Most modern gas furnaces are fitted with a type of igniter known as a hot surface igniter. These igniters are very simple, and therefore very reliable, but they can cripple your entire system if they do break down. In many cases, faulty hot surface igniters cause a problem known as short cycling.

What Are Hot Surface Igniters?

Hot surface igniters are small, ceramic rods located within your furnace's combustion chamber. When the furnace is activated, an electrical current is passed through the igniter. This creates electrical resistance, heating the igniter until it ignites the surrounding gas.

Why Do Hot Surface Igniters Fail?

In most cases, hot surface igniters fail because of basic wear and tear, usually after many years of use. As the igniter heats up and cools down, it expands and contracts slightly, which stresses the brittle ceramic the igniter is made of. Over time, expansion and contraction cycles can cause the igniter to crack. Cracks interrupt the electrical current that passes through the igniter, preventing it from heating up.

Hot surface igniters can break down more quickly if they regularly become coated in soot. If your gas furnace creates large amounts of soot while burning gas, solid soot deposits can build up on the igniter. These deposits act as heat insulators, preventing them from igniting the surrounding gas. 

How Do Hot Surface Igniter Faults Cause Short Cycling?

If your furnace has a faulty hot surface igniter that isn't heating up correctly, the gas flowing into the combustion chamber will fail to ignite. After a few seconds, the furnace's gas shutoff valve will kick in to prevent a dangerous gas leak, and the furnace will shut down. If the furnace is left active, it will try to start up again a short while later, only for the problem to repeat itself. 

This is known as short cycling and is bad news for any gas furnace. With no ability to burn gas, your furnace will not produce any heat, but that isn't your only problem. If your short cycling furnace is left running, it will consume large amounts of energy, and the constant stopping and starting will cause excessive wear to moving components. Short cycling can leave you both out in the cold and out of pocket.

If your furnace has a digital readout screen, it will probably show an error code if and when this problem occurs. If you are reasonably familiar with furnace maintenance, you can also test the igniter directly, by attaching it to a multimeter and checking whether electrical current can flow through the igniter.

For more info, contact a local heating contractor.