Duct Heater kW Calculator and Sizing Formula

A duct heater kW calculator estimates the electrical heat output needed to raise a known airflow by a chosen temperature difference. Enter airflow and temperature rise, and it returns the ideal heating power in kilowatts; actual equipment selection also depends on site conditions and manufacturer limits.

Formula

The ideal sensible-heat formula for air is:

Power (kW) = 1.2 × airflow (m³/s) × temperature rise (°C)

• Airflow is the volume of air passing through the heater each second, in cubic metres per second (m³/s).
• Temperature rise is the desired outlet-air temperature minus the inlet-air temperature, in degrees Celsius (°C). A temperature difference of 1°C equals a difference of 1.8°F.
• 1.2 is an approximate air heat-capacity factor in this unit combination, representing typical air density and specific heat. It is an engineering approximation, not a universal constant for every pressure, altitude, humidity, or temperature.

The calculator also converts the result using 1 kW ≈ 3,412.142 Btu/h and 1 m³/s ≈ 2,118.88 cubic feet per minute (cfm). These are unit conversions, not additional estimates of heater performance.

Worked Example

With the default airflow of 0.5 m³/s and temperature rise of 20°C:

1. Multiply airflow by temperature rise: 0.5 × 20 = 10.
2. Apply the approximate air factor: 1.2 × 10 = 12.00 kW.
3. Convert to Btu/h: 12 × 3,412.142 = 40,945.704, which rounds to 40,946 Btu/h.
4. Convert airflow: 0.5 × 2,118.88 = 1,059.44, which rounds to 1,059 cfm.

So, under the assumptions in the formula, the required ideal heat output is 12.00 kW for 0.5 m³/s of air and a 20°C temperature rise.

How to Use the Calculator

Use airflow measured at or representative of the heater and the temperature increase you need across it. The inputs are metric: m³/s for airflow and °C for temperature rise. If your readings use other units, convert them before entering values.

• Convert litres per second to m³/s by dividing by 1,000. For example, 500 L/s is 0.5 m³/s.
• Convert cfm to m³/s by dividing by about 2,118.88. For example, 1,000 cfm is about 0.472 m³/s.
• Find temperature rise by subtracting inlet temperature from the intended outlet temperature. For example, raising air from 10°C to 30°C means a 20°C rise.
• If temperatures are given in Fahrenheit, subtract them to get the Fahrenheit difference, then divide that difference by 1.8 to obtain °C. For instance, a 36°F rise is 20°C.

The headline result is thermal output in kW, not a complete specification for a heater or its electrical supply. The Btu/h line expresses the same heat rate in another unit; the cfm line helps check your airflow conversion.

What the Estimate Means

The calculation is based on the sensible heat needed to warm a moving stream of air. It assumes a steady airflow, a uniform temperature rise, and typical air properties. It does not account for heat lost from ducts, leakage, changing airflow, heat absorbed by ductwork, controls, or the way the heater cycles.

For a more exact calculation, the general relationship is heat rate = mass flow rate × specific heat capacity × temperature rise. Mass flow rate depends on air density and volume flow. Density changes with pressure, altitude, temperature, and moisture, so a project engineer may use actual site conditions and equipment data instead of the approximate factor 1.2.

A useful comparison: doubling airflow while keeping the same temperature rise doubles the estimated kW. Doubling the temperature rise at fixed airflow does the same. If both double, the estimate becomes four times as large. This proportional relationship is a quick way to spot a mistaken input or surprising result.

Choosing a Duct Heater

Use the estimate as a preliminary heat-output calculation, then check a heater’s published capacity at the intended airflow and operating conditions. Electrical duct heaters commonly have elements that convert electrical input to heat, but the installed system still has to move the specified air safely and distribute it as intended.

Selection requires more than a kW figure. Confirm the heater’s allowable airflow and temperature rise, duct dimensions, installation orientation, control method, and compatibility with the ventilation or HVAC system. The equipment documentation may specify minimum airflow, temperature limits, clearances, interlocks, and other installation conditions. A qualified HVAC and electrical professional should verify the design and the required supply, protective devices, wiring, and controls against applicable requirements. This calculator is not a method for sizing wiring or breakers.

Common Errors and Limits

• Using the wrong airflow unit: entering a cfm number as though it were m³/s produces a result thousands of times too large. Convert first.
• Entering a target temperature instead of a rise: use outlet minus inlet temperature, not the outlet temperature alone.
• Mixing Fahrenheit and Celsius: convert the temperature difference, not merely one temperature, when working from Fahrenheit readings.
• Treating the estimate as an equipment rating: the formula estimates ideal heat delivered to air. Losses and operating constraints can change the capacity needed in practice.
• Assuming all conditions are standard: unusual altitude, pressure, or air conditions may make the approximate factor less representative.

A result of zero occurs if either airflow or temperature rise is zero. A negative rise is not a heating requirement; it describes cooling under the stated inlet and outlet conditions. Check signs and units before using any result for design decisions.

Frequently Asked Questions

How many kW do I need for a duct heater?
Estimate it from airflow and required temperature rise: kW ≈ 1.2 × m³/s × °C rise. Then have a qualified professional match the estimate to equipment performance and system requirements.

How do I calculate duct heater kW from cfm?
Convert cfm to m³/s by dividing by about 2,118.88, then multiply by 1.2 and the temperature rise in °C. Alternatively, use kW ≈ 0.000566 × cfm × °C rise.

How do I calculate kW from Btu/h?
Divide Btu/h by approximately 3,412.142 to convert a heat rate to kW. For example, 40,946 Btu/h is about 12 kW.

Is duct heater kW the same as electrical input?
For a resistance electric heater, electrical input is generally converted to heat at the elements, but the rating and delivered performance depend on the specific product and conditions. Check its documentation rather than assuming the calculator specifies electrical requirements.

Can I use this formula for a gas duct heater?
The airflow-and-temperature-rise relationship estimates heat delivered to air regardless of heat source, but it does not calculate fuel input, combustion efficiency, or venting requirements. Consult the gas heater’s performance data and a qualified professional.

Related

• Convert Story Videos to a Consistent FFmpeg Loop
• How to Calculate Oxygen Saturation With a Pulse Oximeter
• CS2 FPS Calculator: Estimate Counter-Strike 2 FPS

in airflow | Airflow | m3/s | 0.5 | 0
in rise | Temperature rise | °C | 20 | 0
out kw | Required heating power | kW | 1.2 * airflow * rise | 2
out btuh | Heating power | Btu/h | kw * 3412.142 | 0
out cfm | Airflow | cfm | airflow * 2118.88 | 0