Why Is My Furnace Not Blowing Air Through Vents?

It can be a frustrating day when you come home only to find that your furnace isn’t blowing air through the vents. Maybe you can hear the flames roaring within the furnace and blower motor rotating. But nothing comes out of the vents.

But, there’s no need for panic. Sometimes the furnace may stop blowing for a non-issue, such as a piece of furniture blocking the vents. It will resume operation as soon as you remove the furniture and clear the air paths.

That said, though, you must also take the lack of warm air seriously as it may signal a massive underlying problem. Below, we look at the most common cause and ideal solutions. First, though, let’s attempt to understand how the furnace works.

How Does a Furnace Produce Hot Air?

The furnace burns fossil fuel to produce heat, which is circulated throughout your home to keep you, your family, and your guests warm. It all starts when you hit the ON switch on the furnace. As soon as you press or flip the ON button, the furnace’s circuit/control board is activated.

The board then invokes the pressure inducer motor to remove poisonous gases possibly trapped within the furnace’s combustion chamber and exhaust system.

At the same time, part of the control circuit will check with the thermostat to see if there’s a need to light the burners. The control board requests burner ignition if the current (indoor) temperature is lower than the thermostat setting.

According to the Department of Energy, most people set the thermostat between 68°F and 72°F. Whichever the setting, the control board will request heating as soon as indoor temperatures drop below the setting. The first thing that happens when the thermostat calls for heating depends on the type of furnace.

Traditional furnaces have a continuous pilot system that instantly lights the burners right away. However, electronic ignition models must first light the pilot. Then the pilot lights the burners.

After several seconds, when the furnace is warm enough, the blower motor springs into action. Located in the bottom chamber of the furnace, the blowing motor helps pull cool air from the home into the furnace via the return vents. The cold air passes through a filter that traps airborne particles, then via the blower and a strategically positioned heat exchanger.

Several more seconds can pass before the furnace is warm enough to put out warm air into the home. However, as soon as it’s ready, the heat exchanger helps safely transfer the heat from the combustion chamber to the exterior areas where the warm air mixes with passing cold air.

The blower motor works alongside the heat exchanger to ensure that the warm and cold air mix thoroughly.

Meanwhile, the blower fan continues to blow, forcing the now-warm air mixture into the supply ducts and your home via the supply vents.

Why Your Furnace is Not Blowing Air Through the Vents

The above process flow should help you diagnose potential issues whenever you’re not getting warm air from your supply ducts. Assuming that you have both power and gas supply, you want to troubleshoot the furnace for the following issues.

1. The Thermostat isn’t Calling for Heat

As we’ve seen, everything starts at the thermostat. When you push the ON button on your furnace, the control panel checks with the thermostat to see if you need heating and will only request heating if the thermostat says okay. Now, imagine a scenario where you’re dealing with a faulty thermostat or a wrongly set one.

In both cases, the misleading or absent temperature readings may cause the furnace burners to remain off even when you need heat.

For instance, if someone went behind your back and changed the temperature setting from 70°F to 60°F, your furnace will only produce heat when temperatures drop below 60°F – which rarely happens in most areas.

It means that you may remain in the cold perpetually. If the thermostat is broken, you won’t get heating however cold it gets.

Solution: Begin by checking whether the thermostat is working. Can you see any readings? Are the readings stables? If all you see is a blank screen or the readings keep jumping, the thermostat is defective and must be replaced. However, if it’s working, make sure it’s set correctly. The DOE recommends setting your thermostat to 68°F-72°F.

2. The Furnace isn’t in “Heat” Mode

Modern furnaces can be a little confusing given the numerous settings, timers, and everything in between. Timers are especially interesting.

Although they make your work easy by letting you come home to a warm home and switching off the furnace when you’re away to save energy, you need to memorize the settings. Otherwise, you’re in for a few shocks.

A particularly common confusion arises when you set the timer to run for a specified period but forget about it. The furnace will automatically go off (or sleep) after the pre-defined period.

However, if you’re still home and awake by the time it goes off, you may be convinced that the furnace is broken – when all you need to do is reset the timer.

You may also experience the same issues if the date and time on your thermostat are incorrect. Another confusing setting is “Fan” mode.

Many people mistakenly think that the furnace will still provide heat even in “Fan” mode when the opposite is true. Furnaces keep the burners off in “Fan” mode, only powering the blower to provide air conditioning.

