Standing vs. Intermittent Pilot Lights
A pilot light is the simple name for the flame required to light the gas used in a natural gas furnace or water
heater. For example, when a furnace is directed into action by a thermostat, a valve releases gas into the
burner, enabling the pilot light to ignite the gas and provide the necessary heat. Attached to the pilot light is
a tube with a valve on it that shuts off if the light goes out, a safety feature to prevent the continuous
release of gas even when the light--and thus the furnace--is inactive.
Types Of Pilot Lights
Most old furnaces were made using a standard pilot, which maintained a burning light at all times. This design
comes in one of two types, the first being a thermocouple system that uses a pair of different metals to generate
the electricity that keeps the aforementioned pilot valve on the gas valve open once the light has been lit.
Second is a Milli Volt system by which the pilot heats a second, larger device that generates sufficient
electricity to operate the main valve as well as the pilot valve. The rationale for this design is that if the
pilot isn't lit, the main burner won't light either. Unlike the thermocouple system, which is driven by a
24-volt transformer, the Milli Volt system doesn't require one, which makes it ideal for mobile homes and
swimming pool heaters because it doesn't require a utility power source.
Newer furnaces are electric and
usually come with an intermittent pilot system, either through a hot surface ignition system or a direct spark.
In an intermittent pilot system, the pilot responds to the thermostat's call for heat with a high voltage spark
that ignites the light. The main burner senses the pilot flame is lit through a flame-sensing rod, lighting the
main burner. While considered very reliable, should one fail they are considerably more costly than replacing a
part on a standard pilot system. In the intermittent spark-ignition pilot furnace, a thermostat's call for heat
triggers a pilot flame that, when detected, directs the electronic module or circuit board to signal the gas
valve's main valve to open, lighting the main burner until the thermostat's target is reached. At that point,
all ignition processes, including the pilot, will stop.
While a straight hot surface ignition system uses no
pilot light, an intermittent hot surface-ignition pilot has been designed by
Honeywell, by which a small hot
surface igniter is installed on a pilot light assembly with the flame sensor. The circuit components are inside
the gas valve, rather than on a separate board, and the hot surface igniter is triggered by the same calls for
action as the spark-ignition model, only without the pilot light.
Replacing Or Repairing A Pilot Light
Repairs are obviously dependent on the system. The nature of the pilot light setup is such that a repair will
usually be for one component of the assembly rather than the entire thing. For example, if a pilot won't stay
lit after you release the button on the valve knob, it's likely that it will be necessary to replace the
thermocouple. If that doesn't fix the problem, replace the thermocouple and the gas valve. Each of these parts
are available at most home supply stores. For an intermittent system, a pilot problem will usually mean the
replacement of the entire control box, which runs roughly $100.
Who Makes Replacement Parts For Pilot Lights?
Valve knobs and thermocouples, the primary repair components for standing pilot light assemblies, are made by a
broad array of plumbing companies. You should be able to find one at any major home improvement store.
Honeywell's line of universal thermocouples, for example, are found at many stores and online distributors. The
control box for the intermittent system is made primarily by Honeywell, Robertshaw, Johnson Controls, Fenwal or
White-Rodgers. Hot surface igniters are made by a host of HVAC companies including, but not limited to Honeywell,
Nordyne, Carrier, York, Rheem/Ruud and International Comfort Products. Igniters are often specific to the
furnace manufacturer, so be careful about selecting the right one. Ask a heating
professional for what you need.