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== Thermal Coupler ==The thermal coupler is primarily a safety device that allows the pilot light to stay light, however, in the event the pilot light does go out, the thermal coupler prevents gas from leaking into the air. The thermal coupler shuts off the gas supply to the pilot of the pilot goes out. The heat from the pilot light keeps the valve in the thermal coupler open. If the pilot goes out, the metal cools and closes the valve. When a thermal coupler fails it tends to close the valve even when the pilot is burning and the metal warm. A thermal coupler is easy to replace and you will likely have to replace one on a regular basis.''note: If your gas furnace does not have a thermal coupler, then it uses an electronic igniter. The thermal coupler pilot light ignition system has been superseded by a modern electronic igniter system so that it is no longer necessary to have a pilot light.''=== How to Install a Gas Furnace Thermal Coupler ===If the pilot light on a gas furnace will not stay lit the problem may be with a part called the thermal coupler. The thermal coupler is screwed into the gas control valve and has a thin wire which sets in the flame of the pilot. If there is no pilot light burning the thermal coupler tells the gas valve to shut off the gas supply.# Locate the shut off valve on the gas line which goes to the furnace and shut it off.# Locate the thermal coupler on the furnace. Look at where the pilot light is and trace the wire that sets in the flame of the pilot back to the gas control valve.# Unscrew the nut which attaches the thermal coupler to the gas control valve. Remove the old thermal coupler and take it to the hardware store to make sure the right size replacement thermal coupler is purchased.# Tighten the nut by hand which holds the thermal coupler onto the gas control valve. Use an adjustable wrench to tighten the nut the rest of the way.# Place the wire end of the thermal coupler in the pilot the same way as the old thermal coupler. Usually only about the top 1/3 of the thermal coupler wire should be heated in the pilot light.# Turn on the gas supply and re-light the pilot. == Electronic Ignition ==The electronic ignition system in a gas furnace is a more efficient way to light the burners than the old pilot light system. The electronic ignition occurs typically in one of two ways, Intermittent Pilot, or Hot Surface Ignition. The intermittent pilot system uses an electronically controlled high voltage electrical spark to ignite the gas pilot and then subsequently the main burners when the thermostat calls for heat.The hot surface ignition system uses an electronically controlled resistance heating element, not unlike a light bulb filament (and shown in the photo above), to ignite the gas burner.[[File:Norton20165703S.jpg]]<BR>Illustration:The Norton Model 601 is an example of a hot surface ignitor. == Thermal Coupler ==== Troubleshooting ===== How to Install a Gas Furnace Thermal Coupler ====== Electronic Ignition ===If the pilot light on a gas furnace will not stay lit the problem may be with a part called the thermal coupler. The thermal coupler is screwed into the gas control valve and has a thin wire which sets in the flame of the pilot. If there is no pilot light burning the thermal coupler tells the gas valve to shut off the gas supply.Here is an [http://homeguides.sfgate.com/tell-ignitor-gas-furnace-working-23432.html erroneous source] that claims you can not manually light a furnace that uses an electronic ignition system. They are wrong. This proves that the Internet is full of misinformation. This [https://youtu.be/UO73p3GxWlk youtube video] shows me successfully lighting a furnace that has a faulty electronic igniter. * [http://www screenshot of web site with false (or in some cases false which is why they should not state it as an absolute) claim.]* [http://www view this youtube video of me manually lighting this furnace by flame.] There are safety concerns with doing this so do not try it yourself. On this particular furnace design it was simply one step in a multiple step troubleshooting procedure. Other safety components had already been verified and the control board tested. On the hot surface ignition gas furnace check the igniter using an Ohm meter to make sure it has resistance. If no resistance is read (open circuit) then the igniter is broken or cracked and needs to be replaced. Also ensure there is a proper ground to the board so that is not "floating" from one leg to the other. There should be 24 volts on a single leg which is what you need on the electronic ignition gas furnace controls. === Furnace Pressure Switch ===The switch is near the draft inducer motor, which is responsible for moving combustion byproducts out. The pressure switch is activated by vacuum pressure. Early systems have a single vacuum pressure switch. When vacuum pressure is present it closes the connection and when there is insufficient vacuum pressure the connection is open. You can draw air though the vacuum line while an Ohm meter is connected to see if the switch is mechanically functional. You can connect the Ohm meter and then turn on the furnace. Once the draft inducer is active then the resistance value of the switch will change, typically from an open circuit to a resistance value relatively low. If the switch is good but not activating during attempted operation of the furnace, then the draft inducer motor might have failed to turn on (if your furnace has one) or there could be penetration in the heat exchanger causing loss of induced vacuum pressure in the chamber. === Draft Inducer Motor ===The induced draft or forced draft motor is used to force toxic gas out the vent. Some furnaces have a forced draft system while others have an induced draft design. The difference being the forced draft blower blows air through the heat exchanger while the induced draft motor pulls air through the heat exchanger. === Furnace Control Board ===The furnace control board is responsible for governing the operation of the furnace. Your furnace may have one or more control boards depending on age and design. Some old pilot light furnaces do not use a furnace control board. The furnace control board design was introduced late in the thermal coupler pilot light furnace type. Basic control board operation:# triggered by the thermostat to control furnace (on/off/blower stage)# interprets safety sensors such as the furnace vacuum pressure switch# starts the ignition system, whether its a spark generator, glow coil or pilot light# turns on or off gas supply to the burners# governs operation until triggered by thermostat, a problem is detected, or a timed cycle duration is achieved# Locate the shut off valve on the gas line which goes to the furnace and shut it off.# Locate the thermal coupler on the furnace. Look at where the pilot light is and trace the wire that sets in the flame of the pilot back to the gas control valve.# Unscrew the nut which attaches the thermal coupler to the gas control valve. Remove the old thermal coupler and take it to the hardware store to make sure the right size replacement thermal coupler is purchased.# Tighten the nut by hand which holds the thermal coupler onto the gas control valve. Use an adjustable wrench to tighten the nut the rest of the way.# Place the wire end of the thermal coupler in the pilot the same way as the old thermal coupler. Usually only about the top 1/3 of the thermal coupler wire should be heated in the pilot light.# Turn on the gas supply and re-light the pilot.[[Category:Heating and Air Conditioning]]