Most new furnaces are now equipped with an diagnostic indicator light on the circuit board that displays the unit’s operating status and/or fault codes. Do not remove the blower door or unplug the furnace until you have observed and noted the status and/or fault codes. If the blower door is removed or the furnace is unplugged; all codes will be erased. Normally, a label is attached to the outside of the furnace that will give an explanation/break down of what the diagnostic indicator light code means. Follow the manufacturer’s instructions for the use of the built in diagnostic features.

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On furnaces with an induced draft motor, the motor must first come up to speed creating a negative pressure for the pressure switch. This switch must only be closed after the induced draft motor comes up to speed or the motor will not start. If this switch is jumpered (bypassed by the technician) prior to a call for heat the furnace unit will give an error code of “pressure switch contact failed in the closed position”. If the motor does not start check for induced draft motor wheel stuck or seized, lack of 115-volts to the induce draft motor, bad start capacitor on induce draft motor, defective inducer motor.

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On furnaces with an induced draft motor, the motor must first come up to speed to create a negative pressure. The pressure switch reads this pressure differential between atmospheric pressure (outside) and the exhaust vent pressure (internally). This difference in pressure will close the pressure switch’s contacts. This switch must only be closed after the induced draft motor comes up to speed. If this switch is jumpered (by the technician) prior to a call for heat the furnace unit will give an error code of “pressure switch contact failed in the closed position”. If the pressure switch does not close check for the following, cracked heat exchanger, proper vent sizing or pitch, vent restrictions or high winds, cracked or blocked pressure switch tubing, defective inducer motor, induced draft motor wheel stuck or seized, lack of 115-volts to the induce draft motor, bad start capacitor on induce draft motor, defective pressure switch or inadequate combustion air.

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The furnace limit switch is a safety switch that will shut off the main burners and turn on the blower motor to protect the furnace from overheating. A limit switch might be tripping due to dirty filters; filters should be maintained on a regular basis, dirty indoor air filters would cause restricted airflow across heat exchanger. Duct restricted; a duct supply/return duct that is kinked, smashed, pinched, crushed, flattened or any anyway restricted will have the same negative effects on the airflow as dirty filters. Dampers partly closed; dampers either motorized or manual are designed to restrict the airflow to areas that do not need or have too much air volume. Dampers must be used in moderation. Check if a damper has come loose or is positioned incorrectly thus unknowingly restricting the airflow. If the blower has adjustable speed taps, verify the blower is at a heating speed. If it is set too low, use a faster speed tap. The limit switch is located in the control compartment. The furnaces flame rollout switch is designed to shut the furnace down immediately if flame/excessive heat is sensed in an area they don’t belong. This can happen if there is an improper installation or when there is inadequate combustion air. If the rollout switch is tripped a qualified service technician must diagnose and fix the problem with the furnace. The rollout switch is a manual reset safety and must never be reset until the problem that caused the rollout has been found and repaired. Never remove a safety from the control circuit.

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Hot Surface Igniters (HSI) will heat themselves up to 1800º and above. It may develop tiny difficult to detect cracks that can cause failure. Handle with care the HSI is very fragile. Do not touch the surface. Only handle by the wires or its base. Sometimes a white area will develop around a crack. If this is seen, replace the HSI. An ohmmeter (a meter used to measure resistance) can be used to find other hard to see cracks. The HSI can be checked in the furnace. Simply unplug it and ohm out the wires. 45 to 95 ohms – Good 110 ohms and above – Cracked At room temperature, the resistance range should be 45 to 90 ohms. A resistance higher than 110 ohms indicates a crack. If a crack is detected, replace the HSI. If 115-volt is applied to the HSI and it does not glow; replace the HSI.

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	Verify furnace is connected to a power supply. Check for high voltage at the electrical outlet to the furnace, or at any disconnect-switches, fuses or breakers prior to the furnace.
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Check for voltage at the transformer.  Transformers will transform high voltage 230, 208,115v to a lower voltage (typically 24-volts on residential systems). If the transformer is not working or has less than 10% (21.6 volts) it will not supply enough voltage to the thermostat and thus the gas valve will not have enough power to operate.  If the transformer is supplying proper voltage to the thermostat; and the thermostat is calling for heat; check for an open safety in the control circuit.

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Check all wiring and wirenut connections. The control circuit is the low voltage wires between the thermostat and the gas valve. This is where the safeties either manual reset; auto reset; or one time thermal links are located. Safeties are put in the control circuit to protect the furnace from excessive temperatures in the heat exchanger, flame roll out in the control compartment and excessive vent temperatures. If any safety is open, find out why the safety is open before resetting or replacing it. Manual reset safeties are designed to hold the unit off until a technician arrives to diagnose the problem with the installation or operation of the system. Never remove a safety from the control circuit.

