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Gas furnace ignition controls
This page covers the repair of gas furnaces.
These appliances use flammable and explosive gasses and lethal voltages. If you are not confident in your abilities, do not proceed.
Many years ago, if you wanted heat from a gas appliance, you held a lit match near the burner and turned on the gas. If you turned on the gas then lit the match, you got a face full of fire or a kaboom!
The first ignition control that was safe, was the thermocouple safety. It consisted of a pilot light that warmed a thermocouple that would hold a solenoid open, allowing gas to flow to the main burner.
For a more complete explanation of the pilot safety click here.
The pilot safety was a reasonably safe flame safety control but it 3 problems:1. To operate it had to have a continuous pilot burning. The gas used by the pilot lowered the efficiency of the furnace.
2. If the pilot went out for any reason, It had to be manually relit or the appliance would not work.
3. If the pilot went out, it took up to 3 minutes for the pilot safety to drop out and not allow gas to flow to the main burners. If there was a call for heat during that time, gas would flow to the burners without lighting.
When efficiency became important, getting rid of the pilot was seen to be the first way to reduce gas usage.
The first electronic ignitions were called intermittent pilot ignition.
They would light the pilot with a spark when there was a call for heat and when the call for heat was over, the pilot would be shut off.
These systems had to have some way of proving that the pilot was lit before allowing main gas to come on.
The video below covers one of the early ignition controls that used the intermittent pilot.
The first ignition control that was safe, was the thermocouple safety. It consisted of a pilot light that warmed a thermocouple that would hold a solenoid open, allowing gas to flow to the main burner.
For a more complete explanation of the pilot safety click here.
The pilot safety was a reasonably safe flame safety control but it 3 problems:1. To operate it had to have a continuous pilot burning. The gas used by the pilot lowered the efficiency of the furnace.
2. If the pilot went out for any reason, It had to be manually relit or the appliance would not work.
3. If the pilot went out, it took up to 3 minutes for the pilot safety to drop out and not allow gas to flow to the main burners. If there was a call for heat during that time, gas would flow to the burners without lighting.
When efficiency became important, getting rid of the pilot was seen to be the first way to reduce gas usage.
The first electronic ignitions were called intermittent pilot ignition.
They would light the pilot with a spark when there was a call for heat and when the call for heat was over, the pilot would be shut off.
These systems had to have some way of proving that the pilot was lit before allowing main gas to come on.
The video below covers one of the early ignition controls that used the intermittent pilot.
Below is a troubleshoot of the early intermittent pilot used on the Carrier, Bryant, Day Night and Payne.
The mercury pilot switch
Another type of intermittent pilot was the mercury switch pilot. It had a device that looked like a thermocouple on the pilot. However, this was a piece of tubing connected to a diaphragm that was filled with mercury. The pilot had a spark ignitor that lit the pilot when there was a call for heat. The pilot warmed the mercury which expanded and activated a switch that opened the main gas flow. The video below gives a sequence of operation of this system. This video will be available 2-20-15.
The video below disassembles the mercury switch. This video will be available 2-22-15.
The next advance was electronic controls that used an electronic circuit to start the spark and prove that it was lit. The spark would be initiated and the pilot valve would open. If the pilot lit, an electronic circuit would prove that it had lit and the main gas would come on. The spark would stay on usually for 90 seconds before locking out on safety if the pilot did not light. Some models did not have a shut down time and would spark continuously until the pilot lit.
The video below looks at a number of spark ignitors, how they work and some troubleshoot.
The video below looks at a number of spark ignitors, how they work and some troubleshoot.
One of the ignition systems used is called the Honeywell Smart Valve system. It uses a small hot stick ignitor to light the pilot, the pilot is then proved and main gas is turned on. The video below covers the Smart Valve system.
Up to this point, all controls still used a pilot.
There was no real necessity to have a pilot with electronic controls, so the next step was to ignite the burner directly.
This is when the hot surface ignitor came into use.
This part is essentially a small heater that heats up to over 2000 degreesF.
The video below demonstrates how the HSI works.
There was no real necessity to have a pilot with electronic controls, so the next step was to ignite the burner directly.
This is when the hot surface ignitor came into use.
This part is essentially a small heater that heats up to over 2000 degreesF.
The video below demonstrates how the HSI works.
Because the HSI lights the main burner, the trial for ignition is much shorter than for intermittent pilot.
Once the HSI is up to temperature, the main gas comes on for 4 to 7 seconds. If the presence of flame is sensed, the flame continues to burn. If not, the gas is shut off and the ignition sequence begins again. Most of these furnaces will try for ignition 3 times and if the flame is not sensed, the control will lockout and wait for an hour, then try again.
Most furnaces use a flame rod to sense that the flame is present.
The video below explains how the flame rod works.
Once the HSI is up to temperature, the main gas comes on for 4 to 7 seconds. If the presence of flame is sensed, the flame continues to burn. If not, the gas is shut off and the ignition sequence begins again. Most of these furnaces will try for ignition 3 times and if the flame is not sensed, the control will lockout and wait for an hour, then try again.
Most furnaces use a flame rod to sense that the flame is present.
The video below explains how the flame rod works.
Below is a video about cleaning the flame rod.
The control board, IFC and circuit board are all names for the electronic control used in modern furnaces to control all operations of the furnace, including the flame safety. Below is a video about troubleshooting the control board.
When troubleshooting this board or other safeties on the modern gas furnace, care must be taken to be sure all safeties are operating normally. Some repairs can involve the traces on the board. Whether you do these repairs rather than replace the board depends on your level of experience and your confidence in your abilities. If you are not sure, call a professional.
When troubleshooting this board or other safeties on the modern gas furnace, care must be taken to be sure all safeties are operating normally. Some repairs can involve the traces on the board. Whether you do these repairs rather than replace the board depends on your level of experience and your confidence in your abilities. If you are not sure, call a professional.