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Why Does Your Heater Keep Turning On and Off?

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As colder weather arrives in Texas, people will turn on their furnaces to keep their home’s heater to a comfortable temperature. So it is natural to wonder why your heater keeps turning on and off and whether you need furnace maintenance.

How often should a heater cycle on and off?

If your furnace is working properly and is sized correctly for your home, turning on and off is a normal heating system function. Usually, the furnace will cycle on and off about three to eight times an hour, depending on:

  • Your Thermostat Setting
  • How Well Your Home Is Insulated
  • How Cold It Is Outside

As outdoor temperatures drop, your furnace can run longer or cycle more often to keep your home nice and toasty. However, you need to be concerned if your furnace keeps turning on, runs for about five minutes or less, and shuts off.

If you notice your furnace is doing this, it is short-cycling, which is not good for your furnace. If the problem persists, you could require costlier furnace repairs or even replacement. Plus, your energy bills will skyrocket.

What could be causing your heater to turn on and off rapidly?

Hand turning a home thermostat knob to set temperature on energy saving mode.

Before you call your HVAC technician and tell them you have a short cycling furnace, there are a few things you can check to help see if that is the problem.

#1. Dirty Air Filter

Low airflow problems can cause your furnace to shut off before fully cycling. First, check your air filter and, if it is dirty, replace it. Some furnaces also have a secondary filter next to the unit, so you will also want to check that air filter.

#2. Thermostat Batteries Need Replacing

Many digital thermostats use batteries to store certain functions. When the batteries need to be replaced, it can cause short cycling. If it has been more than a year since you replaced the batteries, install new ones.

#3. Air Vents Are Closed or Clogged with Dust and Dirt

Airflow is essential to the operation of your heating and air conditioning system. If you have too many air vents shut or they are dirty, the system will not operate properly. In the rooms you do not want to heat as much, leave the vent slightly open to allow for minimal airflow.

#4. The Thermostat Is In the Wrong Location

Check your thermostat and the location where it is installed in your home. It should be on an interior wall that is not near other heat sources like an oven, stove, or fireplace. It also should be in a location where it does not get direct sunlight.

Thermostats installed near another heat source or that get direct sunlight will sense it is warmer in the house. As a result, they will read these results, short cycle, and not run as long as they should.

What else could be causing your furnace to short cycle?

Other causes why your heater keeps turning on and off and does not run a full cycle could include:

  • Thermostat Not Wired Correctly – If you recently upgraded your thermostat to a smart model and it was not wired correctly, it could cause short cycling.
  • Thermostat Malfunctioning – Thermostats do wear out. When they near the end of their useful lifespan, they can start to malfunction and cause heating cycles to shut off abruptly.
  • Furrance Is Too Big – An oversized furnace that is too big for your house will not function correctly. It will turn on, heat up quickly, and shut off.
  • Flame Sensor Problem – There is a flame sensor on natural gas furnaces. If this is dirty or corroded, it can have problems detecting when there is a flame and the gas valve is open. When it malfunctions, it closes the gas valve and shuts off the furnace.
  • Overheating – Furnaces have built-in safety features to shut down if internal components become too hot. If the temperature sensors in the furnace are dirty or corroded, they can malfunction, causing the furnace to shut down mid-cycle.
  • Damaged Heat Exchanger – Overheating can also occur when the heat exchanger is damaged and has cracks in it. Heat is released inside the furnace and can rapidly overheat the furnace.
  • Air Drafts – Sometimes the reason for short cycling is simply because the doors and windows in the home are not properly sealed. If you notice air leaks around doors and windows, have the windows sealed and weather stripping installed around the doors.
  • Electronic Ignition System Failure – If the electronic ignition system is having problems, it will shut down the furnace to prevent excess gas accumulation inside the furnace.
  • Flue Blockage – If there are leaves, twigs, nests, or even an animal that has taken up residence inside the flue, it will prevent your furnace from running a full cycle. A clogged flue causes the furnace to shut down to prevent carbon monoxide from being released into the home.

When should you call an HVAC technician for furnace maintenance?

Technician hands holding a manometers on equipment for filling air conditioners.

You will want to call your HVAC technician to further diagnose the problem after:

  • Checking the Air Filter
  • Replacing the Batteries
  • Opening the Air Vents
  • Cleaning the Air Vents
  • Or When Having the Thermostat Moved to a New Location

As you can see, there can be several issues and problems that could cause your furnace to turn on and shut off quickly. Furthermore, if your furnace is too big for your house, the only way to resolve this issue is by replacing it with a correctly sized furnace.

For help diagnosing your furnace and resolving short cycling problems in New Braunfels, Temple, San Antonio, or the Austin Metro area, please feel free to contact Christianson Air Conditioning & Plumbing at 512-246-5400 today!

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