Why Furnace Keeps Running and No Heat?

A furnace is a vital part of any home, and when it doesn’t work as it should, it can be incredibly frustrating. If your furnace is running but there’s no heat, what could be causing this? 

Your furnace can run without producing heat because of a thermostat error, airflow issues or clogged condensate lines. You can easily troubleshoot and fix these problems by resetting the thermostat, unclogging the condensate lines, or correcting airflow errors.

In this article, I’ll discuss why your furnace might be running but not producing heat. I’ll also 

provide you with possible solutions to these issues. Let’s dive right in!

1. The Furnace Has Thermostat Issues

Thermostat issues can be a reason your furnace runs but doesn’t heat your home. The issues range from minor ones you can easily fix to those you need to consult a technician.

The most common thermostat issues stem from:

Settings Changes

If you share your house with someone else, they may have adjusted the thermostat settings without your knowledge. For instance, they may have accidentally turned on the thermostat’s fan. This would cause the fan to blow cold air into the room.

Furnaces don’t heat a house continuously but take breaks in between. When you adjust your thermostat, it will send warm air through the ducts. Once the air is at your desired temperature, the furnace will stop heating it but will kick in again when the temperature drops. 

If you’ve accidentally switched the fan on, it will blow cold air when the furnace is dormant and not actively heating the air. This could be why you feel cold in the room even when the furnace is on.

Read: High-Efficiency Furnace Venting Options

Low Thermostat Battery 

Low batteries can cause the thermostat to lose its connection to the heating and cooling system, resulting in inaccurate temperature readings. If your thermostat’s battery is low, it could cause your furnace to stop heating the room even when it’s on. 

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You may experience this problem if your thermostat runs on a self-contained battery unit. The battery could be completely dead or running very low, thereby impeding the furnace’s thermostat function.

Read: Cracked Heat Exchanger In Furnace – How To Troubleshoot?

Thermostat Is Incompatible With Your Heating Unit

A thermostat issue could also be because your furnace isn’t compatible with it. This problem can happen when you DIY a new thermostat installation or install an incompatible model. This will cause your furnace to run but not produce heat, resulting in a cold house.

How To Fix?

You can solve thermostat issues by doing the following:

  • Ensure the fan settings are correct. Set the fan to run on “Auto.” This setting ensures the fan runs when the furnace is active. This will ensure that the furnace won’t blow cold air.
  • Replace the thermostat batteries. Thermostat batteries are built to last a long time. So, if you’ve had them for a long time, they may stop working, and you’ll need to replace them. However, if the thermostat is fairly new, it’s possible that the batteries aren’t the problem.
  • Turn up the thermostat. This is an easy fix and involves turning up the temperature on your thermostat. Doing this may trigger the furnace to start a heat cycle and should correct the furnace’s problem if the settings were the issue. 
  • Consult a thermostat technician. If none of the above fixes worked, you may need to call an HVAC technician. They will troubleshoot and fix the problem as necessary.

Read: Why Furnace Blowing Cold Air? – Troubleshooting Guide

2. HVAC Airflow Problems

Another reason your furnace keeps running but doesn’t produce heat is an HVAC airflow problem. The problem could be the furnace itself or ductwork with airflow challenges.

A common cause of HVAC airflow issues is dirty air filters. Dirty filters limit the hot air that blows into the room, and the living areas could be chilly even while the furnace runs.

How To Fix?

You can solve your HVAC airflow issues by doing the following:

  • Check the vents. Examine the vents for any signs of clogged dirt and dust. Ensure that the louvers aren’t closed, and if they are, remove the vent cover and clean them. You can call a professional to replace the vent cover if the louvers are jammed.
  • Check the air filter. Make sure that the air filter isn’t clogged with debris. The best way to keep HVAC filters in good condition is to replace them when dirty. This will encourage rich air circulation in the filters that disable your furnace’s heating capabilities.
  • Inspect the duct system. Check the ductwork for any leaks where hot air from the furnace escapes. Ask an HVAC professional to seal duct gaps, if any. Doing so will prevent cool air from flowing through the vents due to hot air escaping via damaged ducts.
  • Remove obstructive objects from the HVAC registers and vents. Remove large objects like furniture or appliances that block the HVAC vents and registers. Also, check if something has fallen into the vent and remove it. This will help the furnace’s hot air to circulate more effectively.

Read: How To Troubleshoot And Reset Furnace?

3. The Condensate Lines Are Clogged

Clogged condensate lines could also make your furnace run without heating the room. The condensate lines require routine maintenance because they can easily accumulate dirt and grime. The more the HVAC unit runs, the more the lines accumulate dirt.

The condensate lines can become clogged over time when you don’t clean them, resulting in the float switch tripping, eventually leading to the heating system’s shutdown. 

How To Fix?

Follow these steps to unclog the condensate line:

  1. Turn off your heating unit. Leaving the furnace on exposes you to electric shock and can cause serious electric damage to the unit. You should turn it off before working on the unit.
  2. Look for the drain pipe. This is a PVC pipe with a plastic top. You can find it in the same closet as the furnace if you live in an apartment. If you live in a single-family unit, you can find the drain pipe outside near the condenser unit.
  3. Remove the drain pipe’s plastic top. Removing the cap with your hands is easy, so you don’t need any tools for this step.
  4. Remove debris from the pipe. Manually remove any dirt that you see in the drain pipe. Avoid pushing it further into the pipe because removing it will be more difficult. You can use a wet or dry vacuum to remove the dirt if you haven’t unclogged the pipe in a long time.
  5. Pour vinegar into the line. Once you have removed the debris, pour distilled white vinegar into the pipe to help clear the grime. Allow it to dry. 
  6. Replace the plastic cap. Return the cap and wait for about 30 minutes before you turn on the heating unit. It should run perfectly after this step.


When the furnace runs but doesn’t heat the room, it could indicate:

  • An underlying issue with the thermostat
  • Airflow challenges in the heating unit 
  • Blocked condensate lines

As you’ve seen above, various reasons can cause thermostat, airflow, and condensate issues. That’s why it’s critical to understand why they happen and what you can do to solve them. If you cannot solve the problem, you can ask an HVAC technician to troubleshoot and fix the problematic heating unit.