You wake up in the middle of a cold night and feel a chill. As you drag your feet to your thermostat, you turn on the furnace to blow hot air. After a couple of minutes, you notice a lack of hot air circulating throughout your home, and you immediately start to wonder why your furnace isn’t blowing hot air.
At Monkey Wrench Plumbing, Heating & Air, we’ve worked on hundreds of furnaces throughout Los Angeles and the surrounding areas. A lack of hot air is a recurring issue we’ve seen homeowners experience with furnace systems. Since 2007, we’ve inspected, repaired, or replaced furnaces that experienced a lack of hot air.
Here are some of the most common reasons why a furnace may not be able to blow hot air.
- Gas ignition system issues
- Obstructed airflow
- Lack of gas
- Thermostat problems
- Furnace overheated
We’ll go over each of these issues in detail so you can get a better idea of what these issues are and what a heating, ventilation, and air conditioning (HVAC) technician can do to address these problems.
Let’s jump right in!
Why Don’t I Have Hot Air With My Furnace?
There are various reasons why a furnace may not blow any hot air, and while this list won’t give you DIY tips on how to fix the issue, it can help give you an idea of what’s going on. Furnace systems are complex and often located in hard-to-reach areas. Contacting a certified technician to diagnose your furnace can help you pinpoint the exact cause of the issue.
Although electric-only furnaces exist, we will review the lack of hot air issues associated with gas furnaces.
Issues With the Gas Ignition System
An issue with the gas ignition system could cause a furnace to experience a lack of hot air. Furnaces have several components that make up the ignition system, such as.
- Flame sensor: This sensor is a built-in safety device inside a furnace that detects whether the equipment is producing a flame or burning a gas. A dirty or broken flame sensor may lead to a lack of hot air.
- Gas burner: The gas burner is the area inside a furnace ignition system where burning gas and air are mixed to produce heat. A home may experience a lack of hot air if the gas burner is malfunctioning.
- Heat exchanger: The heat created by a gas burner is absorbed by the heat exchanger. If there’s a lack of hot air circulating, it may be caused by a defective or broken heat exchanger.
- Pilot light or electric igniter: Similar to a pilot light in a conventional water heater, a furnace pilot light produces a flame that generates heat. Pilot lights are more common in older furnaces, and newer systems tend to have an electric ignition source. The furnace will not produce hot air if the pilot light or electric ignition source isn’t working.
- Gas valve: This part allows gas flow into a furnace ignition chamber. If the valve is broken or obstructed, the furnace will not have any gas fuel and will be unable to create hot air.
- Limit switch: Like the flame sensor, the limit switch is a safety component of a furnace system that constantly checks the temperature of the furnace. If the system overheats, the limit switch is activated, and the furnace shuts down.
Professional technicians can inspect these components and identify which parts are causing a lack of hot air from a furnace. Even if there aren’t any issues with the parts of the furnace’s ignition system, the parts will not work if there’s a lack of natural gas as a fuel source.
Lack of Gas in the Furnace
A furnace cannot produce hot air without natural gas fueling the flames. Gas leaks, gas shutoffs, or broken or blocked gas lines can prevent the furnace from creating heat inside its combustion chamber.
If there’s a leak, homeowners may identify the smell of gas near the furnace or a home’s gas lines. Leaking gas is flammable and should be addressed immediately by a certified professional.
Any issues with the gas supply should only be addressed by a technician. The technician can verify any problems with the gas supply that fuels a furnace and other fixtures throughout a home. Once any issues with the gas are fixed, the technician can perform a test to see if hot air is flowing through the vents or if there is an obstruction in the airflow.
Furnace Airflow is Obstructed
Furnace systems may be able to create hot air, but if there are any obstructions with the airflow, then the hot air will not blow. The ductwork, blower motor, and air filters each play a role in transporting hot air throughout a house.
Notably, air filters need to be replaced every three to six months. Although several factors can affect a filter’s lifespan, it’s still vital to replace them to avoid any issues with airflow. Dirty air filters can suck in dirt and debris, which can clog up and restrict airflow. 
A Monkey Wrench Plumbing, Heating & Air technician applies hardware cloth to the ductwork to protect it from animals.
A furnace’s ductwork is crucial in providing hot air a pathway to flow. Rodents can chew through the ductwork, or the ductwork may deteriorate over time, causing hot airflow restrictions. Even if the airflow isn’t obstructed, a malfunctioning blower motor cannot blow hot air throughout the ductwork.
The thermostat is the device where homeowners control their furnaces. In the next section, we’ll examine how some thermostat issues can lead to a lack of hot air.
Furnace Thermostat Issues
The thermostat controls and regulates the temperature of a furnace system. Although other factors can affect hot air flow, a furnace may not be able to kickstart the heating process if there’s an issue with the thermostat.
The following thermostat issues may contribute to a lack of hot air from a furnace.
- Old thermostat
- Failing batteries in thermostat
- The thermostat sensor does not detect ambient temperature
- Issues with wiring connecting thermostat with furnace control board
Some thermostat issues can be easily fixed, such as replacing dying batteries. A common concern homeowners have with their thermostats is that the temperature of their homes doesn’t match with what’s displayed on the device.
A Monkey Wrench Plumbing, Heating & Air technician is inspecting the thermostat of a home's furnace system.
Other problems may require a new replacement. Furthermore, homeowners can take advantage of smart thermostat rebates and credits available in Los Angeles and the surrounding areas.
The thermostat is just one part of an entire furnace. But if the system overheats, a home will be without hot air until the whole unit is repaired or replaced.
The Furnace Overheated
When a furnace overheats, it is designed to shut down as a safety precaution and will not be able to produce any hot air. The furnace can overheat due to internal damage to parts, restricted airflow, or mechanical failure.
In some cases, the furnace will need to cool down before it can start producing hot air again. Overheating can also affect the heating efficiency of a furnace. So, even if the hot airflow starts running, it may not warm up fast enough.
The system may overheat again, so it’s highly recommended to have an HVAC technician inspect the furnace to see what’s causing it to overheat.
Addressing Hot Air Issues With the Furnace System
We reviewed the different reasons a furnace may not be blowing hot air. With this information, you can start to get an understanding of what the underlying issue is once a technician inspects your furnace. This information also helps converse with the technician as they explain the problem behind a lack of hot air in your furnace.
Every furnace is different, yet the causes behind why it may not blow hot air are similar. Since 2007, we at Monkey Wrench Plumbing, Heating & Air have encountered these issues and helped restore heating for homes across Los Angeles and the surrounding neighborhoods.
If your furnace isn’t blowing hot air, contact us using one of the buttons below or visit our online HVAC repair scheduler to book an appointment.
Once you determine what’s going on with your system, you may have to consider repair versus replacement. Explore the factors that go into HVAC system repairs and what it may be like to consider a new HVAC system.
