need a thermal cut-out and an airflow interlock, plus a run-on period so residual element heat is carried away instead of soaking into the casing. Steam coils need a trap sized for the condensate load, located below the coil with a fall, and a vacuum breaker so the coil drains rather than water-logging when the control valve closes.
Selection Checklist
- Match the service: Use hot water or steam if already distributed; electric only if no hydronic or steam main reaches the door.
- Size the lift: Capacity = airflow ร density ร Cp ร temperature rise, computed at the worst outdoor design temperature.
- Stage with the fan: Never energize the coil without airflow, or the element or tube overheats.
- Check the bill: Electric heat at a busy entrance is a real operating cost that the energy model should capture.
- Plan drainage: Steam coils need a trap and drain; site them where that is practical.
- Do the arithmetic: Use airflow, temperature lift, and duty cycle to size the heat before choosing a medium.
- Protect the coil: Frost stat and continuous circulation on hot water; cut-out and run-on for electric elements.
Why the Medium Choice Matters
The right heating medium keeps the doorway comfortable at the lowest lifetime cost and avoids an outsized electric load on the building's service. Keruite offers air curtain units configurable for electric, hot-water, or steam heating so the curtain matches the services already in the building, and the owner is not paying to extend a plant they do not have.
Doorway heating is one of the few decisions where first cost and running cost point in opposite directions, so the choice should be made against the services already on site. Air curtains built in an 80,000 m² plant by a manufacturer with more than 30 years of air-moving experience are offered with electric, hot-water, or steam coils on a common housing, and the airflow and throw behind each option are measured in an in-house performance laboratory. ISO 9001:2015 covers coil and casing fabrication, CCC certification covers the electrical package, and with deliveries into more than 50 countries units are configured for local supply voltages.
Our engineers respond to selection enquiries within 24 hours, including heating capacity and medium selection for air curtain doorways.