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Pilot light furnaces with a spark ignitor for the pilot
The standing pilot could not always be depended upon to stay on. Wind conditions, or building pressures could blow out the pilot. To keep the pilot on, the 2 wire pilot assembly was made. A bimetal strip located near the pilot burner is kept warm. That keeps open a switch. If the pilot blows out, the switch closes and starts a spark ignitor that relights the pilot. These units were primarily used on rooftop units that are exposed to wind conditions.
The video below covers this pilot assembly.
The video below covers this pilot assembly.
Carrier, Bryant, Day Night, and Payne spark ignitors

The standing pilot ignition system had one glaring deficiency. The pilot gas has to be supplied continuously. That means the unit is always using fuel.
One of the earliest methods of eliminating this fuel usage was the intermittent pilot.
When there is no call for heat, there is no pilot burning. On a call for heat, the pilot is lit, usually by a spark or hot stick. The pilot must then be proved. This means some type of circuit that insures that the pilot has lit before turning on main gas.
The pilot assembly above is one method of pilot proving. This unit uses a bimetal as the 2 wire does. However, the switch actuated by the bimetal is a single pole double throw switch. When in the deenergized position, it will start the spark ignitor and the pilot gas. However this switch will not allow main gas to come on unless it is in the energized position.
The video below describes this pilot assembly.
One of the earliest methods of eliminating this fuel usage was the intermittent pilot.
When there is no call for heat, there is no pilot burning. On a call for heat, the pilot is lit, usually by a spark or hot stick. The pilot must then be proved. This means some type of circuit that insures that the pilot has lit before turning on main gas.
The pilot assembly above is one method of pilot proving. This unit uses a bimetal as the 2 wire does. However, the switch actuated by the bimetal is a single pole double throw switch. When in the deenergized position, it will start the spark ignitor and the pilot gas. However this switch will not allow main gas to come on unless it is in the energized position.
The video below describes this pilot assembly.
The video below gives a view of how the pilot assembly operates.
The video below covers a particular failure related to the pilot assembly that is common with this system.
Electronic spark pilot ignition controls
Some furnaces use an electronic control light the pilot, prove the pilot has lit and turn on main gas.
These controls had some advantages compared to the electric pilot assemblies
shown above.
They could be used with standard pilot burners.
In the early 1980s, these were used sometimes as retrofits to older furnaces to reduce fuel usage. Also some utilities subsidized these installations.
These controls had some advantages compared to the electric pilot assemblies
shown above.
They could be used with standard pilot burners.
In the early 1980s, these were used sometimes as retrofits to older furnaces to reduce fuel usage. Also some utilities subsidized these installations.