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heating furnace parts and air flow, showing the heat exchanger Gas Furnaces & Heater Troubleshooting
Homeowner Checklist gives repair advice for gas-fired furnaces

No heat or weak heat checklist for gas-fired heating furnaces.

This article gives a series of quick and easy checks and repairs for gas fired heating furnaces. The no-heat or weak heat or uneven-heat complaints experienced by most homeowners will be addressed by these easy troubleshooting steps. The text also discusses common sources of indoor condensation on windows or walls where gas fired heaters are installed and includes some important safety warnings where you see condensation indoors.

Some no-heat or inadequate-heat complaints can be easily resolved without having to call a repairman. We include safety warnings and we indicate where a service call for heat repair is almost certainly needed.

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Troubleshooting Gas Furnaces: Quick Checklist for Homeowners

Gas Furnace Troubleshooting Checklist

Watch out: Immediate LP or natural gas safety hazards: if there is evidence of an LP or natural gas leak at a building, gas odors, for example, you should:

Do not do anything that is likely to cause a gas explosion, such as lighting a match, operating an electrical switch, or even using a telephone in the building
Leave the building immediately
Notify other building occupants of the safety concern
Contact the local gas company and/or fire department

No Heat or Not Enough Heat from the furnace
Possible Cause Action Comments & More Help
Furnace won't turn on Check that power is on to the unit

Some WALL-MOUNTED FURNACES can operate without external electrical power.

FURNACE CONTROLS & SWITCHES

HEAT WON'T TURN ON
Thermostat not properly set Set thermostat to above current room temperature. Check that thermostat is set to HEAT mode. THERMOSTATS, HEATING / COOLING
Pilot light is out Check pilot, re-light.
Check that fuel supply is turned on.

Start at

If pilot won't stay lit, check fuel supply.

Check for or replace thermocouple.

THERMOCOUPLE REPAIR / REPLACEMENT
Pilot on, burner won't turn on Check that gas valve is set to ON, not PILOT GAS REGULATORS for APPLIANCES
Burner not operating properly: noisy, incomplete, uneven, weak flame, flame lifts off burner. Service call required. GAS BURNER FLAME & NOISE DEFECTS
 

Insufficient warm air circulation from the gas furnace
Possible Cause Action Comments & More Help
Dirty air filter Clean or replace the air filter

AIR FILTERS for HVAC SYSTEMS
Air supply registers obstructed Clean dirty register outlet grilles; remove blocking furniture, carpet, etc. AIR FLOW IMPROVEMENT, HVAC
Return air registers obstructed Clean dirty register return air inlet grilles; remove blocking furniture, carpet, etc. RETURN AIR, INCREASE
Dirty blower fan Clean dirty blower fan assembly. Small amount of dirt greatly decreases air movement DIRTY A/C BLOWERS
 

Floors are too cold
Possible Cause Action Comments & More Help
Blocked return air inlets   RETURN AIR, INCREASE
Dirty air filter Clean or replace the air filter

AIR FILTERS for HVAC SYSTEMS
Inadequate air circulation: warm at ceiling cool at floor Set blower for continuous operation BLOWER FAN CONTINUOUS OPERATION
 

Noisy Furnace: blower or burner noises
Possible Cause Action Comments & More Help
Noisy blower fan

Blower may need oiling

Blower fan bearings loose, worn

Blower is dirty or air filter is dirty, clogged

AIR FILTERS for HVAC SYSTEMS

BLOWER FAN OPERATION & TESTING
Noisy gas burner Service call required , gas regulator defect, combustion air defect

Watch out: : system may be unsafe. Shut it down.

GAS BURNER FLAME & NOISE DEFECTS
 

Moisture condensing on windows or walls - gas fired heat
Possible Cause Action Comments & More Help
Blocked chimney or exhaust vent

Inspect & clear, repair, replace improper venting.

Service call required.

 Watch out: : the heating system is likely to be unsafe. Shut it down.
Combustion air inadequate Service call required.

Watch out: : the heating system is unsafe. Shut it down.

CARBON MONOXIDE - CO

COMBUSTION AIR REQUIREMENTS
Insufficient ventilation, tight house. Service call required.

Watch out: : the heating system may be unsafe. Shut it down.

COMBUSTION AIR for TIGHT BUILDINGS
Humidifier improperly set Check power humidifier or free-standing humidifier settings. Also see DUCT & AIR HANDLER ODORS

Notes to the table above

Watch out: improper installation and even improper inspection and testing methods involving natural or "LP" gas can involve dangerous conditions and risk fire or explosion.

See DIAGNOSE & FIX HEATING PROBLEMS-FURNACE for complete diagnostic & inspection guides

Adapted from Sears Corporation, "Sears Gas Furnace Service Hints", (1993), provided courtesy Jim Jagger, ASHI, Buckeye Home Inspections, Cleveland OH, Tel: 216-486-4663. Sears Gas-Fired "Space Saver"Furnace Models: 867.765030 - 867.775260.

Direct Vent Gas Furnace Product Sources & Inspection / Installation Information

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-02-11 by Green Leaf Air

Green Leaf Air said:

This article gives a series of quick and easy checks and repairs for gas-fired heating furnaces. The no-heat or weak heat or uneven-heat complaints experienced by most homeowners will be addressed by these easy troubleshooting steps.

Moderator note: Thank you Green Leaf: while our policy does not permit readers to post self-promotions, ads, links, we welcome content criticism, suggestions, comments and are happy to identify and link to such technical contributors.

On 2019-01-24 by john

We have a 16 year old Armstrong condensing gas furnace. When started by the thermostat it would initially run a blower to clear the firebox for a few seconds before the gas would be released and ignited, and then after a short period the main blower would turn on. Now recently the main blower turns on instantly when unit is asked to heat. This sounds like a problem to me??

On 2019-01-06 by - by (mod) -

friedman (mod)

This sounds as if the system is overheating and being shut down by the fan limit controller. Replace the air filter, clean the blower fan and confirm that the ductwork is not crimped or disconnected, and that there's plenty of return air, and let me know if that fixes the trouble.

On 2019-01-04 by willie

I have an old gaffers & sattler u80 the burnners light for about 30 sec.then shutts off then the fan kicks on for 60 sec.then shuts off then starts again about 5 min. later

On 2018-12-09 by (mod) - Pilot light flickering

Dirty orifice

Loose connection

Improper placement

Low fuel?

On 2018-12-09 by kathy

Pilot light flickering

On 2018-12-01 by - by (mod) -

friedman (mod)

Jerry

See BLOWER FAN WONT STOP

inspectapedia.com/heat/Furnace_Fan_Wont_Stop.php

where we List the common causes of that problem. Take a look at that article and let me know if I can be of further help.

On 2018-12-01 by Jerry

What caused the furnace blower to run not shut off

On 2016-11-07 by Anonymous

the burner on my wall furnace will go out unexpectedly


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