Why Lennox Furnace Is Not Staying ON

There are quite a few things that can make your unit short cycle.

A Lennox furnace won’t stay on, in case there is low airflow, the flame sensor is dirty, and the draft inducer motor, pressure switch, control board, and thermostat are faulty. An oversized furnace can also end up turning on and off way too often.

Why Does My Furnace Keep Shutting Off After a Few Minutes?

‘Short cycling’ is a special HVAC term that is used to describe a situation when an air conditioner or a furnace turns on and off too frequently.

The biggest issue with short cycling is that it makes the system work much harder than it actually should which decreases the lifespan of your furnace and its individual components. 

Why Lennox furnace not staying on? There might be a few reasons for that.

Low Airflow

Without sufficient airflow, the unit’s heat exchanger might easily overheat. This, in its turn, is going to trip the high-limit switch which will shut the furnace down.

It’s an important safety measure as if the unit doesn’t turn off, the furnace is going to overheat and break down (in the worst case – it might cause a fire).

Here are the most common causes of a low airflow:

  • A clogged air filter
  • Closed air supply vents
  • A dirty blower wheel

Read: Why Gas Furnace Is Overheating?

Dirty or Faulty Flame Sensor

A flame sensor’s main job is to send a current of electricity as soon as it ‘senses’ that the gas got ignited.

If the flame sensor is dirty or faulty, it won’t be able to make a correct reading. In such a case, the element is not going to create a current and the system will shut down due to safety issues.

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Read: Furnace Flame Sensor – Troubleshooting Guide

Faulty Draft Inducer Motor

This motor pulls in the air into the burner and then – out the flue. 

If it brings in enough air, the pressure switch is going to close. This activates the ignition process.

However, a defective motor won’t be able to create the right circumstances for the system to operate.

Faulty Pressure Switch

The draft inducer motor might have brought in enough air, but if that still didn’t make the pressure switch close, then it looks like the component had gone bad.

Defective Control Board

A control board is an element that powers your furnace. If the board is defective, the ignition system is not going to get any power supply.

Faulty Thermostat

The thermostat is the device that controls your heating system. It will tell the unit when to turn on and off.

However, if the readings of the thermostat are inaccurate or if it had gone bad, the furnace might end up turning off too quickly.

Read: Why Does Home Thermostat Say HEAT ON But There Is No Heat?

Oversized Furnace

Finally, your Lennox furnace might simply not be properly sized for your house.

A furnace that is way big for the space is going to heat up the house too quickly which will cause it to turn off more often.

Of course, homeowners can try calculating the size of the unit that they need on their own. But the most reliable way to determine the perfect match for your house is to invite a local Lennox dealer.

The expert is going to take into account not only the square footage of your home, but also the state of the doors and windows, the house’s insulation value, and a lot of other factors.

What Do You Do When Your Furnace Won’t Stay On?

There are a few simple tricks that you can try out for yourself before giving an HVAC expert a call.

  • Change the air filter

A dirty or clogged air filter will obstruct the airflow. As a result, the heat exchanger can get too hot and this is what will make the system shut down.

Thankfully, all you would have to do to get rid of the problem is change the filter.

Replace this component at least every 3 months, but make sure to check whether the filter is dirty or not every month or so.

Read: Why Does My Furnace Filter Get Dirty So Quickly

  • Keep the vents open

Closed vents are another thing that can restrict airflow. It might seem logical to close the vents in the rooms that you don’t use, but the truth is that such a measure is going to create pressure in the system and force it to work harder.

This can easily lead to higher energy bills and even a breakdown, so try to keep the air vents open.

  • Relocate the thermostat

There might be nothing wrong with your thermostat, but it would still be causing the furnace to shut down too soon if the device is placed in the wrong location.

For example, you shouldn’t put the thermostat in direct sunlight or close to the heat register. In such a case, the device won’t be able to measure the house’s average temperature.

Hint: don’t forget to regularly change the batteries in your thermostat. This is yet another thing that can sometimes cause short cycling. 

  • Check the flue pipe

The flue pipe, or the exhaust vent, is usually located on the roof. 

It can get blocked by leaves, dirt, snow, birds’ nests, and sticks. Such an obstruction will make the furnace short cycle, so check the flue pipe and get rid of the blockage, if there is such a need. 

  • Clean the flame sensor

The flame sensor can become corroded or covered in soot. In such a case, the element is going to close the gas valve and shut the furnace down.

A dirty sensor will not be able to sense the flame, but the great news is that you can attempt to clean the component yourself.

Lennox Flame Sensor Cleaning

Lennox gas furnaces have a flame sensor that is located on the left side of the burner assembly and looks like a thin metal rod.

It will take you only a few minutes to clean the sensor, but don’t do that if you don’t feel confident performing such tasks.

  1. Turn your Lennox furnace and the gas off.
  2. Lift the access panel (in some models, you would have to remove two screws). The panel is located right on the front of the unit.
  3. You’ll find the flame sensor on the left side.
  4. Remove the screw that is holding the element.
  5. Use steel wool to carefully get rid of any soot that is covering the flame sensor.
  6. Wipe the element with a dry tissue.
  7. Reattach the flame sensor and put the access panel back in place. Turn the furnace on.

Read: Lennox Furnace Error Codes – Troubleshooting Guide

Lennox Flame Sensor Replacement

Ideally, you would want to replace the sensor every 2-3 years. This will help ensure that the element keeps working properly.

How can you find out, if the sensor needs to be replaced?

Simply take the flame sensor out and inspect it. If you notice soot, then all you would have to do is clean the component.

However, if there is a crack or a missing part, then you would have to replace the sensor

You can change the sensor yourself if you feel comfortable doing it. A new part will usually cost you up to $40, and you can easily find the right sensor in a local furnace supply store.

Read: Why Lennox Furnace Not Igniting?

How Do You Reset a Lennox Furnace?

If the furnace trips the circuit breaker (this happens when the system overheats), you might have to manually reset your Lennox furnace.

  1. Switch off the power to the furnace.
  2. Make sure that the circuit breaker is in the ‘off’ position, if not – flip it.
  3. Lift the control access panel and set it aside. You’ll find it either on the bottom or the top panel of your furnace.
  4. To the right of the ignition control box, you’ll find the limit control switch. Press and release it to reset the unit.
  5. Put the control access panel back in place and turn the furnace on.

Read:HVAC UV Light: Everything You Need To Know