Calculator Inputs
Formula Used
SI heat rate: kW = ρ × Cp × airflow × ΔT ÷ efficiency.
Imperial reference: kW = 1.08 × CFM × Δ°F ÷ 3412.142.
Current: A = kW × 1000 ÷ volts. Three phase uses √3 × volts.
The calculator converts airflow and temperature units first. It estimates air density with elevation and average air temperature. It then applies efficiency and design margin. The result is required electric heater input power.
How to Use This Calculator
- Enter the airflow through the heating section.
- Select the airflow unit used by your design data.
- Enter entering and target leaving air temperatures.
- Add efficiency, elevation, margin, stages, voltage, and phase.
- Press Calculate to see heat size above the form.
- Use CSV or PDF buttons for saved results.
Example Data Table
| Airflow | Entering Air | Leaving Air | Margin | Approximate Result |
|---|---|---|---|---|
| 800 CFM | 55 °F | 90 °F | 10% | 9.75 kW |
| 1,200 CFM | 55 °F | 95 °F | 10% | 16.72 kW |
| 2,000 CFM | 50 °F | 100 °F | 15% | 36.40 kW |
Understanding Leaving Air Electric Heat Sizing
Leaving air temperature is a useful design target. It shows how warm supply air becomes after the electric heater. The target depends on comfort needs, duct losses, and equipment limits. A higher target needs more kilowatts. A lower target saves energy and reduces thermal stress. This calculator links airflow, temperature rise, density, and electrical input. It helps designers compare heater banks before final selection.
Why Airflow Matters
Airflow carries heat through the duct system. More airflow needs more heat for the same temperature rise. Low airflow can overheat a heater. High airflow can leave supply air too cool. Correct airflow protects coils, controls, and safety switches. Many field problems start with dirty filters or weak fans. Always verify real airflow when possible. Nameplate airflow can differ from measured airflow.
Using Leaving Air Targets
Entering air temperature is the air condition before heat is added. Leaving air temperature is the design supply condition after heat is added. The difference between them is temperature rise. Electric heat capacity is based on that rise. For comfort heating, moderate rise is usually preferred. For process air, a higher rise may be required. Check the heater maker for maximum discharge temperature.
Density and Elevation Effects
Air density changes with elevation and temperature. Thin air carries less heat at the same volume flow. That means high altitude systems may need careful review. The calculator estimates density using elevation and average air temperature. This improves the result compared with a fixed rule. It is still a planning estimate. Final values should match project specifications and product data.
Voltage, Stages, and Current
Electric heaters are usually divided into stages. Staging improves comfort and reduces cycling. The calculator divides total kilowatts by stage count. It also estimates line current. Single phase and three phase circuits use different current formulas. Power factor is included for flexibility. Resistance heat is often near unity. Local electrical codes still control wiring and protection.
Practical Design Notes
Use a design margin when conditions are uncertain. Do not oversize without reason. Oversized heaters can short cycle. They may also raise discharge temperature too fast. Compare calculated load with standard heater sizes. Then check airflow proving, limit controls, and clearances. For installed heaters, enter actual kW. The tool estimates achieved leaving temperature. Always confirm final selections with qualified local professionals.
FAQs
What does leaving air temperature mean?
It is the air temperature after the heater. It is measured downstream of the electric heat section. It helps show whether the heater can meet the desired supply condition.
Why is airflow required?
Airflow sets how much air must be heated each minute. Higher airflow needs more heat for the same temperature rise. Low airflow can cause unsafe heater temperatures.
What formula does this calculator use?
It uses sensible heat transfer. The main form is kW equals air density times specific heat times airflow times temperature rise. Efficiency and margin are then applied.
Can I use Celsius values?
Yes. Choose Celsius in the temperature unit field. The calculator converts values internally. The final temperature rise is shown in both major units.
Why include heater efficiency?
Most resistance heaters are close to one hundred percent. Efficiency is included for advanced checks, unusual systems, or conservative planning. It adjusts required electrical input.
What is design margin?
Design margin adds extra capacity above the calculated need. It covers uncertainty in airflow, controls, altitude, and load. Use reasonable margin to avoid oversizing.
How are heater stages calculated?
The calculator divides total required kW by the number of stages. This gives an equal stage size. Real equipment may use unequal or sequenced stages.
Does elevation affect the answer?
Yes. Higher elevation lowers air density. Lower density means each volume of air carries less heat. The calculator applies an estimated density correction.
Can this size electrical wiring?
No. It estimates current for planning only. Wiring, breakers, fuses, and disconnects must follow local electrical codes and equipment instructions.
What if installed kW is entered?
The calculator predicts achieved leaving air temperature. It also compares installed kW with required kW. This helps review existing heater capacity.
Why is the result different from quick rules?
Quick rules often assume standard density and fixed units. This tool also considers elevation, efficiency, margin, stages, voltage, and phase.