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Gas furnaces are essential for keeping your house warm in winter. They have an efficient design that cycles warm air throughout your home and draws out cold air. Gas furnaces are usually reliable, but occasionally they start to malfunction and don’t heat up properly.
To fix a gas furnace, check if: the blower is blocked, the air filters are blocked, the blower capacitor is working, the flame sensor is dirty, the pilot orifice is clogged, the pilot flame setting is too low, the thermostat is working, and if the vents are open.
In this detailed guide, we will cover every reason why your gas furnace might not be heating up, and what you can do to fix it. So, let’s get started.
How Does a Gas Furnace Work?
There are a few steps in the heating cycle of a gas furnace:
- The burner/ignition switch ignites natural gas or propane inside the combustion chamber.
- Gas jets allow gas to come into the combustion chamber.
- The pilot light (older models) or hot surface ignitor (newer models) ignites the gas jets.
- This fire heats up the metal heat exchanger. Exhaust leaves through the flue and is pushed outside the home.
- The heat exchanger transfers its heat to moving air.
- The blower pushes the heated air into ducts, which move it through the house.
- While warm air is pushed into the rooms, cool air is sucked back into the return ducts and then the furnace. It is heated by the heat exchanger, repeating the cycle.
- When your house gets to the temperature that you chose on the thermostat, it shuts off the gas. This ends the heating cycle.
Check if the Blower Is Blocked
If you’ve been using your gas furnace for a while, it’s possible that there is an accumulation of dirt and waste around the blower. Because the air isn’t moving properly, the blower can’t efficiently blow hot air into your ducts and heat your house.
It can also damage your heat exchanger by preventing the flow of cool air that stops it from overheating.
How to Fix
- Switch off the electricity that powers your gas furnace. Do this directly at the unit and also with your circuit breaker.
- Remove the screw on the unit to lift away the outer panel, exposing the inside of the furnace.
- Unscrew the panel that covers the blower motor.
- Disconnect any wires that will stop you from removing the control panel or blower.
- Remove the bolts of the shelf that the control panel sits on. Lift out the control panel and move it to one side.
- Unscrew the bolts that hold the motor and fan in place. Remove the motor and wheel.
- Use a handheld vacuum with a brush attachment to clean the wheel fins. We recommend the Shop-Vac Contractor Vacuum.
- Reassemble the furnace.
If you prefer a visual run-through, this video takes you through the step-by-step process of cleaning your blower:
Check if the Blower Capacitor Is Working
The blower capacitor is like a battery that helps the furnace’s blower operate with more power. When the blower capacitor isn’t working properly, the blower may stop working completely, make loud noises, or only work intermittently.
The following steps show you how to check if the blower capacitor is working.
How to Fix
- Switch off the electricity at the circuit breaker on your fuse box and turn off the gas.
- Pull out the blower so you can reach the capacitor.
- Remove the wires and discharge the capacitor by placing a screwdriver with an insulated handle horizontally across the terminals. If it is a dual capacitor, touch a screwdriver from the FAN to the Common terminal, then the HERM to the Common terminal.
- Hook up each terminal to the probes of a multimeter that can read capacitance levels. We recommend the AstroAI Digital Multimeter.
- Compare the reading to the rated value printed on the side of the capacitor.
- If the reading is far from the rated value, you will need to buy a replacement capacitor.
This video shows you how to check if your blower capacitor is working properly:
Check if the Flame Sensor Is Dirty
A dirty flame sensor means that the furnace can’t light its burners. It will be unable to sense any flames and so will turn itself off.
How to Fix
- Unscrew the front panel.
- Locate the small metal rod on the right side of the furnace.
- Use a nut driver to loosen the closest screw and remove the rod (the flame sensor).
- Use a piece of rough paper, light sandpaper, or a dollar bill to rub the residue off the flame sensor.
This video is a great reference for cleaning the flame sensor:
Check if the Air Filters Are Blocked
If your air filters are blocked or closed, air will not be able to flow easily from the furnace. Continual use can create a buildup of dirt and grime that blocks the filters.
You should replace your air filters every 30-90 days for optimum performance.
How to Fix
- Open the door panel of the furnace.
- Remove the old filter.
- Slide in the new filter, making sure that airflow arrows are pointing towards the blower.
- Replace the door panel.
For a simple visual guide, check out this video:
Check if the Pilot Orifice Is Clogged
In older gas furnaces, your unit has a pilot light that should stay lit. If there is any waste matter clogging the pilot orifice, then it can prevent the pilot light from igniting.
How to Fix
- Turn off the gas and the electricity on the unit and at the circuit breaker.
- Poke a thin piece of wire through the hole where the pilot light burns.
Check if the Pilot Flame Setting Is Too Low
It’s possible that the flame setting of the pilot light is too low, not allowing it to generate enough heat.
How to Fix
- Find the screw near your pilot light that has “pilot adjust” written in small letters. It will be next to a small indent.
- Remove the screw and insert your screwdriver.
- Light the pilot light.
- Turn the screwdriver until the pilot light increases in size.
You can use this video for reference:
Check if the Thermostat Is Working
Your thermostat controls the temperature and heat settings of the gas furnace. If you have the wrong settings or the thermostat is faulty, the furnace won’t heat up.
How to Fix
- Set the thermostat to “Heat” if you have a dual cycle system.
- Raise the temperature to 5 degrees more than room temperature.
- Set the fan setting to “On” or “Auto.
- Replace batteries in the thermostat.
- Open the thermostat and use a soft paintbrush to dust around the coils and contact plate.
Check if the Vents Are Open
It’s possible that your gas furnace is heating up, but the warm air isn’t able to get into your house.
How to Fix
- Fully open all vents.
- Move any curtains or furniture that are blocking vents.
In this article, we’ve covered all the common reasons why a gas furnace might not be heating up, including issues with the blowers, filters, pilot light, thermostat, and vents.
Make sure to check each issue if your gas furnace isn’t heating up, but always remember to switch off the gas and electricity first to keep yourself safe.