Gas Furnace Troubleshooting and Repair
It's getting cold outside. If your furnace fails or malfunctions, diagnose and fix the situation yourself with our DIY Furnace Troubleshooting Guide.
On those cold winter nights when the heat goes out, it can get pretty scary. Within a matter of hours, the house will be significantly colder, with the young and elderly suffering the worst of it. Time to get out some space heaters to delay the encroaching cold and begin the furnace troubleshooting process.
The best way to avoid a malfunctioning furnace in the first place is to schedule professional heating maintenance every fall. Sign up for a home maintenance plan so you never forget this important home maintenance task.
Common Furnace Problems and How to Fix Them
Before you call the professionals, there are some quick things you should check first. Sometimes, all that is needed is a filter or battery replacement.
1. Thermostat Troubleshooting
Many times, when people think they have a broken furnace, it’s really a thermostat problem. Time to troubleshoot your thermostat:
- POWER – Make sure your thermostat has enough juice to send signals to your HVAC system. Look at its faceplate to see if there is a low battery signal. The most difficult part of changing your thermostat’s batteries is getting that darn faceplate off. Watch this video to learn how to replace your thermostat’s batteries, and don’t hesitate to call Hiller if you are having trouble:
- DUST – Older electromechanical thermostats sometimes need a little dusting. If your thermostat isn’t working, open it up using the tips from the video and do some light cleaning with a Q-tip. Make sure you are going around the bimetal coil and contact plates. If your Q-tips can’t get in the small crevices, a small piece of soft paper should do the trick.
- LOCATION – If your thermostat is near a heat source, such as a heating vent, light fixture, or cooking equipment, the temperature reading will be off and wrong commands will be sent to the furnace. Make sure your thermostat is away from direct sunlight and other sources of heat.
- BALANCE – For your thermostat to work properly, it is important that it is completely level. Use a level device to check if your thermostat is straight. This is especially important for older thermostat that rely on mercury switches. Reposition the thermostat if necessary.
- ADJUST ANTICIPATOR – If you have a non-digital thermostat with a mercury switch, you may need to adjust the anticipator. This will require removing the faceplate and locating the small metal tab that goes from shorter to longer. If your furnace is cycling on and off too frequently, try adjusting the anticipator toward the “longer” setting. If the HVAC system isn’t cycling enough, turn the anticipator toward the “shorter” side. Sometimes a little trial and error is needed, so be patient.
- SETTING – This may sound obvious, but make sure your thermostat is set to “heat.” If the furnace still won’t turn on, turn off the furnace at the source and switch the thermostat over to “auto” and wait about 30 minutes. Turn the furnace back on and switch the thermostat back over to “heat” to see if this “reset” did the trick.
Learn how a thermostat works from Mr. Wizard:
If you do have any problems with your thermostat, don’t hesitate to contact Hiller.
If you have checked all of the common thermostat problems and the furnace still isn’t working, it’s time to move on to the next steps.
If your furnace isn’t getting power, go over to your breaker box and check for any blown fuses or tripped breakers. If power at the breaker/fuse box is on, make sure your HVAC system hasn’t been unplugged for any reason. There may also be a separate power switch near the furnace that looks like a light switch. Turn it on.
After making sure everything is plugged in and switched on, reset power at your breaker box. If restoring the circuit doesn’t work, then you could have faulty wiring between your thermostat and furnace. If this is the case, call Hiller.
If you have power, but the furnace is still not working properly, move on to step 3.
3. Dirty Air Filter
If both your thermostat and furnace are getting the power they need, the next thing to check is your furnace filter. Some HVAC systems have a safety device in place to turn off the furnace in the case of a dirty or clogged air filter. It’s important that you replace your HVAC filters every 30-90 days depending on the type of filter you have and how often you use your system. Changing your air filter improves airflow and indoor air quality while lowering heating costs and extending the lifespan of your unit.
Watch this video to learn how to change your furnace filter (make sure the arrow is pointing in the direction of airflow):
4. Relight Pilot
If your furnace has power and a clean air filter but still won’t work, there may be a problem with the gas. If you aren’t getting gas or the pilot won’t light, make sure the gas valve is fully open and the pilot light is fully depressed.
