If a furnace isn’t turning on or shuts down immediately, this indicates some serious operational issues. Start with furnace troubleshooting basics, like checking to make sure your thermostat has the right settings. If nothing obvious appears wrong, it’s time to take a closer look at the system itself to find why the furnace is not igniting. Here’s what could be going on.
The pilot light has gone out.
The pilot light is a very important device on gas furnaces and fireplaces that acts as an automatic starter for the gas. A small amount of gas is channeled to the pilot light, which burns a constant flame. This way, when gas enters the primary burner, it is automatically ignited by the pilot light.
Pilot lights were once very common in all home gas furnaces. They were cheap, easy to use and helped prevent gas build-up. Today, new gas furnaces tend to use smart ignition systems that spark the gas from the main burner – a bit like an automatic gas stovetop.
Sometimes pilot lights are turned off for the summer to save money. Sometimes they accidentally go off on their own due to mechanical issues. Either way, that means the furnace won’t ignite no matter the settings. Find where the pilot light is in your furnace, and check to see if it’s burning. If it is not on, consult your owner’s manual to see where the pilot light ignition is and how to use it. If it still won’t light up, call in an expert to see if the pilot light needs to be cleaned or adjusted. It may also be time to think about a full furnace replacement.
The furnace limit switch isn’t working.
If the pilot light is still wrong, then there’s a more complicated reason the furnace won’t ignite. Start by taking a look at the limit switch. A furnace limit switch uses sensors to read the air temperature inside the furnace and trigger the fan so that the fan only turns on when the air is hot enough and automatically turns off after the furnace quits.
If a limit switch stops working, it may never send a signal for the fan to stop working: If the furnace is not blowing hot air but the fan is still pumping cool air into the house, this may be the issue. Other times, the limit switch may stop the furnace from heating at all.
This can be hard to diagnose. Fortunately, many thermostats and LED indicators on furnaces include error messages when the furnace won’t turn on. If one of these error codes is connected to the limit switch, you’ve spotted your problem. Otherwise, one of the key signs that this is the source of your issue is a fan that constantly runs whenever the furnace is on. Call a furnace repair service like Gainesville Mechanical to replace this component.
The flame sensor is broken.
The flame sensor is an important safety mechanism to prevent fire hazards. It detects if the primary gas burner is, well, burning! If the gas line is open but the fire isn’t ignited, the flame sensor will immediately shut down the furnace entirely. This prevents gas from building up in the heat box before it is ignited, which can cause disastrous explosions.
If the furnace is not turning on at all or turns off right away when it tries to turn on, the flame sensor could be the cause. The sensor is located close to the main burner: If you can find it, you can inspect it to see if it is broken, misaligned or too dirty to work properly.
Basic cleaning and adjustments of the flame sensor can be DIY, but when in doubt always call a local professional to perform any required maintenance or replacements.
The furnace is clogged and overheating.
When asking, “Why won’t my furnace turn on?” always remember to look at your filters and vents. A furnace needs proper airflow to move heated air via the heat exchange and stay cool enough to operate. If air can’t reach inside the furnace, heat will build up too quickly and the furnace sensors will shut everything down to avoid damage or fire hazards. A common symptom of this is short cycles where the furnace only runs for a few minutes before shutting off again.
There are several potential causes for this kind of overheating:
- The HVAC filter is old and clogged: Most home furnaces share a filter with the entire forced-air system. This filter is located close to the fan and helps trap debris in the air before it can damage the fan system. These filters are generally designed to be replaced (or washed) every few months, otherwise, they can grow clogged. This reduces airflow to the furnace, and a neglected filter can eventually make it hard for the heat exchanger to get rid of heat fast enough. Replacing these filters is inexpensive and easy, and you can always arrange professional maintenance visits to take care of it when you are busy!
- The furnace flue is clogged: Most furnaces have flues or exhaust pipes that channel extra fumes outside while they work. These flues can become clogged with dirt and debris, especially if there are nearby trees or bushes. This in turn can cause problems with overheating. Always keep exhaust vents clear and check them if your furnace starts short cycling.
Supply vents are closed in the house: Air supply vents in each room can be closed or opened as needed. In colder months, it’s a common practice to close supply vents in unused rooms so all the heat is focused in areas where people actually are congregating. That’s smart, but there’s a risk: If too many supply vents are closed, the furnace won’t have anything to do with the heat it’s creating and will overheat before shutting down. Only shut supply vents for limited spaces around the house.