If the spark gap is too large the spark may not be hot enough to ignite the pilot gas. Natural gas will ignite at approximately 1100 degrees. If the spark gap is to small it may not be in the path of the pilot gas. Readjust, if necessary, so that the gap between the electrode tip and pilot hood is as shown in the installation instructions (typically1/8" to ¼").If the spark electrode is dirty it will not produce a hot enough spark. Clean any dirt, scale, soot and/or carbon using a soft bristle brush. Do not scratch or score electrode. The ceramic that surrounds the electrode may become cracked. If this happens it may allow the high voltage spark to "leak" or occur prior to the electrode. If a cracked ceramic is found, replace the pilot electrode assembly. Check for loose or broken wiring (looks like a spark plug wire) at and between spark generator and electrode. Replace wire or tighten connection as necessary. Check fuse or circuit breaker for 115-volt supply to furnace. Check blower access panel or door for proper installation. If this door is not properly installed the door switch behind it will not complete the 120-volt electrical circuit, therefore the furnace will appear to not have power. Check 24-volt input to spark generator. Be careful, the spark generator may only have a 24-volt input but it has an internal transformer that will put out a 10,000 to 20,000-volt spark output. If you read 24 volts input to the spark generator and the above steps have been completed, replace spark generator assembly.
Check to verify the igniter is not shorting out or lying against the burner. If it is touching the burners in any way adjust away from the burner and toward the hood of the pilot assembly (typically1/8" to ¼") or as specified in the installation instructions. For the pilot to ignite, the pilot gas must pass thru the path of the spark.
Clean any dirt or corrosion from the pilot orifice by using a soft wire to dislodge any deposits from inside the pilot or by blowing the lint out with your breath. Be careful not to damage or nick the orifice. If damaged, replace the pilot orifice. A properly burning pilot flame has a soft blue color with some yellow at the tip and has sufficient height to impinge on the thermocouple. It extends above the burner carryover ports so that it will properly ignite the main burners. Check for 24-volts on the gas valve when the furnace is calling for heat. Check for broken wires or loose connections. If the transformer is not working or has less than 10% (21.6 volts) it will not supply enough voltage to the pilot valve to operate properly. A gas valve is a non-serviceable item that cannot be disassembled. If the gas valve has 24-volt and gas supplied to it but it still does not work; replace the gas valve.
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.
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.
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.
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.
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.
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.
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 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.
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.
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 (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 secondsOR
"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.
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; readjust.
Look for signs of a problem, poor venting, or rusted/damaged vents and vent connectors, excessive moisture in the house. Check burner flame, it should be blue, not yellow. Check for heat marks or soot on furnace or water heater, soot on vent or any other debris. Check the air shutters on burners, they may be closed or restricted; adjust until there is a soft blue flame. Check for restriction in the heat exchanger furnace venting system. The furnace venting system carries the products of combustion from the furnace to the outdoors. The type and size of the vent system must be carefully matched to the furnace. An undersized or restricted vent can limit the flow of gases, resulting in poor furnace performance and possible unsafe conditions. Lack of combustion air; see installation instruction for required amount. Also don’t block air ducts or door openings (undercuts or louvers) in furnaces or water heater closets. An over fired furnace can cause premature failure of heat exchanger; reduce gas pressure to the required amount. Check to verify main burner orifices are sized properly. The correct orifice size should be listed on the furnace’s nameplate. If the heat exchanger is cracked, replace it. If the furnace is located in a garage, make sure the return ducts are sealed to prevent vehicle exhaust fumes from being pulled into the furnace.
Check to make sure the thermostat is calling for heat or "closed". 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 sub-base are dirty the thermostat may still be open. Also check for bad transformer or broken/cut thermostat wires.