If you need to relight your pilot, follow the manufacturer’s instructions exactly (hopefully the manual is fastened to the furnace somewhere). If you can’t find the exact instructions, follow these steps:
- Locate the pilot light assembly and turn the dial to the “off” position.
- Wait 5-10 minutes until all the gas has dissipated.
- If you still smell gas after 10 minutes, evacuate the household and call your local gas company and fire department.
- Look for the ignitor button next to the gas valve with “on,” “off,” and “pilot” settings.
- Turn the valve over to the pilot setting.
- Hold a long lighter or match to the pilot opening while you press down on the reset button (usually red).
- Keep the button depressed until the flame comes on.
- Switch the setting from “pilot” to “on.”
- Learn more
Bad Pilot Signs
- Yellow, purple, orange, or red flames
- Split, flickering, or wavering flame
- Flames higher than 2 inches or shorter than 1 inch
You can adjust your pilot light flame height by tightening or loosening the screw located on the pilot valve body. Refer to your manufacturer instructions for proper location. Tightening the screw (clockwise) should decrease the flame size while loosening the screw (counter clockwise) should increase it.
If your pilot light won’t stay lit or is any other color than blue, call the professionals at Hiller to diagnose the situation. You may have a dangerous combustion situation or a faulty thermocouple (safety device that shuts off the gas).
Sometimes, to fix the thermocouple, all you need to do is tighten the nut with a wrench. Be careful not to over-tighten! Only tighten it a little bit. Then, try to light the pilot again. If tightening the thermocouple nut doesn’t work, you probably need to replace the thermocouple. This is a fairly easy repair, but unless you know exactly what you are doing, we recommend calling a pro.
5. Cleaning, Lubrication, and General Maintenance
The best way to avoid furnace problems in the first place is with regular maintenance. Most heating maintenance is accomplished by remembering to schedule a professional tune-up in the fall (or signing up for a home maintenance plan) and replacing your air filters every 30-90 days.
Your technician will lubricate all moving parts, clean the insides and outsides, as well as checking for venting and electrical safety. The technician will clean off all the dust and dirt that have accumulated over the summer, as well as sealing and leaks and tightening bolts hold any components in place. If there’s a bigger problem brewing, he can spot it and schedule a repair session; he’ll also run the heater to ensure that everything is functioning as it should. This helps the heater use less energy in the winter months, reducing your bills as well as running less of a risk of major repairs. These are things you should not attempt to DIY.
Read our Air Filtration Guide to learn more about the importance of HVAC filters.
To recap the furnace troubleshooting process:
- Check your thermostat
- Check for power
- Replace the air filter
- Relight pilot light
- Make sure all vents and registers are unblocked
- Make sure access panel doors are fully closed
- Restart your furnace
Although some malfunctions are obvious—such as a furnace that won’t turn on at all—others are a bit harder to diagnose, such as changes in the sound the furnace is making. Below are some of the warning sounds that your furnace needs professional assistance you shouldn’t let slip by.
- Booming: If you gas furnace makes a “booming” sound when it comes on, it usually means there is an issue with the burner, such as dirt or rust, that is blocking the gas jets. You should only trust the job of cleaning the burner to professionals. The booming might also be due to a defective gas valve.
- Clicking: A clicking noise as the furnace turns on sometimes indicates problems with the electrical ignition. If the clicking sound occurs after the blower fan turns off, it can point toward cracks in the heat exchangers, which can lead to carbon monoxide leaks in gas furnaces—so do not hesitate when it comes to calling for repair work.
- Screeching: This sort of mechanical sound can mean issues with the blower motor, possibly with the bearings. Have technicians look into the sound and see if the motor needs replacement.
- Clanging: If you have an older furnace that still uses a blower motor fan belt (rather than a direct motor), the eruption of clanging noises from the cabinet might mean the belt has snapped and is starting to strike other parts of the cabinet. Although repairs can fix this, in most cases you should give serious consideration to replacing the furnace with a newer model.
Because there can be more than one cause behind a particular furnace noise, you will need repair technicians to diagnose the problem first and then apply the right solution. The dangers of DIY furnace repair include fire, carbon monoxide poisoning, refrigerant leaks, and expensive repairs. If you have any doubts about what you are doing, stop and call a professional.
Troubleshooting the issue on your own is extremely risky, so don’t attempt it: just contact the award-winning team at Hiller. We are familiar with all makes and models and can get your heat back in no time.