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	Check to see if main gas valve is operating. Verify that all manually operated gas shut off devices external to the furnace are open. Look for loose or broken 24-volt wiring connections. If the gas valve has 24-volts but will not energize, replace gas valve.
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If the burners light but only remain on for a limited time check the following. To prove the presence of a flame the control board uses a flame sensor. To check for the proper signal from the flame sensor use a “micro-ammeter” wired in series with the electrode, this can measure the DC electrical signal. Some early circuit boards used a 24vac signal to the electrode. Some later versions have a transformer on the board to generate a 120vac signal. Most circuit boards with LED’s generate a 120vac signal. 120VAC board 0.5 to 4.0 microamps (DC)

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If the burners light but only remain on for a limited time check the following: low gas pressure at the gas pressure manifold. The furnace may have enough pressure to light but not enough to remain lit. This will happen when there is to long of a gas line running to the furnace or the piping is undersized / restricted internally. A water or oil manometer is used to measure gas pressure in inches of water column pressure (WCP). A manometer is clear and shaped like the letter ”U”. In most locations, natural gas entering the home is regulated between 6 and 7 inches WCP. LPG gas is regulated to 11 inches WCP. The appliance pressure regulator reduces the incoming pressure to the required amount, which is usually 3.5 inches WCP for natural gas installations and 10.5 for LPG installations. To measure gas pressure (clock the gas meter) follow the instructions below, make any major adjustments by changing the main burner orifices. Clocking the gas meter a. Obtain average yearly heat value from local gas supplier. b. Check and verify orifice size in furnace, NEVER ASSUME ORFICE SIZE. c. Turn off all other gas appliances and let the furnace run for 3 minutes. d. Measure (in seconds) for gas meter to complete 1 revolution. Step 1. Gas flow rate= Cubic feet per revolution x 3600 (seconds per hour) Time of one revolution in seconds OR “Example” Gas flow rate= 2.0 cubic feet x 3600 (seconds per hour) 48 seconds = 150cubic feet per hour Step 2. Heat input rate= cubic feet per hour x BTU/cubic feet    =    BTU/hour “Example” Heat input rate= 150 cubic feet per hour x 1050 BTU/cubic feet    =    157,500 BTU/hour Adjusting the gas input a. Remove regulator adjustment seal cap. b. Turn adjusting screw counterclockwise to decrease input. Turn adjusting screw clockwise to increase input. Do not set manifold pressure less than 3.2 inches WCP or more than 3.8 inches WCP for natural gas. Make any major adjustments by changing main burner orifices. c. When correct input is obtained, replace regulator seal cap. Main burner flame should be clear blue, almost transparent.

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	If the burners light but only remain on for a limited time check the following. Verify the burners are not being affected by wind / breezes during start-up. If the burner flame is being blown away from the flame senor it will not be able to sense they are lit.
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	If the unit works fine in cooling, but doesn’t work in heating; check for reversed 115-volt polarity. Reversed polarity can be fixed by shutting off all 115-volt power prior to the furnace at the breaker panel. Unplug the furnace, and then reverse the 115-volt power leads at the furnaces 115-volt junction box. Remember to turn the power back on at the breaker panel when finished.
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On furnaces with an induced draft motor, the pressure switch’s contacts must stay closed (read a negative pressure) for the duration of the call for heat. The pressure switch reads this pressure differential between atmospheric pressure (outside) and the exhaust vent pressure (internally). This difference in pressure will close the pressure switch’s contacts. This switch must only be closed after the induced draft motor comes up to speed. If this switch is jumpered (by the technician) prior to a call for heat the furnace unit will give an error code of “pressure switch contact failed in the closed position”. If the pressure switch does not remain closed check for the following, cracked heat exchanger, proper vent sizing or pitch, vent restrictions or high winds, cracked or blocked pressure switch tubing, defective inducer motor, induced draft motor wheel sticking or dragging, a drop in 115-volts to the induce draft motor, defective pressure switch or inadequate combustion air.

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The furnace limit switch is a safety switch that will shut off the main burners and turn on the blower motor to protect the furnace from overheating. A limit switch might be tripping due to dirty filters; filters should be maintained on a regular basis, dirty indoor air filters would cause restricted airflow across heat exchanger. Duct restricted; a duct supply/return duct that is kinked, smashed, pinched, crushed, flattened or in anyway restricted will have the same negative effects on the airflow as dirty filters. Dampers partly closed; dampers either motorized or manual are designed to restrict the airflow to areas that do not need or have too much air volume. Dampers must be used in moderation. Check if a damper has come loose or is positioned incorrectly thus unknowingly restricting the airflow. If the blower has adjustable speed taps, verify the blower is at a heating speed. If it is set too low, use a faster speed tap. The limit switch is located in the control compartment. The furnaces flame rollout switch is designed to shut the furnace down immediately if flame/excessive heat is sensed in an area they don’t belong. This can happen when there is in an improper installation or when their is inadequate combustion air. If the rollout switch is tripped a qualified service technician must diagnose and fix the problem with the furnace. The rollout switch is a manual reset safety and must never be reset until the problem that caused the rollout has been found and repaired. Never remove a safety from the control circuit.

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Check to make sure thermostat is calling for heat or “closed”. The thermostat may be satisfied, have the heat anticipator set to high, or have a cycle lock activated to limit the amount of cycles per hour. The thermostat may look like it is calling for heat on the outside, but if the thermostats contacts either internal to the thermostat or on the subase are dirty the thermostat may still be open. Also check for bad transformer or broken/cut thermostat wires.

