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How can I test individual elements in my electric furnace?

How can I test individual elements in my electric furnace?

Building Supplies , HVAC
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Manage My Life
One of the most difficult things to detect is burned out elements on electric furnaces. Because these furnaces may have up to six elements, a normal operating test will not usually reveal whether all of the elements are operative.

This is an electrical or mechanical failure.

Under cold weather conditions, adequate heat may not be available in the home.

You won't usually be able to tell just by cycling the furnace and doing a visual examination. You will get heat at the supply registers and the temperature rise (if you measure it) will not be excessive. It might be slightly on the low side.

Clamp-on ampmeters

can be used without disturbing electrical connections. As a result, they are relatively safe devices. These measure the current flow through wires by sensing the electrical field around the wire. They are not particularly expensive and are relatively easy to use. We recommend that you have an electrical specialist introduce you to this device and show you how to operate it, if you choose to perform this kind of test. Clamp-on ampmeters are useful for measuring the current flow through 240-volt circuits. You have to measure the current flow through one leg or the other. You can't do both at the same time.

With the furnace at rest, the ampmeter can be clamped around either the black or the red conductor powering the furnace. This can be done at the furnace or at the electrical panel. Without turning the thermostat up, you may want to turn the furnace fan on using the fan switch or an auxiliary switch on the thermostat. This allows you to isolate the fan current from the electric elements. The fan current will draw through either the red or black wires if it is 120-volt, and will draw through both if it is a 240-volt fan motor.

We want to determine the current draw of the fan and then ignore it. The typical current draw from a blower fan is two to five amps. These fans are most often 120-volts. The two to five amps will be drawn through either the red or the black wire. With your ampmeter you can figure out which wire is powering the blower. For this test, you should clamp your meter onto the other wire. This is the wire that does not power the blower.

When you turn up the thermostat, you should watch the ampmeter and look at your watch. What you should see, assuming a 6 element furnace, is the following: 1. When the thermostat comes on, the meter reading will jump from zero to 21 amps as the first element comes on (assuming a 5 kilowatt element, which is the most common). 2. After 30 to 90 seconds, the current draw should jump to 42 amps. This indicates that the second element has come on. 3. After another 30 to 90 seconds, the current draw should jump to 63 amps, indicating that the third element has come on. 4. After yet another 30 to 90 seconds, the meter reading should jump to 84 amps, indicating that the fourth element has come on. 5. And, after another 30 to 90 seconds, the current reading should jump to 104 amps, indicating that the fifth element has come on. 6. After another 30 to 90 seconds, the meter should jump to 125 amps, indicating that the sixth element has come on. If the time delay between elements is usually 30 seconds, and there is a 60 second delay between any of the steps, you can reasonably assume that one element is not working. If the total current draw is less than the rated current draw, you know that at least one element is not working. You may or may not be able to determine which element isn't working, depending on how well the test is performed, although this part isn't terribly important.

Safety Note:

As always, you should not test electrical equipment if you are at all uncertain what to do. Better to contact a professional than to risk electric shock.
by Manage My Life
April 26th, 2007
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