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Electric furnaces
When troubleshooting, understand you are working with potentially lethal voltages. If you do not have the ability to do these operations safely, do not attempt them.
These furnaces use electricity for the heat.
The information below give some ideas how they work and how to diagnose and repair failures.
They are characterized by one to three heavy 240v wire circuits.
The heaters are usually Nichrome wire which is a high temperature wire.
Each of the elements usually are 5 kilowatt and the furnace may have from 2 to 6 of these elements.
The elements are controlled by contactors or sequencers which turn the elements and the circulating fan on.
The video below gives an overview of how the electric furnace heat strip is put together.
Below is a demonstration of the elements sequencing on and off.
Electric furnaces use high temperature limits as as safety to shut off the elements if the temperature exceeds a preset number. The limits below are rated at 145 degrees F (63 degrees C). They will shut off when the temperature exceeds the rating and automatically reset when the temperature drops below the rating. The video below shows the limits in operation.
The large amperage draw of the components of an electric furnace often causes any loose terminals to overheat. All connections need to be as tight as is practicable. Never the less, terminals will overheat. The result is burned wires and melted components. The video below discusses how to find potential problems and prevent failures.
Sequencers are time delay switches often used in electric furnaces to control the heating elements and the circulating fan. They operate by low voltage (24V) heaters that warm bimetal discs that time on and off the elements in sequence. The video below explains their operation.
The video below shows what the stack type sequencer looks like inside.
The video below covers replacement of a furnace sequencer.
The most common problem with electric furnaces is overheat of components. Wire terminals commonly overheat due to resistance from poor connections.
Poor airflow due to plugged filters may cause the furnace to overheat and high temperature limits open to protect the furnace. The high temperature limits were not made to open numerous times and will fail either open or, more dangerously, closed. The limit itself may burn up. The furnace may also have high temperature fuses that will open if the limit sticks closed. These fuses open permanently to protect from fire.
The video below shows an electric furnace that has a number of burned wires and components that give an idea of what happens to these furnaces.
Poor airflow due to plugged filters may cause the furnace to overheat and high temperature limits open to protect the furnace. The high temperature limits were not made to open numerous times and will fail either open or, more dangerously, closed. The limit itself may burn up. The furnace may also have high temperature fuses that will open if the limit sticks closed. These fuses open permanently to protect from fire.
The video below shows an electric furnace that has a number of burned wires and components that give an idea of what happens to these furnaces.
On some newer electric furnaces, there is an off delay timer that includes a fan relay as well as an off delay timer for the fan in cooling. The video below shows how this part works.