6 Reasons Why Furnace Blower Keeps Running But No Heat?

It’s a fairly common problem. Maybe you’re coming home from work and hope to escape the winter cold when you finally get home, only to find that the furnace blows cold air. You can hear the blower humming. However, you can’t feel the heat.

Perhaps you’ve even checked at the supply vent and confirmed that the furnace only produces lukewarm air. So, what could be wrong, and what can you do about it? Let’s find out.

How does Furnace Blower Works?

To understand why the blower motor may run without the furnace necessarily producing warm air, you need first to understand how the blower motor works.

Furnace blower motors are ordinary blower fans that rotate to move air. They comprise an electricity-powered motor that rotates hundreds of times per minute.

The motor, in turn, connects to a blade system. This way, the blades rotate in tandem with the motor. The rotating blades create a motion that moves air in a specific direction. This motion helps to draw cold air from your home into the furnace and warm air from the furnace into your home.

Blower motors derive power from the furnace’s control board, not the home’s main supply. However, since the power from the board isn’t enough to start the motor, the fan has a special capacitor that stores extra change for starting. It’s also worth noting that a limit switch controls the blower fan’s on-off operation.

The limit switch has three different settings, i.e., the lower limit, ON, and high limit settings. The lower temperature limit, typically 90°F, is the furnace temperature (not thermostat temperature) when the blower motor goes off. As soon as the furnace temperature drops to the lower limit, the fan will go off – even if the furnace is still running.

Meanwhile, the ON setting, typically 100°F, is the furnace temperature at which the fan kicks on. This means that the fan will remain off even after switching on the furnace until the thermostat reads 100°. Finally, the higher limit, typically 200°F, is the furnace temperature at which the furnace will go off and shut down the furnace too to prevent overheating.

If you’re wondering, furnace temperature here refers to the temperature of the hot air as it exits the supply plenum. The vents, working alongside the blower and thermostat, regulate temperatures in your rooms by limiting the amount of hot air entering your home.

Under normal circumstances, the furnace blower will kick on within a few seconds of switching on the furnace and keep running until you switch off the furnace. It will also consistently blow warm air through the supply vent until you shut the furnace.  

6 Reasons Why Furnace Blower Keeps Running But No Heat?

If your blower seems to run, but the furnace isn’t producing warm air at the supply vent, you may have an issue with the fan, the limit switch, the thermostat, or the control board. The following are the most common causes and what to do.

1. The Furnace is Set ON

You’re probably aware that the furnace takes instructions from the thermostat. It only heats when the thermostat says so. Otherwise, it stays off. So, it could be ON but not heating.

Meanwhile, the furnace blower will run as long as the furnace is at 100°F, whether the furnace is heating or not.

So, you could have the blower running even when the furnace isn’t heating. This is especially true when you set the furnace fan ON at the thermostat. The fan will run as long as the thermostat is ON, whether or not it’s heating.

Solution: Make sure that the fan setting is in AUTO. This way, it will only run when the furnace is heating and switch off when the burners go off.

2. Wrong Thermostat Settings

If the fan is set to AUTO, then maybe the temperature setting is misleading. As we’ve mentioned, the furnace only does what the thermostat says. Now, imagine that the thermostat setting is below room temperature.

Or, say it’s 60°F in your home (before heating). So, you set the thermostat at 70°F to beat the cold. However, perhaps one of the kids tampered with the setting, leaving it at 55°F.

In this case, you’re essentially telling the thermostat that you’re already getting more heat (60°F) than you requested (55°F). So, it will request the furnace to keep the burners off perpetually.

Solution: Always double-check to ensure the thermostat setting is above room temperature. According to the Department of Energy, your thermostat setting should be between 68°F and 72°F, depending on your geographic location.

3. The Fan Limit Switch is Stuck “Closed”

We’ve already explained the role of the fan limit switch. What we didn’t mention, though, is that the relay switch can malfunction. For instance, if there’s an electric short circuit, it may get stuck or overheat. Sometimes some of the wires may even melt.

A particularly common issue is when the switch gets stuck in a closed position. This can happen if the switch is dirty or due to internal damage. A blower fan stuck “closed” will run continuously as long as the furnace is switched on. It will keep running even when the furnace isn’t heating.

Solution: You need to find out why the switch is stuck closed. If it’s dirty, clean it. If it’s internal damage, consider repair. Alternatively, replace the fan.

4. Airflow Challenges 

Airflow issues are perhaps the most common cause of furnace problems. As soon as the furnace experiences airflow blockage, it will react and may even stop. One of the signs of poor or impeded airflow is a humming blower fan that doesn’t blow warm air.

This problem usually points to blocked filters, a blocked flue, or leaky air ducts. Whichever the case, it means that air cannot move around the furnace as originally intended. Maybe it’s leaking out or unable to reach the burner/outlet vents.

Solution: Ensure regular maintenance to keep the furnace healthy and clean. More importantly, always watch out for air leaks, blocked/bent ducts, and dirty filters. You need to replace the filters at least once every two weeks.

5. Gas Supply Issues 

You may also have the furnace blower running with no heat if the furnace isn’t getting enough fuel/gas or getting none at all. In this case, the blower will keep running, especially if it’s on AUTO, but there’ll be no heating because there’s no fuel to burn.

So, the question you need to ask is why you don’t have gas. Is it a supplier issue? Do you have blocked gas lines? Or maybe one of the gas lines is leaking? You may also have no gas at the burners because the main valve is turned off.

Solution: The first step is to make sure that your gas supply is open. Verify that all the necessary valves are open. Secondly, make sure you don’t have gas leaks or blocked pipes. You may need to hire an HVAC professional to probe the pipes.

6. The Condenser Pan is Full

Finally, you should also check your condenser drain pan. Nearly all furnaces in use today are condensing units. They produce vapor as part of exhaust gases, then condense the vapor to prevent early rusting and the creation of toxic acids that may erode the furnace surfaces.

The water then collects in a dedicated pan and eventually drains out automatically. However, sometimes the drainage system can block, resulting in overfilling.

Since an overflowing drain pan is bad for the health of your furnace, furnaces have a dedicated drain pan switch that immediately shuts down the burners when the drain pan is full. Meanwhile, the blower will continue to run.

Solution: Make sure the drain pan is not overflowing. If it is, find out why. Is it blocked? Unblocking a blocked drain pan is a simple DIY process.

Summary

If your furnace blower keeps running, but there’s no heat, you immediately check the thermostat to ensure it is in AUTO mode and set correctly.

If that doesn’t fix the problem, troubleshoot the furnace for possible airflow, gas supply, limit switch, and condenser pan issues. It’s almost always one of the six issues.