HVAC Training: 8 Reasons Why Hot Surface Ignitors Fail
To technicians and furnace manufacturers everywhere, the sequence of events that takes place for a gas furnace to start a flame that jets into the tubes of a heat exchanger is like a well-choreographed dance.
We’ll talk about an integral part of the sequence to bring heat into the home – the Hot Surface Igniter.
As the winter season approaches, we’re going to be using our gas furnaces more and more. A lot of those furnaces won’t be lighting up when they’re turned on for the first time this season.
The Hot Surface Igniters that start the flame in a gas furnace fail so often that it’s almost as common as replacing a capacitor in an air conditioning system.
Hot Surface Igniters are resistance elements made of Silicon Carbide or Silicon Nitride. Anywhere from 80 to 240 volts are applied to the wires that are attached to the igniter.
A ceramic base insulates the wire connection to the carbide element which looks like the letter M on most applications.
Another shape that we see in are spirals.
Most nitride igniters are formed in the shape of a one and a half-inch flat stick or a two-inch long cylinder.
When the voltage is applied to the wires, the element starts to glow because of the resistance the carbide creates from one wire to the next.
When it glows long enough, gas is poured over it and the flame ignites.
Hot Surface Igniters are resistance heaters. The element itself glows orange when the voltage is applied. Now, how hot that element gets depends on the voltage being applied to it.
A 120-volt Hot Surface will glow at around 2500 degrees Fahrenheit. Most gas fuels will ignite around 1100 degrees, so 2500 degrees is a little excessive.
A 240-volt igniter burns even hotter. Several control boards these days are made to support an 80-volt igniter. This way, the carbide breaks down slower adding life to the system.
Before hot surface igniter and spark ignition was around, we had gas pilots that would stay lit burning a one to two-inch flame year-round, whether the heat was on or not.
When the heat was turned on, the gas valve would flow more gas over the pilot to ignite the burner assembly that carried the flame.
For a pilot to stay lit all year, it could cost up to $150 depending on your location. Some of you have pilot lights still going strong on your 35-year old furnaces. Although super reliable when you need it, and not a truly major expense, it’s not a great use of resources to just let that gas flame burn all year either.
You might be okay with a small pilot burning in your furnace all year but it freaks out some of our customers.
A lot of people don’t even know that the furnaces that come out these days don’t have pilots anymore. They come with on-demand ignition components like a Hot Surface Igniters.
Meaning the igniter only comes on when it’s needed and even then it only comes on for less than a minute at a time.
Silicon carbide is one of the most common components that makes up a hot surface igniter. Not only are these igniters used to light gas furnaces but they’re used to light stoves, boilers, and other appliances that heat things around your house.
Carbide is also used as an abrasive, as a cutting tool, and has some automotive applications as well. The first carbide igniters were produced in 1969. From then until now, they’ve become one of the main choices for manufacturers to use as their ignition source.
Just like most components on your HVAC system, these parts last about five to ten years. Yes, you can get lucky and have one that lasts for 20 years but it’s far and few between.
Different hot surface igniter lasts longer than others. The trend over the last five to ten years has been to use the more durable Silicon Nitride igniters because they seem to be a little bit less brittle, making them better able to stand the test of time.
So, why do these Silicon Carbide igniters break so often?
The fact is a gas flame pours over these igniters which applies a lot of damaging heat to them. The same thing that makes them work also destroys them.
8 Reasons Why Hot Surface Igniters Fail
So, here are the reasons why Hot Surface Igniters break down prematurely.
1. They’re super brittle.
A technician may accidentally break Hot Surface Igniters as they were cleaning the burner assembly on routine maintenance. It happens.
If you took your index finger and thumb and brought them together even somewhat quickly, it would be enough force to break the carbide tip of a hot surface igniter to pieces.
A furnace that cycles on and off more than it should, will reduce the lifespan of a Hot Surface Igniter. Making sure this system is properly sized for the house is probably a good idea and we say it all the time but an improperly sized unit is going to cause all kinds of problems.
Maybe not in the first year of its life but long after the contractor who installed it is gone and not responding to customers’ phone calls anymore.
3. High voltage.
If an HOT SURFACE IGNITERS is exposed to higher voltages than it’s supposed to receive, they will surely break sooner than they should.
An 80 volts HOT SURFACE IGNITERS should have about 80 volts applied to it. Having 120 volts applying to that HOT SURFACE IGNITERS will cause it to break and sometimes almost immediately.
On the other hand, having too low voltage may not let the igniter burn hot enough.
Based on experience, one of our technicians once replaced a 240 igniter for a packaged unit with a 120 igniter. Not only did the HOT SURFACE IGNITERS break on the first startup but the high voltage backfired to the control board and took it out as well.
Some field experts say the oils on the hands of technicians will cause the carbide tip to break down earlier than it should while other experts say it won’t.
One thing is for sure though, the fewer contaminants that touch the surface of this red-hot igniter, the better.
Other contaminants around the house that can get on the hot surface igniter; our sheetrock dust, condensation, dirt, rust, and fiberglass.
5. Gas valve pressure
An over-fired gas valve will cause the flame to be hotter than it should be. Any kind of heat is going to break down the HOT SURFACE IGNITERS naturally, but making sure the system is set up properly can make its parts last longer.
6. A Faulty Control Board.
In most cases, the igniter on the furnace is lined up with the flame that shoots into the heat exchanger. It starts glowing red-hot when the control board tells it to come on. If the control board doesn’t tell the HOT SURFACE IGNITERS to turn off, it’ll continue to glow red hot.
You’d likely have a faulty board in this case and that wouldn’t be good for the igniter either. The normal time for that hot surface igniter to be energized is about a minute.
Most igniters achieve the maximum temperature in less than 15 seconds. Some ignition sequences can leave the igniter burning for about a minute.
The less the igniter has to be on, the longer the lifespan of the igniter. Some things just can’t be changed on a furnace such as this designed ignition sequence, so sometimes we’re just stuck with what we’re given.
7. Propane gas.
Propane is a very viscous gas. If you were to compare a natural gas furnace to a propane gas furnace after just five years of use, you would see that the burner assembly on the propane system looks like it needs to be cleaned more than the natural gas burners.
We’ve seen Hot Surface Igniters that stand in the stream of a propane flame, have the top half of the carbide tip burned off after just three to five years.
8. Radiant heat.
A heat exchanger that is overheated and is shutdown could radiate extra heat onto the igniter and damage it or its ceramic base. Especially in closed combustion systems like the Coleman Furnaces or the Intertherm Downflow Furnaces that you find in most modular and mobile homes.
A fan cools the heat exchanger after the call for heat has been satisfied. Making sure the fan stays on for more than 90 seconds might be a way to correct this.
Standing pilot lights are a thing of the past. Furnaces these days have spark igniters or Hot Surface Igniters.
Hot surface igniters usually need to be replaced every 5 years or so. Spark igniters have their own downfalls though, which is why the industry hasn’t dedicated itself to one technology or the other.