Tips for Troubleshooting Your Electric Furnace

To most people an electric furnace is a complicated beast. Full of wires and heating elements and gadgets too many to number. But it isn't all that bad once you understand what goes on and why.

A furnace is like any other piece of equipment. It works on well understood principles and like most other devices, knowledge of the basic functions allows understanding of the rest of the workings to sort of fall into place.

Just recently the electric furnace in my home began malfunctioning. I could hear the thermostat mounted on the wall behind me click but the furnace failed to respond. As the temperature slowly fell in the house I also noticed that while the furnace had not kicked on, it was making some noise. A loud click to be precise. So what was going on in my heating unit?

A furnace works this way. Let us assume the thermostat has clicked. This is a demand call for heat. The thermostat has simply closed a switch and this is the click we hear. The wires trailing from the thermostat lead directly to the furnace. Their is a voltage on those wires and when the thermostat closed the switch, this voltage is now free to set in motion a chain of events that will activate the furnace.

So where did that voltage come from? It comes from a transformer mounted within the furnace. This transformer takes the 110 volt house voltage and transforms it to somewhere between 24 and 70 volts. Depends on the manufacturer. This voltage is then applied to the wires leading to the thermostat. If the thermostat is open and there is no demand for heat, that voltage travel stops. It can't go any farther and nothing happens. If the switch is closed, the voltage returns to the furnace and goes directly to a device called a sequencer.

A sequencer is a device that operates on heat. Inside of the sequencer is a tiny heating coil, a spring loaded metallic contactor and two contact points. Here is the "sequence" of events. The sequencer heating coil gets voltage from the transformer once the wall mounted thermostat closes it's switch and the coil heats up. The metallic contactor inside gets hot and deforms in a bending motion thus overcoming the opposing pressure from the spring. Once it bends so far, it come in contact with two metal contactor points, thus bridging the gap between them. One of these contact points is connected to 220 volts and the other is connected to the heating element. So with the sequencer bridging the gap of these two contact points, voltage is thus applied to the elements and we have heat.

But we aren't done yet. Our sequencer hasn't quite completed it's job. Many sequencers do double duty. They can activate more than one function. My furnace has a set of stacked sequencers. Two devices mounted on one body and two separate bodies. Not only can they activate more than one device, they can be set so that one function will always happen first, or last. For example, my furnace is set up so that the heating elements come on first. And then the blower comes on after the elements are good and hot. This prevents a prolonged blast of colder air before the heat arrives.

We have seen how the process unwinds to produce heat. The next function of our sequencers is to bring on the blowers. A second sequencer can be brought online by voltage from the thermostat and transformer combination or, a second sequencer can be activated by the first sequencer in line. This second sequencer also heats up, makes an internal contact and gets the blower motor going.

So the heating elements and blower have done their job and the thermostat responding to the temperature changes relaxes and opens the circuit. The first sequencer loses voltage and begins to cool off. The internal switch gets forced back into it's off position by the spring and the voltage is removed from the heating elements. All this takes a few seconds to occur. Regardless of how it is connected the second sequencer also will lose power and begin cooling and will disconnect thereby shutting off the blower as well. This is where the first or last/on or off feature comes into play again. By turning on last and shutting off last, the blower can extract the most heat and efficiency from the heating system.

So once again the "sequence of events is as follows. The thermostat closes and makes a "demand call". Voltage is applied to a sequencer which heats up internally and activates the heating elements. A second sequencer is also turned on and once it trips, it activates the blower motor. When the thermostat responds to the desired room temperature and opens the demand switch,voltage is removed from sequencer number one and the heating elements are turned off. Sequencer number two is now turned off and disconnects the voltage from the blower and the heating system shuts down until the next cycle.

With some voltage tracing performed in my misbehaving furnace I determined that while sequencer number one was working, the sequencer for the blower was not. A visit to a local parts supplier got the parts and I had them installed that evening. The clicking noise I had heard was the safety limit switches turning off the heating elements when the blower fan failed to turn on.