We’ve talked a lot about the safety systems keeping your furnace from burning down the building or blowing up your building. This all begs the question: how do we get the flame started in the first place? It’s actually not a fully straight forward answer, and it varies by fuel source and furnace design.
The Old Fashioned Pilot Light
In days long-gone, a furnace needed a constant flame to light it’s burner. This was called a Pilot Light. It was just a tiny, constant little flame like a lit candle. When the burner started, it simply had to turn the gas on and the pilot would ensure that the whole burner lit afterwards. The solution worked well enough, but by modern standards is an incredibly wasteful way to run a furnace.
In systems with an always-on pilot light, fuel would always be getting burned, even if there wasn’t heating anything. Overtime, this adds up to hundreds and thousands of gallons of wasted fuel. It certainly worked for a time when we had no better alternatives but it’s a relic in today’s high-efficiency world.
One of the major hurdles of moving on from a pilot was creating enough energy to light the fuel. It takes more than just a spark for ignition, it can take significant voltage. Between the fuel mixture, spark size, spark temperature, and everything else, it’s a difficult ballet to directly, electrically light a large burner. On the other hand, it’s very easy to light a small flame.
Systems using Intermittent Pilot Lights have an electronic ignition system that lights the pilot, then uses it to light the main burner. This creates a best of both worlds scenario where fuel isn’t wasted and where the actual burner ignition is as simple as possible. There’s no need for complicated electronics like those seen in a purely electronic ignition system, just a simple spark to start a small flame.
Hot Surface Ignitors
As technology as rolled forward, we’ve created systems which don’t need a spark or a traditional pilot light. In many gas-fired systems, the gas will ignite around 1,200º Fahrenheit. All we need is to make something that hot to ignite the fuel with. Luckily, materials such as Silicon Carbide will survive past 2,000º Fahrenheit and conduct electricity. With these systems we push low voltage, high current power into the igniter. It’ll become so hot that the interior of the furnace will be lit with heat. When the gas is turned on, it strikes the igniter and catches fire.
Modern systems will have Hot Surfaces more often than pilot or intermittent pilot lights due to the increased efficiency. The intermittent pilot still wastes some fuel for the start up process, it adds steps to the ignition process, and it still relies on an electronic ignition system. A Hot Surface by contrast, just needs to be turned on before the burner gas valve is opened. This simplicity gives it an advantage in efficiency and cost for gas fired systems.