Oceanside Air Conditioning & Heating Knowledebase >Troubleshooting a Gas Furnace - Furnace Repair

The Gas Furnace

The gas furnace is an appliance looking like a large box that does the following:

takes in cold air,
cleans it with an air filter,
heats it up with a gas burner using a stainless steel heat exchanger,
distributes the warm air with a blower motor through your home's ductwork

The heated air then cools down in your home's various rooms and returns to the furnace through return air grills and ductwork. The cold returning air enters back through the air filter into the furnace to complete another heating loop. Sometimes there is a humidifier mounted on the furnace or the return air ductwork. Furnaces come in different efficiencies measured in AFUE.

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.

Let's take a look at the more common problems and furnace repairs you may have to make.

PROBLEM: Gas Furnace Produces No Heat

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

PROBLEM: 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.

PROBLEM: 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
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.

PROBLEM: Gas Furnace Blower Does Not Turn Off

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

PROBLEM: 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.

PROBLEM: Furnace Pilot is Out - Relighting a Standing 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:

Please call a specialist such as Comfort Air Zone Oceanside Air Conditioning & Heating

PROBLEM: 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.

PROBLEM: 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.

PROBLEM: Furnace Problems Caused by Thermostats

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.