Furnace Not Blowing Hot Air

You’re not the only one asking “Why is my furnace not blowing hot air?” Every winter, households across the country discover the furnace is blowing but no heat is coming out of the registers. 

If it’s extremely cold outside, you may be asking “Why is my furnace blowing cold air when the heat is on?” You’re in the right place to find answers to why your furnace is not heating, and tips on how to get your home warm and toasty again. 

When the furnace is blowing warm air not hot, it’s time to investigate. Explore each possible cause of your furnace not working and make adjustments yourself, or call an HVAC professional for help, based on the problem.

Your thermostat is the brain of your HVAC system. It signals the furnace when to heat and how warm to make your house. It can be an easy fix if your thermostat is to blame for your furnace not blowing warm air. Common thermostat issues and solutions include:

The problem: The thermostat is set to Cool instead of Heat 

The fix: Whoopsie. Easy fix. Set the thermostat to heat.

The problem: The fan is set to On instead of Auto. If you’re wondering why “my furnace is blowing cold air” here’s your answer. The fan will blow cold air when the furnace isn’t running. 

The fix: Switch to Auto and put the furnace back in control of the fan.

The problem: The temperature is not set correctly.

The fix: Check the thermostat and adjust the temperature upwards to a comfortable level.

The problem: The thermostat has dying or dead batteries,

The fix: Replace the batteries or consider upgrading to a wired smart thermostat.

The problem: You did a bad DIY thermostat installation. It happens. If the furnace problem is happening after a recent thermostat installation, you may have chosen a thermostat that isn’t compatible with your furnace, or you missed a step. 

The fix: You may want to call an HVAC professional to straighten things out.

The problem: A dirty furnace filter restricts airflow to your furnace, which can restrict the circulation of warm air in your home. A dirty air filter can also cause the furnace to overheat and shut down as a safety precaution.

The fix: Other than thermostat issues, this is the easiest problem to fix. Turn off the furnace, then replace the dirty and clogged furnace filter with a new one. Turn the furnace back on and see if this resolves the problem. Get in the habit of changing the filter every 30-90 days, depending on the quality of the filter. Consider ordering air filters online for convenience.

The problem: No fuel is making it to the ignitor.

The fix: If you have a propane or oil furnace, ensure you have fuel in the tank. If you have a natural gas furnace, make sure the furnace gas valve is open. You’d be surprised. Life happens and you forget to order fuel. Or the gas valve gets bumped. It’s worth checking.

The problem: High-limit switch failure. Your furnace has a safety feature called a high-limit switch. One of its jobs is to sense if the furnace is overheating and shut things down. A dirty filter can cause repeated overheating, and repeated overheating can cause the high-limit switch to fail altogether. So your furnace does not burn fuel and instead continually blows cool air.

The fix: Get professional HVAC help. Call an HVAC technician to troubleshoot and replace the switch. And get in the habit of changing your filter to prevent problems in the future.

The problem: A clogged condensate drain line. High-efficiency furnaces, or condensing furnaces, produce condensation during the heating process. Without regular maintenance, the drain line or drain pan can become clogged with dirt, debris, cobwebs, etc. A clog in the condensate drain line triggers a safety measure that prevents the burners from igniting, sending cool air blowing through the registers.

The fix: Get an HVAC technician to clear out the drain line and drain pan, and check the condensate pump, if you have one. 

The problem: Furnace not igniting. The furnace pilot light keeps going out or the furnace electronic ignitor isn’t working. 

The fix: If you have an older furnace with a pilot light that has gone out, you will need to reset the furnace and relight the pilot light. Follow the instructions in your owner’s manual or instructions you find for your heating system online. If the problem persists, call for professional help.

Newer furnaces have an electronic ignition system that will have to be evaluated and fixed by an HVAC professional.

The problem: Blocked or clogged burners. Over time, dirt and grime can accumulate on the furnace burners, preventing fuel from getting through. Without no fuel to burn, the furnace can't produce heat. Since a blocked burner doesn't affect the blower fan, the system will push cool air out through the registers and into your home.

The fix: Clean the burners. A handy homeowner may be able to do this themselves, but we would recommend calling an HVAC professional. If there is that amount of grime on the burner, your furnace system is likely overdue for full maintenance.

The problem: Dirty or failing flame sensor. The combustion process creates grime that can collect on the flame sensor and keep it from detecting the flame and initiating the ignition process. The blower fan may keep going, sending cool air throughout your home.

The fix: Clean the flame sensors. If the flame sensor is failing, you’ll need an HVAC professional to replace it. 

The problem: Leaky ductwork. If you have leaky air ducts, your HVAC system will pull cold air into the ducts in the winter and leak warm air. In the summer, it will pull hot air into the ducts and leak cool air. This creates sub-optimal conditions that may be especially noticeable in one room that stays cold all winter long or hot in the summer. 

The fix: This issue is bigger than just your furnace (or your AC unit). You will need to have an HVAC company assess your ductwork for leaks and determine if duct sealing will solve the problem or if you need to replace some or all of your ductwork system to achieve the best efficiency.

Take care of your furnace and it will keep you warm and cozy. Practice regular furnace maintenance to keep your furnace running in peak condition. Change the furnace filters regularly - every 30-90 days depending on the quality of the filter and if you have pets. 

Remember, if your furnace is under warranty and you attempt a DIY fix (we know all about those DIY videos on YouTube), you run the risk of voiding your warranty. Most warranties specify that you should have routine maintenance and needed repairs done by an HVAC professional.

Schedule a furnace tuneup each fall. During the service visit, the HVAC technician should:

• Clean and lubricate the blower motor and other furnace components
• Inspect and tighten electrical connections
• Test and calibrate the thermostat
• Test the furnace operation
• Clean the condensate line if applicable
• Make minor repairs or replace parts

Many HVAC companies offer yearly HVAC maintenance plans that include fall and spring tune-ups for your heating and cooling systems. You might also be placed on a priority list for HVAC repair.

Help ensure your furnace is always heating your home when you need it. Opt into Trane Diagnostics for peace of mind. You can give your dealer remote monitoring capabilities through the Trane Home App. Your Trane smart thermostat sends data in real-time to your dealer, alerting them to any issues, even when you’re on vacation. 

They can run furnace troubleshooting tests to identify the issue(s). In some cases, they can solve the problem remotely. In this situation, you would grant them one-time dealer remote access (DRA). 

If they need to schedule an on-site service call, the technician will arrive with the knowledge gained from remote access. This enables shorter service calls and ensures the technician has the proper replacement parts.

Anne Fonda, Content Writer at Trane Technologies

A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.

She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism. When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.

Expert review by John Kim, Senior Product Manager, Furnaces

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