It can be anything from dirty filters to blocked vents/ducts, a faulty flame sensor, dirty burners, or even a blown control board.
Ideally, you need to conduct further tests to find the root cause of the problem. Or, you can call your HVAC technician to help point out the potential problem. However, it’s also handy to keep a list of “likely causes” so you know where to start.
Below, we discuss the most common reasons your furnace may blow without necessarily generating heat and how to troubleshoot each potential cause.
First, though, let’s try to understand what happens between the moment you push the ON button on your furnace and when warm air comes out of the supply vent.
How Does a Furnace Heat a House?
Furnaces are electrical appliances that burn fossil fuels to generate heat, then disperse the heat throughout your home to keep you and your loved ones cozy and warm.
The entire process begins way in advance when you set the thermostat setting. Most people set the thermostat between 68°F and 72°F during winter.
By so doing, you’re telling your furnace to commence heating whenever temperatures drop below the selected setting.
With the setting in place, and assuming that the furnace breaker is ON and the main gas supply valve (the one next to the furnace) is open (parallel to the gas line), the heating process goes on as follows;
You turn on the furnace
Pretty much all furnaces have an onboard ON/OFF switch that you need to turn ON to kickstart the heating process. When you do so, the furnace’s control board springs to life.
The pressure inducer fan kicks on
Although most people don’t know it, the pressure inducer fan is the first thing that comes on when you turn on the furnace. It begins to run right away to clear out exhaust gases potentially trapped within the furnace.
The thermostat requests heating
Simultaneously, the control board engages the thermostat to check whether your home is sufficiently warm. So, the thermostat compares current temperatures to the settings you input earlier. If it’s higher than the setting, nothing happens. However, if the temperature is below the setting, the thermostat requests heating.
The ignition system is energized and lights
This step doesn’t apply to traditional furnaces as their standing pilots remain ON round the clock even when the furnace is OFF. However, if you have a modern electronic ignition furnace, the call for heating will close a circuit, allowing electricity to flow through the ignitor. Then the pilot lights.
The pilot system ignites the burners
Several processes take place between the time the ignition system is energized and when the burners are lit. However, if all goes well, the burners ignite within at most five seconds after the pilot system ignites.
The burners heat the heat exchanger
The heat exchanger helps contain the flames to prevent potential fires while also ensuring maximum heat transfer.
The heat exchanger heats return air
Return air comes from the return air plenum at the base of the furnace and passes through an air filter before it’s forced past the heat exchanger. It absorbs the heat from the flame as it passes through the heat exchanger.
Warm air exits at the supply air plenum
The warm air coming out of the exchanger, now known as supply air, gets out of the furnace through the supply air plenum.
Exhaust gases exit via the exhaust vent
Exhaust gases are harmful gases, including carbon dioxide, carbon monoxide, and silicon dioxide.
Warm air is dispersed throughout your home
Typically, the supply air is forced into supply ducts that direct it into different rooms throughout the house. The pressure/speed helps generate convectional currents that ensure even heating throughout the allocated spaces.
Why is My Furnace Running But Not Heating?
If your furnace seems to be running but isn’t producing heat or the heat isn’t strong enough to keep you warm and comfortable, there’s a problem in one of the ten steps. The following are nine things that can go wrong and what to do about each.
As we saw earlier, heating officially commences when the thermostat calls for heat. Therefore, the thermostat is the first place to check when the furnace is running but not producing heat. Is the thermostat set correctly? Or is it too low? Is it even working in the first place? Can you see digits on the screen?
Solution: Check whether the thermostat is working and correctly set. It should be set between 68°F and 72°F. Additionally, make sure the fan is set to “AUTO” rather than ON. Finally, make sure the battery is strong enough.
Ignitor/Pilot Light Issues
Next, check the ignitor and pilot light. Ignitors and pilot lights are critical to furnace ignition. If either is compromised or dead, the furnace cannot light up. Common ignitor issues include dirt buildup, loss of sensitivity, and short-circuiting. Meanwhile, pilot lights can become blocked with dirt or get blown off by the wind.
Solution: Make sure the ignitor is in excellent working condition. If not, test and repair or replace it as appropriate. The same applies to the pilot light. Try to clean it. But if the problem doesn’t go away, replace it.
