Short Cycling Furnace – What To Do?

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A furnace may short cycle over time and this is usually because of different problems. When this happens, you may wonder what you can do to tackle the problem. Fortunately, we have done the legwork for you, and here is what we found.

You can fix short cycling problems easily without the help of a professional. Here are things to do when your furnace starts to short cycle:

  • Check the air filter for blockages. 
  • Check the thermostat setting.
  • Clean or replace the dirty flame sensor.
  • Check for a clogged flue.
  • Check for a faulty fan limit switch.

Whenever you notice your furnace is short-cycling, make sure to solve the problem as soon as possible so it doesn't get worse. Keep reading to get detailed information on how to tackle a short cycling furnace.

A home high energy efficient furnace in a basement, Short Cycling Furnace - What To Do?

What Is Considered Short Cycling For A Furnace? 

Short cycling occurs when the normal 10-minute cooling cycle in a furnace reduces to less than 5 minutes. This prompts the compressor to cycle on and off more frequently than normal. This repeated stopping and starting cause additional wear on the compressor while reducing the level of cooling in your home.

A short cycle occurs when the heating element turns off before the appliance reaches the target temperature. If you set your thermostat to 81 degrees, the furnace may turn off at 68 or 71 degrees, but it will not reach 81 degrees.

Technician repair maintenance on a furnace while check circuit using a flatscrew

What Should I Do When My Furnace Short Cycles?

When you notice your furnace is are coming on and off frequently without reaching the temperature that you have set on the thermostat and there are no other issues, then your furnace may be short-cycling. 

Fixing certain problems that cause the furnace to short-cycle is simple while tackling others may require a professional. Do these when you have a short cycling furnace:

1. Check The Air Filter For Blockages

A clogged filter can cause your furnace to short cycle. It is necessary for the fan on the furnace to move adequate air over the heat exchanger. When air has to pass through a clogged air filter, it reduces the air quality. This can also cause the fan limiter to become active.

You can fix his problem easily. All you have to do is take out the air filter, clean and then replace. Check to see that the arrows that are printed on the frame of the filter point in the same direction as the airflow produced by the system. 

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2. Check The Thermostat Setting

The thermostat may have been set incorrectly, the batteries may be weak or dead, or the thermostat itself may be damaged. To solve this problem, remove the old batteries from the thermostat and replace them with new ones. Check the settings and inspect the thermostat for any damage. 

3. Clean Or Replace Dirty Flame Sensor

The flame sensor in the furnace is a metal rod shaped like L. It notifies the furnace when it detects a flame or the gas valve has been left open.

The heating system will turn off if the flame sensor does not detect any sign of a flame. Over time, flame sensors get dirty. When this happens, flame sensors may not detect a flame accurately. 

To fix this, clean the greasy flame sensor. When the furnace is turned off, you can remove the sensor using either a socket wrench or a hex driver. Clean the flame sensor by gently rubbing it with fine steel wool or sandpaper with fine grit. Sometimes, you may need to replace the flame sensor with a new one. 

4. Check For Clogged Flue

The furnace's flue is meant to remove potentially harmful gases. The heating system will automatically turn off if any object is obstructing the chimney, such as a bird's nest, a dead rodent, or leaves.

To fix this, switch off the furnace and disassemble the flue. With a flashlight, inspect each part of the chimney. Remove any obstacles that are in the way. After reassembling the flue, turn the furnace back on. 

5. Check For Faulty Fan Limit Switch

The furnace fan is equipped with a fan limit switch that determines when the fan should turn on or off. The fan limit switch acts as the gatekeeper to regulate hot air flow through the system. Since it is a safety switch, the fan limit switch will turn off the furnace if it detects that it has reached an unsafe temperature. 

Limit switches for fans are hermetically sealed devices, and they should not be fixed. Changing the switch should rectify this issue.

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Is It Bad For A Furnace To Short Cycle? 

It is bad for a furnace to short cycle. Short cycling may lead to more serious problems. A system that keeps turning on and off, producing heat, and then turning off again, would eventually stop working properly.

This problem is likely to result in premature wear and tear and eventually shorten the system's life span. It may also bring about the need for frequent repairs. 

A Brand new furnace installed on a basement

What Causes Furnace Short Cycling? 

Furnace short cycling happens because of various problems and may even boost your electricity bills. Your furnace could get totally damaged and may need to be replaced if the issue is neglected for too long. Here are common problems that lead to furnace short cycling:

Dirty Air Filter

Male hands are installing a furnace air filter

An unclean air filter can cause short cycling. Your heat exchanger will not receive the correct amount of airflow if the air filter in your unit is blocked with dirt. 

This causes the furnace to reach an unsafe temperature, which triggers the high limit switch and may turn off the furnace to prevent further damage. After it has cooled down, it begins again, and the process keeps repeating.

Faulty Thermostat

If after examination, you discover that the air filter is not the cause of the problem; the thermostat is the most likely reason for the short cycling. The thermostat is not completely reliable. It may need to be checked at regular intervals. If the thermostat in your home is malfunctioning, it may cause your furnace to turn off and on.

Bad Flame Sensors

When the fuel in your furnace burns, a little amount of water is produced by the system as a byproduct of the heating process. Over time, this may cause metals to corrode and rust. As a result, corrosion accumulation causes brief cycling, which can be dangerous.

Click here to see this furnace flame sensor on Amazon 

Clogged Flue

The chimney or vent pipe could get clogged by a foreign object or a nuisance like a bird that flies into it. As a protective measure, the furnace will automatically turn off so that you are safe from the potentially harmful effects of carbon monoxide. This will result in short cycling. So, rectify this issue as soon as possible.

A person changing an air filter on a high efficiency furnace

How Do I Know When My Furnace Is Short Cycling?  

The most reliable sign that a furnace is short-cycling is when the two temperature readings on the thermostat — room temperature, and the temperature set on the thermostat — never coincide. 

The furnace may be short-cycling if the room temperature reading consistently climbs during the heating cycle but stops short of the temperature that has been set. Other symptoms of furnace short cycling include a frequent heating cycle, a short heating cycle, and a cold house. 

How Long Should A Furnace Run During A Normal Heating Cycle?

The typical amount of time it takes for a furnace to complete a heating cycle is between 10-15 minutes if there aren't any extreme conditions. Your furnace may need to run twice to three times an hour in order to keep the temperature in your home at a suitable level.

Is It Normal For A Furnace To Cycle On And Off?

It's normal for a furnace to cycle on and off. The cycle should only take place 3-8 times every hour, resulting in heating cycles of 7-20 minutes. This number might vary depending on the condition of your furnace system, the temperature outside, and other factors. 

On the other hand, if your furnace turns on and off at an unusually high number of times or operates for longer than a minute or two before turning off, then there might be a problem with your furnace. 

To Wrap Up

A home high energy efficient furnace in a basement

You should not overlook a furnace that short cycles regularly. As soon as you suspect there is an issue, do simple troubleshooting or consult a qualified HVAC technician. If you fail to do so, the problem can lead to costly repairs or require a complete replacement. 

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