Troubleshooting Electric Furnaces
Electric furnaces can be intimidating if you haven't had a lot of experience in troubleshooting them when you're responding to a "not heating enough" or "blowing cold air", or simply a "not heating" complaint. However, when you break an all-electric, forced-air heating unit down, it really becomes one of the simplest pieces of equipment to repair.
Consider the operating parts...the only things there are:
1. Heating Elements
3. Blower Motor
4. Main Line Circuit Breakers (or Fuses if it's an older unit)
5. Over-Temperature Protection Devices
That's really all there is. Of course, we could break #5 down into two categories such as a Fusible Link and a Bi-metal Limit Switch, but even considering that, it still doesn't make the electric furnace so complicated that it can't be understood, providing you have the skill to read a schematic diagram, understand the sequence of operation when there is a call for heat, and you either have a general idea what a component would test like (or specific information from the manufacturer), regarding how much resistance you should read on a given load, such as a heating element, motor winding, or sequencer operating element.
As an example, we'll consider an electric furnace that, according to the customer, is "blowing cold air".
This brings is to the first step in troubleshooting....taking the information we get from the person who is requesting service and figuring out what that really means in specific terms relative to the operation of the equipment. For example, when this particular problem is described this way ("blowing cold air"), it means we can automatically eliminate some of the components as being the possible source of the equipment failure.
First, we know that the problem can't be the transformer. The system is trying to do something, and that couldn't be occurring unless the transformer was OK. And, then there's the thermostat. Without it calling for heat like it should, the condition we've described in this particular problem could not exist. And what about any main line protective devices (circuit breakers, for example) inside the unit? Since we can expect the indoor air handler to be operating on the same voltage as the heating elements (240 VAC), then we can also set aside any consideration of those devices being part of the problem.
What we're saying here is that before you even show up to check this equipment, you've already eliminated some of the possibilities. Of course, when you arrive and confirm the customer's description of the problem is when you really 100% eliminate those components as possibilities, but still, when the troubleshooting process has begun for you before you're even on site, you're already on your way to finding out what needs to be done in order to repair this equipment.
Here's what the schematic looks like for the particular problem we're going to solve:
Like any dual-voltage (240-volts for operation and 24-volts for control) diagram, this one is built on the symbol for the transformer, and that fundamental symbol has simply been expanded to allow the placement of the equipment operating components. And, to make sense of the diagram, the next thing to consider is the legend. (See the illustration below)
When you match the legend up with the diagram, you gain a better understanding of the symbols being used and the components they represent. In this case, for example, you can see that the letter "E" is being shown as identifying heating elements, and you can see the "E" near the symbols for the heating elements in the area identified as "TOP" (meaning that this system has two elements in that location), but it isn't visible in the area identified as "BOTTOM". Instead, this manufacturer has provided us with specific information (the kilowatt rating of the element) while neglecting to show the "E". No problem, we can still identify all the heating elements in this equipment (there are three) by simply understanding the schematic symbol for resistance heat....the sharp peaks on the symbol.
The legend also helps us identify the two sequencers in this equipment. Seq 1 and Seq2. As we would expect, the control element of the sequencers are shown on the control side of the diagram, and the switches of the sequencers are shown on the operating voltage side of the diagram.
Since we're dealing with a "blowing cold air" complaint, we'll go straight to what is supposed to be our heat source (the elements) and we'll make some tests:
....With a voltmeter, we check the terminals of the bottom element, and we find that there is 240-volts applied.
....Next, with an ammeter, we check the current draw on the line feeding the bottom element, and we find it to be within the manufacturer's specifications.
So, what the above means is that the bottom element is not the problem since it is drawing current, and therefore providing heat like it should.
Hmmmmm....didn't our customer say that the equipment was "blowing cold air"?
Yes, they did, but what we just found out with our first two meter tests is that their concept of "blowing cold air" is really "blowing air not as warm as they are used to". A fine point, but one worth noting.
At any rate, now that we've eliminated the bottom element as a possible source of the problem, we'll move on to the top elements.
When we use our test equipment to check these two elements, we get the following results:
...o-volts at each element
...240-volts at each set of normally open contacts at Seq1
...24-volts at the Seq1 control segment.
And, with this series of tests, we've just isolated the failed component as Seq1...Sequencer #1.....because:
....There is no energy being applied to the heating elements, and therefore they can't provide any heat.
....There is a reading across the normally open switch, and that tells us that the switch, even though it's supposed to be closed when 24-volts is applied to the control segment of the sequencer, it is in fact open because we can read voltage by testing the two points of the switch.
....Which means that the sequencer, even though we are applying 24-volts to the control segment, is not closing in order to allow a complete circuit to the top elements, and this results in the complaint of "blowing cold air" (which is really "not heating enough" but the customer doesn't know the difference between the two. But, no matter, it's our job to figure that out when we get to the site to solve the problem.
For information on a training video (DVD) on electric furnaces, click HERE.
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