Solution: First, the furnace must be in “Heat” mode whenever you want to heat. Secondly, make sure the date and time are correct.

3. You have Airflow Problems 

If the settings on your furnace are accurate and the thermostat is working, you may want to check for airflow issues. Is the furnace not getting enough air? If so, why? What if it’s getting enough air? Could you have significant leaks within the ductwork? Or are the filters blocked?

We recommend that you specifically diagnose the furnace for the following issues;

  • Dirty or blocked air filters: The furnace cannot draw enough cold air into the unit for heating if the filters are clogged. You should also make sure that you’re using the right filters. The MERV rating is critical.
  • Blocked vents/ducts: You may also get no air from your supply vents if the furnace has blocked vents or ducts. Perhaps debris or leaves found a way into the ducts and are blocking airflow. Or maybe the ducts are bent somewhere.
  • Leaky ducts: According to the DOE, the average furnace loses 35% to 50% of conditioned air via the furnace. However, it could be worse if your ductwork is old and uninsulated. 

Solution: Proper maintenance is the solution to all three problems. Keep the furnace clean and schedule a seasonal furnace tune-up. Additionally, change the filters regularly and keep a close eye on the furnace to catch potential issues early. 

4. The Blower Motor is Compromised or Dead

As we saw at the beginning, the blower motor is solely responsible for pulling cold air into the furnace and blowing warm air back into the room.

Now, consider a situation where the blower is dead or compromised in a way. You won’t get warm air from the supply vents no matter how hard the furnace works.

So, what could cause the blower motor to fail or malfunction? The list is long. For one, the motor itself can die or become defective, effectively killing the blower system. Alternatively, perhaps the blower system isn’t getting power. Maybe the wires connecting it to the control board are loose.

Another possibility is a dead blower motor capacitor. If the capacitor dies, the motor has no source of power to start. Therefore, it can’t run. Other issues to consider are broken blower blades, a worn drive belt, and electrical short-circuiting.

Solution: A simple reset can usually restore blower motor functions. You may also consider parts replacement or professional repairs. However, ultimately, you may need to replace the blower system.

5. It’s a Limit Switch Issue

If the blower motor is fine, but you still suspect that something is “holding” it back somewhere, check the limit switch. Many people only “know” that the blower motor comes on automatically.

However, this isn’t entirely true. Although the motor doesn’t need additional effort to switch on or go off, it relies on a special relay switch known as the limit switch that determines its operational state.

Typically, the limit switch closes, thus switching the fan on, when temperatures inside the furnace reach 100°F. The fan will remain ON until you turn off the furnace, causing temperatures to drop below 90°F. At this point, the switch will close, thus shut down the fan. 

However, a few things may happen that may compromise the normal operation. For instance, the furnace can overheat. Or the switch, essentially an electric circuit, can experience short-circuiting, perhaps during a power surge.

Regular operation is interrupted whenever something happens that throws the limit switch off. For instance, short-circuiting can cause the fan to open even when the furnace is running, thus shutting down the furnace. As a result, you wouldn’t get heat.

Solution: The first step is to ensure the limit switch is set correctly. The lower limit should be 90°F, ON should be 100°F, and the higher limit should be 200°F. More importantly, make sure the fan is operational. If not, replace it. 

6. It’s an Ignition Problem

Finally, and we’re only mentioning this one to cover all bases, you will also not get heat at the vents if the furnace is experiencing ignition problems. Maybe it’s igniting and going off right away. Or perhaps it’s not igniting at all.

In both cases, you’d be waiting for heat when there’s no heating in the first place. So, what could be wrong? It depends on the signs. First, you should check that the pilot light is ON.

Or, in the case of electric ignition systems, make sure the pilot system lights when you turn on the furnace and the thermostat is calling for heat. If the pilot system works, then maybe the burners aren’t getting gas. Alternatively, perhaps the furnace has entered “lockout” after too many ignition attempts.

The furnace becomes unresponsive in “lockout” until you reset it. The two most common causes of “lockout” are overheating and inducer motor malfunction. 

Solution: The solution here will depend on the nature of the problem. For instance, you can clean or replace the ignition system if it fails. Meanwhile, you may need to locate the furnace reset button and press it for at least five seconds if the furnace has entered lockout. However, we strongly recommend seeking professional input to resolve the problem fully. 

Summary

Now you know where to look if your furnace isn’t blowing air through the vents. We recommend that you start by checking the settings before moving to more advanced issues such as airflow challenges and internal blower motor damage. 

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