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Althoff Home Services Blog

Why Is My Furnace Running but Not Blowing Hot Air?

Your furnace clicks on and the blower motor is running, but the air coming from the vents is cold or cool instead of hot. Or maybe the air starts off hot, then turns cool before the furnace shuts down.

Let’s look at some of the solutions to this common furnace problem.

1. Check the air filter.

Has it been awhile since you changed your furnace air filter? If you use disposable air filters, we recommend checking them every month and changing them if they’re dirty.

A dirty air filter affects more than just your home’s air quality (though don’t get us wrong, that’s important). The humble air filter is at the root of many furnace problems.

A clogged, dirty air filter blocks the return air flow to the furnace, meaning:

  • There’s less air to blow out of your vents. This could account for your “cold air” feeling.
  • The furnace can overheat. The furnace is suffocating without enough air, so it has to work harder and run longer to heat your home. Imagine trying to run a marathon with a heavy cloth around your mouth and nose, and you’ll get the idea. Soot builds up on the heat exchanger over time, making it even less efficient. All this can easily cause the heat exchanger to overheat. Your furnace has safety controls that automatically shut down the burners in case of overheating. Though the hot air stops, cool or cold air will continue to blow for awhile in order to cool down the unit to safe level.

Try changing the air filter to see if this resolves your cold air problem. If that doesn’t help but you still suspect your furnace is overheating, it’s time to call in a professional HVAC technician. Repeated overheating will damage the heat exchanger and lead to a big repair bill.

2. Check the thermostat.

If you’re getting cool air instead of warm, the problem may not be your furnace at all. It may be an issue with the thermostat. Thermostats are fiddly things—plus, you never know who in the house might have done some “unauthorized adjustments.”

  • Make sure the thermostat is set to “heat” instead of “cool.” (Yes, this sounds obvious, but it’s been the cause of many a service call!)
  • If your thermostat has a fan setting, set it to “auto” instead of “on.” If it’s set to “on,” the blower fan may run continuously even when the furnace isn’t heating the air up to the set temperature.
  • Open the thermostat panel and gently blow out any dust or debris. A dirty thermostat can sometimes malfunction.
  • If your thermostat runs on batteries, try replacing them. Most thermostats run off your home’s central power with battery backup, but some run exclusively on battery power. Dying batteries can cause the thermostat to send incorrect temperature information to the furnace.
  • Ensure that the thermostat is firmly attached to the wall and none of the wires are loose or damaged.
  • Take note of the air temperature near the thermostat. A thermostat reads the temperature of the air immediately around it. Is the thermostat in a spot that is noticeably warmer than the rest of your house? The thermostat will apply that temperature to the rest of your home, which can lead to cooler air than you wanted as the thermostat tries to regulate the temperature.

If all of these things check out but you still suspect a malfunction, the thermostat may need recalibration or replacement.

3. For gas furnaces: Make sure the gas supply valve is on.

Have you had any repairs done lately? Someone may have shut off the gas supply valve to do a repair and then forgot to turn it back on. Without the gas supply, there’s nothing for your furnace to burn to create the heat. Check that the gas valve handle is inline (parallel) with the gas pipe—that means it’s on. When it’s off, the handle is perpendicular to the pipe.

If this doesn’t fix the problem, and you smell gas, turn off your gas supply and call a professional right away. Gas leaks are dangerous.

4. For gas furnaces: Check the pilot ignitor, flame sensor and/or pilot light.

Problems with your pilot light can cause your gas furnace to blow cold air.

Most modern gas furnaces have an electronic pilot igniter coupled with a flame sensor rather than a physical pilot light. These can malfunction—repairing or replacing them is a job for a professional HVAC technician.

A dirty or clogged pilot ignitor can prevent the furnace from igniting properly. While working with the ignitor is a job for a pro, you can check for a dirty ignitor if you know how and feel absolutely comfortable shutting off power to your furnace, taking off the front panel, and locating the ignitor.

If you have an older gas furnace with a standing (physical) pilot light, the light can go out. This is usually the result of the gas company shutting down the gas lines for maintenance or an emergency. Once you’ve confirmed that the gas line is safely back in operation (a quick call to the gas company is a good idea), try relighting the pilot light according to the instructions for your unit. If it won’t stay lit, call a professional. There may be a problem with the thermocouple or ignition system.

5. Consider the air ducts.

Is your furnace blowing hot air in some rooms but not in others? Does it seem like the airflow is weaker than you remember? Problems with your vents or ductwork are likely to blame.

Take a look at the air vents and make sure that the dampers (the little handles on the front panel) are parallel with the ductwork. This means that that dampers are fully open and allowing for maximum airflow from your furnace.

Still getting cool air from the vents? You may have leaking ducts. Cracks, holes and gaps in your ductwork allow the hot air from your furnace to escape before it reaches the vents. Not only will you have cold spots, but leaky ductwork will run up your energy bills and pull in dust and debris that then gets circulated into your home.

Over time, ducts can actually break due to the stress of fluctuating temperatures. Insulation can break down and fall into the duct space, clogging it. If a duct has collapsed or is severely obstructed, you’ll experience a reduction in airflow. Your furnace’s heat exchanger may overheat due to the lack of air, causing the burners to shut down.

If you suspect a problem with your ductwork, contact a professional who specializes in that area.

Furnace still not blowing hot air? Call in a professional.

A furnace problem won’t fix itself. Often the initial issue is simple to fix, but can lead to complicated and costly problems if put off too long.

If you’re in the Greater Chicago area, schedule your furnace service online or give us a call at (815) 455-7000. Our licensed Chicago HVAC technicians can diagnose the problem and get it resolved fast so you’re not left in the cold.

This information is provided as a general guideline. Althoff Industries does not assume any liability resulting from the provided information.

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