Why Does My Furnace Turn on But Not Heat?

Why does my furnace turn on, but then not heat? There could be a variety of possible problems. Most furnaces will default to just running the blower if it senses a malfunction. Furnaces have an array of sensors that will detect overheating and automatically shut off the heating element if it senses a possible safety problem. This is why you will experience your furnace “running” but not heating.

This is one of the most common questions we hear. Although it is a common problem, there could be many possible reasons why your furnace would turn on, but no heat. Ramon Blachly, an industry expert for more than 35 years, discusses in the video below how and why this may be happening to you.

Gas pressure: a possible, but very rare problem

On extremely bitter cold days, during a time of day where most people would be home, gas pressure could suffer. Lower gas pressure could cause your furnace to lock itself out and stop heating. If you turn everything off, then ask your thermostat to call for heat again and it comes on, this may have been the problem.

Check your filter

High-efficiency air filters do a great job of collecting dirt that would otherwise be sent into your furnace and out into your home’s air. However, if they get too dirty, they could cause a malfunction with your furnace. If your furnace stops working, it may be due to overheating caused by your dirty furnace filter. If you find that your filter is packed with dirt, that may have been the problem. We recommend you change your filter every month if you use a disposable filter.

It is probably not the pilot light

While a pilot light going out used to be a very common reason for your furnace to run, but no heat, it is not a typical problem anymore. If you have a furnace that was installed in the last 15 to 20 years, chances are your furnace does not ignite with the use of a pilot light at all. Instead most furnaces today use electronic starters that eliminate the need for a standing pilot light.

It is possible your thermostat got reset

We’re not going to point any fingers, but it is possible your thermostat got reset without you being aware that it happened. If you wake up and it is cold in your house, it could be that your thermostat was accidentally put in a mode where it is not calling for heat when it usually does. Try resetting your thermostat and see if that was the problem.

Listen for the sequence of operation

If you suspect your furnace is not operating properly, you can listen to the following sequence of operation to determine which component of your furnace is malfunctioning. Note: This is how most furnaces function. Yours could be different.

1. Your thermostat will call for heat

2. Your inducer fan should come on. The inducer fan is tied to your venting components. If the inducer fan doesn’t come on there could be a variety of venting issues. Maybe there is a bird nest somewhere or an ice block somewhere…it would take some detective work to figure this out.

3. After about 20-30 seconds your ignitor will light. You’ll see a glow like a lightbulb.

4. Then the gas will come on. You’ll see burners light and then the ignitor will go out. The furnace will warm up for about 30 seconds.

5. Then the main blower will come on.

6. At the end of the heating cycle, the opposite will happen.

If you listen for all of the above you can loosely diagnose what the problem is. Does the inducer fan come on, but the ignitor doesn’t light? Bingo we’ve narrowed it down.

Is this a serious problem?

Sometimes a freak problem will happen and your furnace will function properly after resetting the thermostat. However, most often, if your furnace malfunctions once, it will probably do it again. Furnaces are made to detect safety issues before they become dangerous and shut off the furnace. It may be safer and more convenient to have a professional come out and fix the problem before it happens again.

How to help your furnace help you in the winter months

Your furnace is already working extremely hard to maintain a comfortable temperature while the temperatures outside feel arctic. There are a few things you can do to be more friendly to your furnace while it is working overtime.

  1. Don’t let your house cool off while you are not there or at night.
    Many of us economically-minded folks try to have our furnace run as little as possible so our utility bills will be lower. Oftentimes while we are not at home we will let our house get colder and then heat it up more when we get home. Ordinarily, that is not a bad idea. However, when it is frigid outside, it is best to just leave your thermostat at a comfortable temperature even when you are not there. Your furnace will struggle to make up the difference in temperature and may it be impossible to recover the temperature in the amount of time you expect.
  2. Open cabinet doors to let the heat in.
    This will warm up your plumbing and make your house a more comfortable and consistent temperature.
  3. Keep your furnace clean and service
    It is remarkable how few of our customers will have a breakdown after they have their furnace maintained. The best way to ensure your furnace will be running smoothly is to clean and check all those components in your furnace that could cause it to malfunction. While you can certainly change your filter we recommend leaving the cleaning of the inside components of the furnace to a trained professional.