The cold returning air enters back through the air filter into the
furnace to complete another heating loop.
Once in a while things don't work quite right and you need to
troubleshoot a gas furnace repair or relight
a standing pilot if it has one. Most newer furnaces have electronic
ignitions and need special troubleshooting.
Possible Causes
- Thermostat set too low and no 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
#2 Gas Furnace Does Not Produce Enough Heat
Possible Causes
Dirty furnace air filter
Gas burners may be dirty or need adjustment
Possible Repairs
#3 Gas Furnace Comes On and Off Too Frequently
Possible Causes
- Dirty furnace air filter
- Blower motor problem
- Thermostat problem
Possible Repairs
#4 Gas Furnace Blower Does Not Turn Off
Possible Causes
- Thermostat set to fan continuous
- Faulty fan limit control switch on furnace
Possible Repairs
- Change thermostat fan setting
- Reset or replace furnace fan limit control switch
#5 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.
#6 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.
#7 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:
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.
#8 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
#9 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.
See the tutorial Troubleshooting
a Home Furnace Thermostat for a great overview of how to repair this
device.
Troubleshooting a Gas Furnace - Furnace
Repair By Bob
Formisano, About.com Guide
| Furnace Repair Service Tips
