Why Won’t My Furnace Turn On? A Portland Tech Explains Why

Nobody wants to deal with their furnace refusing to turn on when it’s cold out. But not to worry! We can help you troubleshoot your furnace issue.

In order to troubleshoot the problem, we first need to diagnose the problem with your furnace.

The four main reasons your furnace could not be turning on include:

  • Your thermostat is set to the wrong settings
  • A tripped circuit breaker
  • A dirty air filter
  • Your furnace has a dirty flame sensor

In this blog, we’ll go into further explanation about how the above reasons can cause your furnace to not turn on. Then we’ll let you know which of the reasons can be fixed by you and which need a professional’s touch.

Rather have a professional HVAC technician check out your furnace ASAP? Schedule a same-day furnace repair with Four Seasons today!

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Reason #1: Your thermostat is set to the wrong settings

If your furnace is not turning on, your thermostat may not be adjusted to the right settings. When your thermostat is not on the right settings, your thermostat won’t tell your furnace to turn on.

To check if your thermostat is working properly, follow the below steps:

  • Make sure your thermostat has power. If the thermostat’s display is off or unresponsive, you will want to switch out their batteries. You can switch out your thermostat batteries with a small, flat screwdriver and either AA or AAA batteries. You’ll have to remove the thermostat’s cover and then pry out the old batteries with the screwdriver.
  • Your thermostat is set to HEAT and ON. When your thermostat is set to COOL, your furnace won’t turn on even if the set temperature on the thermostat is higher than the current temperature in your home.

Once you replace the batteries and fix the thermostat’s settings, check if the furnace is turning on again. If not, you’ll want to contact an HVAC technician to make sure you don’t have a deeper wiring issue with your thermostat.

Reason #2: A tripped circuit breaker

Another thing you’ll want to check if your furnace isn’t turning on is your electrical panel. Your electrical panel can tell you if your furnace’s circuit breaker has tripped. When the furnace’s circuit breaker has tripped, your furnace has its power cut off and won’t be able to turn on until the breaker has been reset.

Examples of non-tripped circuit breakers

The circuit breaker has tripped if the switch labeled “furnace” is in a different position from the rest of the switches (e.g. typically the middle position).

You can reset the breaker by:

  1. Flipping the furnace’s switch OFF (turning it to the right)
  2. Flipping the furnace’s switch ON (turning it to the left)
  3. Waiting 30 seconds to see if the breaker trips again

If your circuit breaker trips again, do NOT reset it. You’ll want to immediately schedule a furnace repair appointment with an HVAC technician. You may have electrical problems that need to be professionally resolved.

Reason #3: A dirty air filter

Whether your air filter is clean can have a big impact on your furnace working. If your air filter is dirty, the clogged filter can obstruct the flow of air entering your furnace.

Not enough air entering your furnace is a big problem because of how furnaces work. Furnaces pull in cool air to be heated by a part called the heat exchanger. Once the heat exchanger has done its job, the heated air is then circulated throughout your home via ductwork. This process repeats itself until your indoor air reaches your desired temperature.

That said, if the clogged air filter is blocking the amount of air entering your furnace, the heat exchanger has to work harder and longer to heat your home. Unfortunately, the heat exchanger can overheat, causing your furnace to shut down to prevent the heat exchanger from cracking.

To check if a dirty air filter is the reason for your furnace refusing to turn on, follow the below steps:

  1. Turn your thermostat off to prevent your furnace from turning on while replacing your filter.
  2. Remove your current air filter. If it looks like the image below, it’s dirty and needs to be replaced. If it’s not dirty, try the other solutions in this blog.
  3. Measure your current air filter so you can buy a replacement. Your existing filter should have its dimensions listed on its frame. Most home improvement stores sell furnace filters.
  4. Replace the air filter by inserting the new air filter into the slots. Make sure you’re inserting the new air filter by following the arrows on the filter frame. If the air filter is inserted incorrectly, the airflow direction will be wrong.
  5. Turn your thermostat back on to check if your furnace is turning on again. If yes, you can prevent your furnace from shutting down in the future by regularly changing out your air filter. The general rule of thumb is to replace your air filter every three months or whenever it’s dirty.
Clean air filter versus clogged air filter

Clean vs Dirty air filter

If you’ve changed out your air filter but your furnace is still not turning on, contact an HVAC technician to diagnose and fix your furnace.

Reason #4: Your furnace has a dirty flame sensor

If you have a standing pilot furnace (i.e. your furnace has a knob on its gas valve labeled with OFF/ON/PILOT), a dirty flame sensor is a common reason for your furnace not turning on. You can tell if a dirty flame sensor is most likely your problem if you hear your gas furnace click on but doesn’t actually turn on.

The flame sensor is a safety measure. Whenever the thermostat tells the furnace to turn on and the gas valve to open, the flame sensor checks that there’s actually a flame to ignite the furnace with and that the furnace isn’t just circulating dangerous gases throughout your home. This is important because the whole point of the gas valve opening and firing gas into the combustion chamber is so that the flame can ignite the gas, producing heat for your home. Without the flame, gas is building up without being converted into heat. So, if the flame sensor senses there’s no flame, it shuts down your furnace.

Typically, the flame sensor becomes dirty because it spends so much time in flame that carbon buildup happens. And because the flame sensor is super sensitive in order to check for a flame immediately, any carbon buildup on the sensor can skew its readings, shutting down your furnace.

If you suspect that a dirty flame sensor is your issue, you’ll want to contact an HVAC professional to clean it.

Want an easy furnace repair in Portland? Contact Four Seasons

We make the furnace repair process convenient. When you contact us for a furnace repair, we offer 2-hour scheduling windows so you can book the appointment the same day. And our technicians can repair any make or model of furnace so you know we’ll get your heat back up and running in no time.

Schedule service today!