Waking up or coming home to a cold house when the furnace should be running is a stressful moment for any homeowner. Your first thought might be of an expensive, complicated repair, but that’s often not the case. Many times, a furnace that isn't producing enough heat—or any heat at all—is facing a simple issue that you can safely diagnose and fix yourself in under 30 minutes.
Before you schedule a service call, working through a few basic checks can often solve the problem and save you both time and money. This guide will walk you through the most common culprits, starting with the easiest fixes, so you can feel confident about what to do next.
Image Alt: A homeowner adjusting the temperature on a smart thermostat.
What Causes a Furnace to Stop Heating?
When your furnace is running but not heating, the issue usually falls into one of three categories: a problem with the thermostat’s signal, a problem with airflow, or a problem with the fuel or power source.
Here are the most common reasons your furnace isn’t working correctly:
- Incorrect thermostat settings: The thermostat might be on the wrong mode, have dead batteries, or simply not be set high enough to call for heat.
- A clogged air filter: A dirty filter can severely restrict airflow, preventing warm air from circulating through your home and sometimes causing the furnace to overheat and shut down as a safety measure.
- Tripped circuit breaker or closed gas valve: Like any major appliance, your furnace needs power. If a breaker is tripped or a gas valve is off, it simply can’t run.
- Blocked air vents: Closed or blocked registers in rooms can make it feel like the furnace isn't working, when really the warm air just can't get out.
Let's work through these possibilities step by step.
A Step-by-Step Guide to Troubleshooting Your Furnace
Follow these steps in order to diagnose why your furnace isn't producing heat.
Step 1: Check Your Thermostat Settings
The thermostat is the brain of your heating system. A wrong setting here is a very common and simple reason for a furnace to misbehave.
- Confirm the Mode: Make sure the thermostat is set to "Heat." It's easy to forget it was switched to "Cool" or "Off" during milder weather.
- Raise the Temperature: Set the temperature at least 5 degrees higher than the current room temperature. This ensures the thermostat is actively "calling for heat" from the furnace.
- Check the Batteries: If your thermostat has a blank screen or a low-battery indicator, replace the batteries. Many digital thermostats use AA or AAA batteries to function.
- Listen for a Click: After you adjust the settings, you should hear a soft click from the thermostat, and then soon after, you should hear the furnace kick on.
If adjusting the thermostat settings and changing the batteries solves the problem, you're all set. If the furnace still isn’t producing heat, it's time to check for an airflow issue.
Step 2: Inspect the Furnace Air Filter
Your furnace filter is designed to trap dust, pet dander, and other particles, but when it gets too full, it chokes off the airflow your system needs to work. This is the single most common cause of furnace problems.
When to Try This Yourself: This is a safe and routine maintenance task that every homeowner should do regularly.
- Turn Off the Furnace: For safety, always turn the power off before opening the furnace. There should be a switch that looks like a standard light switch on or near the furnace unit itself.
- Locate and Remove the Filter: The filter is usually located in a slot between the main furnace unit and the return air duct. It may be behind a small metal door. Slide the old filter out.
- Check If It's Dirty: Hold the filter up to a light. If you can't see light through it, it's definitely time for a replacement.
- Insert the New Filter: Note the arrow on the side of the new filter frame—this indicates the direction of airflow. The arrow should always point toward the furnace blower motor. Slide the new filter into the slot.
- Turn the Power Back On: Replace the access panel and turn the furnace switch back on.
A clean filter can make a dramatic difference in performance. Keeping track of when to change your filter (typically every 1-3 months) is a key part of home maintenance that an app like Casa can help you remember. If the furnace now heats properly, you’ve found your culprit.
Image Alt: A homeowner replacing a dirty furnace air filter with a clean one.
Step 3: Verify the Fuel and Power Supply
If the thermostat is working and the filter is clean, the next step is to ensure the furnace is receiving the fuel and electricity it needs to generate heat.
When to Try This Yourself: These are simple on/off checks. Do not attempt to service gas lines or electrical wiring yourself.
For Gas Furnaces:
- Locate the gas supply line that runs to your furnace. On that pipe, there will be a shutoff valve with a small handle.
- The handle should be parallel to the pipe, which means the valve is open and gas is flowing. If the handle is perpendicular to the pipe, the valve is closed. Gently turn it to the open position if it is closed.
- Safety Note: If you smell gas at any point, do not touch any switches. Leave your home immediately and call your gas utility's emergency number from a safe distance.
For Electric Furnaces (or Gas Furnaces):
- Find your home's main electrical panel (breaker box).
- Look for the circuit breaker labeled "Furnace," "HVAC," or "Air Handler."
- Sometimes a breaker can trip without looking fully off. Firmly flip the breaker all the way to the "Off" position, then back to "On." This will reset it.
After confirming the fuel and power are on, wait a few minutes to see if the furnace starts its heating cycle.
When to Stop and Call a Professional
If you've worked through all the steps above and your home still isn't warming up, it is time to call a qualified HVAC technician. The problem is likely more complex, involving components like the ignitor, flame sensor, or blower motor, which require professional tools and expertise to diagnose and repair safely.
Call a professional if:
- You’ve tried all the steps above without success.
- The furnace makes loud banging, scraping, or squealing noises.
- You smell burning plastic or metal near the vents or furnace.
- The furnace turns on and off very frequently without heating the house (this is called "short cycling").
How Much Does a Furnace Fix Cost?
- DIY Fixes: If your issue is a dirty filter or dead thermostat batteries, your only cost is for supplies. A new furnace filter typically costs between $10 and $30, depending on the size and type.
- Professional Service: According to industry sources, having an HVAC professional come to your home for a diagnostic visit usually costs between $100 and $300. The final repair cost will depend entirely on what part has failed and the labor required to fix it. Getting your system working might be a simple adjustment, or it could require a more expensive part.
You’re Ready to Get Warm Again
Dealing with a furnace that won't heat can be unsettling, but it’s a manageable problem. By checking the thermostat, air filter, and power supply first, you can often resolve the issue yourself and avoid an unnecessary service call. And if you do need a professional, you can call them with the confidence that you’ve already ruled out the simple stuff.
Being prepared for common home issues is a big part of feeling secure in your home. For help with maintenance reminders, repair guidance, and connecting with trusted pros, download the Casa app. We’re here to help you keep your home running smoothly, year-round.
