Duct Heaters

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Forced Air Duct Heaters

Selecting the Proper Duct Heater

1. Establish the requirements of your process heating application. Select the heater assembly type, size, watt density, sheath material and electrical rating best suited for your application.

2. Match the heater watt density (w/in2) to the medium being processed. If the watt density is too high for the application the heater will fail prematurely.

3. Match the sheath material and operating temperature to the medium being processed in order to avoid sheath corrosion.

4. Air flows must never be interrupted. Such events will cause overheating and/or premature heater burnout. Your installation should include high limit temperature controls.

5. Be sure that the heaters are securely mounted and protected from mechanical damage.

6. Select the terminal housing that provides the best terminal protection from the environment surrounding the application.

7. Thermostats are optional. Select the type and temperature rating required for the application. See Section 13.

8. If practical, heaters should be cleaned periodically in order to extend heater life.

9. If you should encounter any problems in selecting and/or installing a process heater, consult Tempco for assistance.

Sizing a Duct Heater

To properly match a duct heater to an application, the wattage, air velocity and element watt density must be determined.
Formulas and graphs on the following pages that will aid you in your design include:

  • Wattage calculation formulas and table
  • Element Watt Density vs. Sheath Temperature and Air Velocity Graph
  • Pressure Drop vs. Air Velocity Graph
In most applications the following design limitations should be adhered to:
  • Maximum watt density of 40 watts/in2 (6.2 watts/cm2)
  • Maximum element sheath temperature of 1400F (760C)
  • Minimum air velocity of 200 feet per minute
    (61 meters per minute)

Installation Recommendations

1. Tempco Duct Heaters may be bolted to the ductwork through the side, bottom or top. Bottom and side mounting are preferred to minimize wiring/terminal enclosure temperatures.

2. Before mounting, consideration should be given to the strength of the ductwork required to support the weight of the heater. Add additional hangers or supports as required.

3. The inlet side of the unit should be at least 48 inches downstream from any change in duct size or duct direction.

4. To minimize pressure drop, mount in the duct with the narrow width of the heater perpendicular to the air flow.

5. Duct heaters may be mounted in tandem to increase the KW that can be installed.

6. Process temperature sensing should be located downstream from the duct heater.

7. All standard duct heaters have a thermowell attached to one element for installing a thermocouple to sense element temperature. Additional protection for the heater from low air flow can be achieved by installing an air flow switch or pressure switch on the inlet side.


1. Power supply conductors must have a minimum ampacity of 125% of the maximum heater load and be rated for the ambient temperature of the heater enclosure.

2. The air handler should run on a time delay after the heater is de-energized. This allows the elements to cool without overheating adjacent areas.

3. Duct heaters drawing more than 48 Amps are divided into smaller branch circuits; each drawing 48 Amps or less. Please note that the number of circuits can be changed to accommodate any wiring requirements you may have.

Note: Before you proceed to make any changes on factory prewired heaters check the heater wiring schematic or consult Tempco.

All electrical wiring must be done in accordance with national and local electrical codes.

Maintenance Recommendations

1. Never perform any type of service on duct heaters prior to disconnecting all power supply lines.

2. After long periods of idle use, clean elements prior to start-up.

3. Periodically clean the elements even during regular use so as not to allow dirt to build-up on the elements.

4. Periodically check that mounting screws or bolts have not become loose from blower vibration.

5. Periodically check that electrical connections are clean and tight.

6. Failed elements are field replaceable, minimizing downtime and saving the cost of a complete new heater.