How do i know if all my heater elements are working in my Norton 21ECM20model furnance

Depending on the unit sometimes you can actually see them with the door off. This isn't a good idea to do because you can trip a limit switch due to not enough air being forced a crossed them. But if it's possible be sure to keep door closed as much as possible. The true way is to take an amp reading on the high voltage wire. Approximately 20 amps per 5kw of heating. 3 heat strips =60 amps. Good Luck.

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Clamp the wires to the elements and see if they are drawing current