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	Move thermostat fan switch to the AUTO or OFF position. Most thermostats will have a fan option for “ON” or “AUTO”. If the thermostat is set to the “ON” position the fan will run continuously. If the thermostat is set to the “AUTO” then the fan will automatically come on with a call for heating or cooling.
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The fusible link is located in the control compartment. If it is blown or “open” it will allow the fan to run continuously. The fusible link is a thermal (temperature) switch; this is a one-time only safety switch. Its purpose is to protect the furnace from excessive temperatures in the control compartment where heat or flames do not belong. If it is open, find out why it is open before replacing, then replace only with the identical part. Never remove a safety from the control circuit.

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The furnace limit switch is a safety switch that will shut off the main burners and turn on the blower motor to protect the furnace from overheating. A limit switch might be tripping due to dirty filters; filters should be maintained on a regular basis, dirty indoor air filters would cause restricted airflow across heat exchanger. Duct restricted; a duct supply/return duct that is kinked, smashed, pinched, crushed, flattened or any anyway restricted will have the same negative effects on the airflow as dirty filters. Dampers partly closed; dampers either motorized or manual are designed to restrict the airflow to areas that do not need or have too much air volume. Dampers must be used in moderation. Check if a damper has come loose or is positioned incorrectly thus unknowingly restricting the airflow. If the blower has adjustable speed taps, verify the blower is at a heating speed. If it is set too low, use a faster speed tap. The limit switch is located in the control compartment.

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	The Heat Relay is a normally open relay that closes (completes the electrical circuit) to bring on the blower. It is typically located on the printed circuit board. If the heat relay gets stuck in the closed position, the indoor blower will operate continuously. In most cases the fan relay is an integral part of the circuit board, therefore if it fails the circuit board must be replaced.
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	There is nothing that can be done to increase a furnaces capacity past what is listed on its own nameplate. All furnaces must be running at + or – 2% of their nameplate capacity. If the furnace is too small for the application, the furnace must be replaced with a properly sized furnace.
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Check gas pressure at manifold. For optimum efficiency of a furnace, the burners should be operated at 100% of their rated input. Under-fired burners can cause the furnace to produce less than its rated heat, while over-fired burners can cause the furnace to overheat. A water or oil manometer is used to measure gas pressure in inches of Water Column Pressure or (WCP). A manometer is clear and shaped like the letter ”U”. Readings from spring type dial gauges are not acceptable. In most locations, natural gas entering the home is regulated between 6 and 7 inches WCP. LPG gas is regulated to 11 inches WCP. The appliance pressure regulator reduces the incoming pressure to the required amount, which is usually 3.5 inches WCP for natural gas installations and 10.5 for LPG installations. To measure gas pressure (clock the gas meter) follow the instructions below, make any major adjustments by changing the main burner orifices. Clocking the gas meter a. Obtain average yearly heat value from local gas supplier. b. Check and verify orifice size in furnace, NEVER ASSUME ORFICE SIZE. c. Turn off all other gas appliances and let the furnace run for 3 minutes. d. Measure (in seconds) for gas meter to complete 1 revolution. Step 1. Gas flow rate= Cubic feet per revolution x 3600 (seconds per hour) Time of one revolution in second OR “Example” Gas flow rate= 2.0 cubic feet x 3600 (seconds per hour) 48 seconds = 150 cubic feet per hour Step 2. Heat input rate= cubic feet per hour x BTU/cubic feet    =    BTU/hour “Example” Heat input rate= 150 cubic feet per hour x 1050 BTU/cubic feet    =    157,500 BTU/hour Adjusting the gas input a. Remove regulator adjustment seal cap. b. Turn adjusting screw counterclockwise to decrease input. Turn adjusting screw clockwise to increase input. Do not set manifold pressure less than 3.2 inches WCP or more than 3.8 inches WCP for natural gas. Make any major adjustments by changing main burner orifices. c. When correct input is obtained, replace regulator seal cap. Main burner flame should be clear blue, almost transparent.

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The furnace limit switch is a safety switch that will shut off the main burners to protect the furnace form overheating. Some reasons a limit switch might be tripping; dirty filter; filters should be maintained on a regular basis, dirty indoor air filters will cause restricted airflow across heat exchanger. Duct restricted; a duct supply/return duct that is kinked, smashed, pinched, crushed, flattened or any anyway restricted will have the same negative effects on the airflow as dirty filters. Dampers partly closed; dampers either motorized or manual are designed to restrict the airflow to areas that do not need or have too much air volume. Dampers must be used in moderation. Check if a damper has come loose or is positioned incorrectly thus unknowingly restricting the airflow. If the blower has adjustable speed taps, verify the blower is at a heating speed. If it is set too low, use a faster speed tap. Verify the “off” setting (the temperature at which the indoor fan will turn off after a heating cycle) on the fan control is set properly. If it is set to low it will shut of the indoor fan before all heat has been dissipated out of the heat exchanger. Reset higher if necessary. Check heat anticipator setting on the thermostat, it may be shutting off the furnace before it has reached set point; if it is readjust higher.

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