Many factors influence the reliability of an electric duct heater, the use of quality parts being the most significant. Contactors, transformers, manual resets and automatic limit switches are all essential to a heater's operation and are fairly easy to replace if they happen to fail.
This is not always the case with a heater's resistive elements. Elements are the most critical component of an electric heater and can be difficult to replace.
Open coil heating elements are made of a nickel/chromium alloy that radiate energy through their outer surface, or skin. The elements' skin can reach temperatures as high as 1700 degrees fahrenheit. When operated at such extreme temperatures, oxidation will begin to degrade the elements. To retard this process, and extend element longevity, surface temperatures of the elements should be kept as low as possible while still obtaining the wattage needed for the application.
"Watt Density" is the rated wattage per unit of an element's surface area, i.e., "watts per square inch". To lower the "watt density", use longer and/or heavier gauge wire to increase surface area. The result will be elements that operate at cooler temperatures and therefore last longer.
High quality resistive heating elements are designed to operate below their maximum rated temperature. When operating at 100% capacity they should appear to be dark or black in color with almost no red glow anywhere on the element.
A heater that is made with low quality materials, or that does not receive adequate air flow, will operate closer to its maximum rated temperature. In that case the resistive elements will appear to be dark orange to bright red in color, an indication that the elements are degrading and will need to be replaced earlier than usual.
ENSURE THAT YOUR HEATER HAS A LONG, REILABLE LIFESPAN
A well designed heater can be expected to deliver 15-20 years of reliable service. Ensure that this is the case by consulting with an HVAC design engineer prior to placing an order for equipment.