Issues with the Gas Valve
If the furnace ignitor and pilot light are working, the next place to check is the gas valve. Gas valves supply natural gas or propane to the pilot system and the burners. Therefore, if the valve is blocked due to dirt buildup, the furnace may not start. The same applies if the valve is leaking.
Solution: Probe the valve for leaks and dirt buildup and act appropriately. DIY maintenance, including cleaning, can help prevent issues of dirt buildup. However, you need professional intervention if it’s leaking. Shut down the furnace at the breaker, turn off the main valve, and call an HVAC professional.
The burner is Clogged
Burners have small “holes” through which gas comes out to burn and produce the heat necessary to warm your home. Now, imagine if the holes are blocked due to dust, carbon, or other forms of dirt. Your burners wouldn’t light.
Solution: Here, too, regular maintenance is the solution. Make sure to schedule professional maintenance at least once a few weeks before the heating season begins. Additionally, take time to wipe the furnace clean at least once weekly to rid it of dust and carbon buildup.
A bad Flame Sensor
The flame sensor is a tiny electrical device with a porcelain base and a long wire tip that constantly tracks the burner flame. Its main purpose is to determine and report on the presence and state of the burner flame, allowing the control board to take necessary action.
For example, if no flame is present, the control board can immediately turn off the gas supply, shutting down the heating process to avert fire risk.
Solution: If your flame sensor is bad, you should clean it and even resistance-test it to see whether it’s working. If it is dead, replace it. Replacement sensors cost about $40.
A bad Blower Fan
The blower fan plays a critical role in pulling cold air into the furnace and pushing warm air out of the furnace and throughout your home. Therefore, you may not get any warm air if the blower fan is bad. Blower fans can malfunction due to internal damage.
Or, it could be dirt buildup, electrical malfunction, or a blown capacitor.
Solution: Begin by toughly cleaning the fan to rid it of dust and debris. Then inspect it closely for damages such as broken blades, loose wires, a dead motor, and a worn belt and replace or repair the parts accordingly. If it’s still not working, test it with a multimeter. If it doesn’t conduct electricity, replace it.
Blocked or Missing Filter
The furnace cannot run with a blocked or missing filter. If the filter is missing, you’ll get an alert that your furnace doesn’t have a filer and the heating process pauses until you replace the filter. However, if it’s blocked, heating may continue, but you’re unlikely to see much warm air from the supply vent.
Also, dirty filters can cause overheating, resulting in furnace shutdown.
Solution: Make sure your furnace’s filter is present and installed properly. Additionally, make sure it’s the right filter. This is especially important when replacing your furnace filter. Above all, ensure to replace the filter at least once every two weeks.
Condensate Line/Pan Issues
Most furnaces today are condensing units that collect the water from the combustion process in a drain pan and drain it out via a drain line. This, too, is a critical process that must go on smoothly. If something blocks the drain lines causing the drain pan to overfill, the water can back up into the furnace, causing untold damage.
So, furnaces have a switch that trips if the drain pan overfills.
Solution: Regular maintenance can help prevent this problem in the first place. However, if you already have a drain line blockage, you need to call the pros.
You may also not get warm air from your furnace if your ductwork is blocked or leaking. For example, maybe a few twigs found a way into the ducts, or a bird was building a nest inside the ducts during the summer, and you didn’t clean the ducts at the onset of the heating season. Or perhaps the ducts have fractured, or connections become loose, thus leaking warm air.
Solution: Unfortunately, probing duct lines for blockages and leaks is extremely difficult. So, you need to call the pros. Ultimately, though, you may need repairs or replacement of certain sections of the duct system.
Limit Switch and Lockout
Finally, you may not have warm air even when the furnace appears to be running if one of the limit switches is triggered or the furnace is in “lockout.” For instance, the high limit switch will engage if the furnace overheats. Meanwhile, the flame rollout switch engages if the burners flames rollout of the flame exchanger.
The furnace will shut down in both cases.
Solution: Check the error codes on your furnace to find out if the furnace is in a lockout or one of the safety switches is engaged. If so, the error codes will tell you why. Fix the underlying issue, then reset the furnace.
Now you know the top ten reasons why your furnace is running but not generating any heat. Don’t hesitate to call your HVAC technician if you feel stuck.