What To Do If Furnace Will Not Turn On With Thermostat

A malfunctioned furnace can spell doom for your family. Apart from causing the discomfort of a cold house, it can lead to costly damages. For instance, it can cause frozen pipes, leading to costly water damages. 

Cold temperatures resulting from a broken furnace can also cause structural damage, leading to cracked walls and foundation shifting. 

Therefore, every homeowner must prep their furnace for the winter and, more importantly, be prepared to handle common malfunctions. This way, you can restore heating function if the furnace goes off in the middle of the night. 

One of the most common furnace issues that you should troubleshoot and perhaps fix is a heater that will run with the thermostat on. Why would a furnace refuse to run with the thermostat connected? 

It will be understandable if the thermostat is disconnected as most furnaces are designed to only run with the thermostat in place and functioning. Why? Because the furnace only gets “Start” and “Stop” signals from the thermostat. Without a “Start” signal from the thermostat, it wouldn’t run. Thus, if the thermostat is damaged, the unit won’t run. Indeed, most furnaces will not run even if the thermostat is in place but positioned or set incorrectly.  

But what if the thermostat is present and working. Why then would the thermostat refuse to run?

Unfortunately, there are several potential reasons. 

Reasons Your Furnace Will Not Turn on with Thermostat

1. Thermostat is not ON

The first place to check when you realize that your furnace won’t run with the thermostat in place is the thermostat itself – and the first thing to check is the thermostat switch. The thermostat is typically placed on one of the main walls in the home around, around the same height as the light switch. 

However, most people forget that the thermostat must be powered and switched on to function. If the thermostat isn’t getting power from your home’s electric supply, it cannot run. The same applies if the thermostat is plugged in but not turned on. 

Therefore, check to ensure that it’s plugged in and turned ON. If you have a programmable thermostat, verify that the display is active and the unit is set to heat. This is very important as many furnaces also have a “FAN” mode. Also, double-check manual thermostats can be easily knocked to an OFF position during standard house cleaning. 

Finally, ensure the temperature setting is one that typically causes the furnace to come on. For instance, if the furnace typically comes on at 70°F, you must set the thermostat to 70°F. Otherwise, you’ll find yourself stuck in the cold and even think your furnace has a malfunction when it hasn’t. 

2. The Gas isn’t Turned ON

It’s also possible to blame your furnace when you haven’t turned ON the gas supply. To prevent this, make sure to check that gas is turned ON. 

Gas furnaces typically have a valve or gas cock located within six feet of the unit. The valves vary in type. However, they characteristically have a little disk with a small handle or lever. Other valves also look like little boxes, while others resemble a lever and can vary in size, though they’re usually about the size of the little finger. Many of them are brightly colored for easy identification. 

Although the gas valve is often open, you may find it in an OFF position for several reasons. For instance, perhaps the technician working on it forgot to turn it back ON.

Or maybe someone in the house noticed a problem earlier and turned it off as a precaution. If your furnace has a lever-type valve, you’ll know it’s OFF if the lever is perpendicular to the gas pipe. 

If you’ve verified that the valve is open but the furnace still won’t work, check if other connected appliances, such as the stove, are working.

If the other appliances don’t work too, you may not have a gas supply to your home. Maybe you have a leak, or the supplier has issues. Call your gas supplier to find out if they’re experiencing problems and, if not, inspect the gas line for leaks. 

3. Check the Furnace’s Power Switch Position

It’s possible that the furnace isn’t turning on because it’s turned off. So, before you begin kicking your feet in rage, check the unit’s power switch position. 

The location of the switch varies from one furnace to the next. However, it’s located on a nearby wall in many cases, though some homes have it on a ceiling or floor joist near the furnace. You’ll be able to identify it by its regular light switch appearance. 

The switch may or may not be labeled. However, the ON position is typically up. If you feel confused, refer to the user manual. The reason this step is important is that someone may accidentally switch the unit off. Unless you know the ON position, you may try, in vain, to turn on the furnace. 

Once you’ve located the switch, turn it to the ON position and try again to start the furnace. It should work. If it doesn’t, then it’s likely a different issue as the switch rarely gets damaged.

Just make sure to give it enough time to come ON. Some furnaces can take several seconds to resume functioning, especially after a lengthy period “out of work.”

4. Is it an Air Filter Problem?

One of the most common issues in furnaces is gas airflow. Restricted airflow can eventually cause the furnace to stop working. So, begin by ensuring that the unit gets enough airflow all around. Remove any blinds and curtains or other obstacles that may impede the free flow of air. 

More importantly, check the filters. A clogged filter can cause restricted airflow within the furnace, resulting in increased heat levels in the heat exchanger. When this happens, the furnace will respond by activating one of the limit switches.

For instance, the flame rollout limit switch is designed to trip and cut the gas supply if overheating in the heat exchanger. This will shut down the furnace. 

The user manual will tell you how often you need to replace the furnace filter. However, the rule of thumb is to change the filter every three months, regardless of heating or airflow issues. Failure to replace it as required can cause the filter to clog with hair, dust, dirt, and other particles. 

The furnace filter is located at the point where airflow into the furnace when it’s working. Slide it out, check the model number and size, and find an appropriate replacement. 

5. Maybe it’s the Ignition Sensor (Flame Sensor)

The flame sensor is a small metal rod that produces a small current to determine the presence of a flame during the first few seconds when you turn ON the heater.

If it detects a flame, it will signal the gas valve to send gas to the burner for proper heating. However, if it cannot detect a flame, it will cut the gas supply, effectively shutting the furnace. 

The flame sensor can malfunction for different reasons. The most common reason is dirt buildup. If the sensor is too dirty, it loses its sensing capacity and may not detect a flame even if one is present.

Cleaning the flame sensor can save a few hundred dollars while helping you restore furnace function without involving a professional. However, you must be very confident in your DIY skills to attempt the process. 

Before attempting the DIY cleaning, shut off the furnace at the gas valve. Next, turn the little lever to the closed or OFF position (usually perpendicular to the gas pipe).

Then, check the area in front of the pilot light for a tiny porcelain object with a small metallic rod. Use a screwdriver to loosen it and remove the screws before sliding the flame sensor out. 

You can clean the sensor with steel wool or a clean, straight-edged knife to remove any buildup. Take caution not to detach it from the base. Once you’re done, return it and screw it into position.  

6. Check the Pilot Light

Finally, we’d also recommend that you check the pilot light to see if it’s ON. The pilot light is a small light on the furnace that allows you to relight the heater without using matches.

It lights permanently and, under normal circumstances, only goes off when there’s no gas supply. However, it does so safely. The gas supply to the pilot is very little, and the light itself is very small. 

However, sometimes you may find that the pilot light isn’t lit even with gas available. When this happens, the furnace cannot light. Common reasons why the pilot light would go off include a loose or broken gas valve, a loose thermocouple, and low gas pressure. Or it could be dirt buildup. 

With the furnace switched off, begin by poking into the holes where the pilot burns with a fine wire to knock out debris. Then turn the power back and try to relight it. If it still doesn’t work, check pressure using your multimeter. If the pressure is okay, it’s time to call a professional. 

Remember that relighting a pilot light is a risky activity. Indeed, experts say it’s way more dangerous than leaving the pilot light on the entire heating season. So, be warned. 

The gas furnace can refuse to turn on for many reasons. If it refuses to turn on with the thermostat in place, check the furnace, thermostat, and gas are ON. If so, check the filter, flame sensor, and pilot light. It’s likely to be one of these six issues.