To start this post, let’s answer the question, What is a Hot Surface Igniter? The hot surface igniter, or glow bar as it is commonly referred to, is a device that lights the burners in your furnace. Just as you used to have a pilot light in the old furnaces, the hot surface igniter replaces that pilot light and provides an energy-efficient way of lighting the burners.
The best way to describe the operation of an igniter is to compare it to a light bulb without the glass outer shell. The igniter is an element or filament that glows and gets hot, just like a light bulb, when power is turned on to the igniter. Since it is not encased in glass, this hot element will then ignite the gas when the gas valve opens. Hopefully, this explanation, though very simplistic, answer how the igniter works.
The other questions I also get is why did manufacturers go to hot surface igniters instead of direct spark ignition. The answer, once you hear it, is pretty obvious — when you have a “spark” that keeps trying to light the gas you get a constant “CLICKING” sound that consumers found annoying, whereas, the hot surface igniter is silent. It really comes down to that simple answer.
So, now that we know how this works, lets look at some diagnostics. Of course, what would cause an igniter to fail and not ignite the burners? Again, comparing the igniter to a light bulb will explain why an igniter would fail to light the burners. If the element were to develop a crack, this would cause a hot spot causing the igniter to burn out just like a light bulb does when it fails.
Can this be checked to prevent “no heat situations” in the middle of winter? Though there are no guarantees in life, a simple resistance check can help determine if the igniter is “GOOD” or possibly going to fail. By using an ohm meter and performing a resistance check of the igniter, when it is COLD, and then comparing it to the manufacturer’s specifications, it can be determined if the igniter is developing problems and could possibly fail in the near future. All igniters have this cold ohm resistance published by the igniter manufacturer. Knowing, or having these values available, makes diagnostic very easy. Attached at the end of this post is a chart we use for the igniters we use currently and have used in the past. Feel free to print it out and keep it with you.
Using this chart, let’s look at an example. The Norton igniter should have a resistance between 40 and 75 ohms when it is “cold”. if this check is performed and the igniter does not meet this specification, chances are the igniter is destined to fail sooner or later.
I also suggest that a resistance check be performed on a new igniter before putting it in a furnace. If you think about it, the igniter has been “bounced around” in the back of the service truck until it is needed to be used. Like a light bulb, the igniter could become broken from this rough handling. Much better to check it before going all the way through the process of installing it and then finding out it is bad out of the box.
One of the main causes of igniter failure is too much cycling. Just like a light bulb will burn out quicker if you keep turning it off and on, so will an igniter if you do the same to it. Too much cycling can be caused by any number of reasons but the most common are:
- the unit cycling on the limit control (improper temperature rise),
- improper gas pressure (unit over fired),
- unit improperly sized for the application,
- Thermostat heat anticipator or cycle rate not properly set to match the controls in the furnace
I’m sure there are others you can think of? These items should be checked whenever maintenance if performed on the unit and any problems corrected to prevent short cycling and possible premature igniter failure.
If the “cycle rate” of the furnace is proper, based on the above criteria, and the igniter resistance is checked periodically, the hot surface igniter is a very reliable and energy-efficient way to ignite the burners of your furnaces. Keep in mind, just because there is no pilot light, that this furnace does require annual maintenance to keep it operating safely, efficiently and trouble-free.