Possible Causes
Thermostat set too low and not calling for heat
Thermostat not working
Circuit breaker or fuse controlling the furnace is tripped or blown
Natural gas or propane control valve is closed
Pilot light is out
Possible Repairs
Check that thermostat is in heat mode and has appropriate temperature setting
Try moving dial setting up or down a few degrees
Clean contacts in thermostat if it is a non-digital type
Reset tripped circuit breaker
Replace blown fuse
Relight pilot light
Gas Furnace Does Not Produce Enough Heat
Possible Causes
Dirty furnace air filter
Gas burners may be dirty or need adjustment
Possible Repairs
Replace dirty air filter
Call furnace service technician to have burners adjusted.
Gas Furnace Comes On and Off Too Frequently
Possible Causes
Dirty furnace air filter
Blower motor problem
Thermostat problem (heat anticipator)
Possible Repairs
Adjust thermostat heat anticipator
See also Troubleshooting a Thermostat
Replace dirty air filter
Oil blower motor lubrication ports (usually at each end of shaft) with light weight oil
Check for proper belt tension. Belt should depress about 1″ at center of span.
If too loose, tighten belt.
Replace frayed belt.
Blower Does Not Turn Off – Always Running
Possible Causes
Thermostat set to fan continuous
Faulty fan limit control switch on furnace (if thermostat has no fan setting)
Possible Repairs
Change thermostat fan setting
Reset or replace furnace fan limit control switch
Gas Furnace Has Noisy Operation
Possible Causes
Higher pitched “squealing” sound may be caused by slipping blower belt or motor or shaft bearings in need of oiling
Lower pitched “rumble” sound may be caused by a poorly adjusted pilot light if this problem occurs with the burners off.
Lower pitched “rumble” sound may be caused by dirty gas burners if the problem occurs with the burners on.
Possible Repairs
Oil blower motor lubrication ports (usually at each end of shaft) with light weight oil
Check for proper belt tension. Belt should depress about 1″ at center of span.
If too loose, tighten belt.
Replace frayed belt.
Adjust pilot light
Call furnace service technician to have burners adjusted or cleaned.
Lighting a Gas Furnace Pilot
Possible Causes
A pilot can go out due to a strong draft, dirty orifice or dirt in the gas tube.
The thermocouple may also be faulty and is shutting off the gas supply.
Possible Repairs
For an overview on the gas furnace pilot and how to troubleshoot, repair and relight it, please see Gas Furnace Pilot and Thermocouple. Also see the tutorial How to Replace a Thermocouple.
Electronic Ignition Furnace Problems
Newer furnaces do not rely on a standing pilot to ignite the gas burners. 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.
See the Tutorial Overview and Repair of the Electronic Ignition Furnace for troubleshooting repair tips for electronic ignition furnaces.
Mismatching the Furnace and Thermostat:
Furnaces and thermostats are not mix and match appliances. Using the wrong type of thermostat with a furnace will cause operating problems and can be dangerous. Although thermostats look similar they are designed vary differently.
There are numerous types of heating systems and thermostat systems and they need to be coordinated for safe and proper operation.
Review the tutorial Thermostat and Furnace Compatibility to understand the three types of thermostat systems used today:
Millivoltage
Low Voltage
Line Voltage
Thermostat Troubleshooting and Repair
Some common symptoms exhibited by the furnace may actually be due to a faulty thermostat. After you have confirmed that the furnace is not the problem, you’ll want to check the thermostat.
Thermostat problems can show up as a furnace that produces no heat, wild temperature swings or cycling on and off too often. As described in the previous section Thermostat and Furnace Compatibility, you need to determine what type of thermostat you have for your furnace.
