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Why does my furnace keep turning off and on | Short Cycling

Why does my furnace keep turning off and on | Short Cycling

Furnace problems are quite common in cold winters. There are many common issues that arise from furnaces and a furnace cycling off and on is one of them.

So if you or someone you know is experiencing any of the following this blog is for you.

Why does my Furnace keeps turning off

Furnace is not turning on automatically

Furnace runs for a few minutes and then stops

Furnace starts and stops and starts again

Why is my Furnace short cycling

Furnace turning off and on repeatedly

Why does my Furnace keep turning off and on repeatedly?

A furnace repeatedly turning off and on in the HVAC industry is known as ‘short cycling’. It’s exactly what it sounds like. The furnace cycle or the ‘routine your furnace takes to continually keep your home the right temperature’ is cut short by something.

Short cycling can be confused by many to the average function of a furnace. A furnace is meant to turn off and on repeatedly, but depending on your furnace, home situation, weather etc, determines how many times and for how long this is supposed to happen.

How do I know if my furnace is short cycling?

A short cycling furnace will turn off and on repeatedly in a very short period of time. This is roughly anywhere from 2 seconds to 2 minutes on and then off again.  This is generally a good rule of thumb to follow.

An average furnace will in fact turn off and on only a couple times in an hour. However, during these times the furnace should be lasting around 5 minutes or longer. If it isn’t that cold outside it’s easier for your furnace to get your home to the desired temperature vs an extremely cold day.

Of course, all of this depends on so many factors, including the size of your home, type of furnace, your insulation, outside temperatures and more…

If your home is older, has poor insulation or windows, and its 30 below 0, it’s almost guaranteed it’s going to work much harder to maintain temperature in your home.

The type of furnace makes a difference.

A high efficiency furnace will usually run longer. These are known as variable speed or modulating furnaces. They can adjust their speeds and energy consumption to any level to always run efficiently and not need to turn off and on nearly as often.

Think of it as a car on cruise control. It can coast when it needs to and speed up when it needs to but it is always maintaining the speed…. Or in this case your home’s temperature.

A single stage furnace is on the lower side of efficiency. If your furnace is one of these guys it will turn off and on more often. This is because it simply has two options. Off or ON. It either runs at 100% power or 0%. It has to kick in every time your homes temperature drops and the thermostat tells it to come back on.

My furnace is short cycling… now what?

We now figured out that your furnace is in fact short cycling and turning off and on more often than it needs to.

There are many reasons that your furnace could turn off so many times, and not all of them are necessarily bad. Technically if your furnace is cycling off and on is somewhat of a good thing….

You might be wondering why this is and I’ll tell you…

Your furnace has multiple key safety features within it that are meant to prevent dangerous situations and are most likely the cause of your furnace turning off and on.

These safety features prevent things like gas build ups, fires, and overheating among others. All of these dangerous situations are clearly not good for your home or your family.

Now a furnace turning off and on is not a great thing for your energy bills or for the furnace itself

Overuse of the motors and internal parts will cause quicker wear and will cause more frequent repairs.

What is Causing My Furnace To run for a few minutes and then stop?

Below are some of the main issues that causes furnace to cycle off and on.

Cause #1

Furnace is Overheating

Depending on if its old or newer

I know it sounds counterintuitive since a furnace is meant to create heat, however It can most definitely overheat.

Overheating is typically caused by restricted air flow which creates more pressure on the furnace. Overuse of the furnace causes parts to wear quicker and leads to more frequent repairs.

This will cause the safety feature known as the high limit switch to flip on.

High limit switch (cause of furnace to be turning on and off repeatedly)

The high limit switch is an internal safety feature of the furnace to prevent your furnace from getting too hot. This is meant to stop dangerous situations like fires etc and this safety feature will shut your furnace off.

Now depending on the age of your furnace your furnace may shut off for a period of time or cycle on and off. If it is an older furnace it will cycle off and on if it’s a new furnace it may turn off for 3 hours before it resets and tries again

What to do?

Check your furnace filters

Your furnace filters are meant to keep your home air clean and also keep the furnace itself clean. As dirt and other particles get built up it restricts air flow and eventually make it extremely difficult for your furnace to properly circulate air. Causing it to heat up.

Dirty filters allow more dirt to build up inside the coils of the furnace and eventually cake on. Not to mention these particles floating through the air.

If your filters are extremely dirty, try replacing them or upgrading to an improved air quality system.

See if this fixes the issue.

Cause #2

Flame sensor

This is a very popular cause of a short cycle in most furnaces. The flame sensor is designed to properly gauge how much gas is getting to the furnace. When it becomes dirty as it naturally does over then it loses it’s ability to do it’s job properly. If this it the case then won’t be able to determine if the gas is being used by the flame and tells gas valve to stop supplying gas to the burner. Once again these are safety features to prevent excess gas buildup.

Try a furnace maintenance from JPS Furnace & Air Conditioning. This includes a full tune up and cleaning of your furnace and flame sensors amongst many other benefits.

Cause #3

Gas is not properly being ventilated out of the furnace

This could potentially be a dangerous situation. If gas build up inside the furnace and it happens to lite, it could cause a fire.

This is where the flue limit switch engages.

Flue limit switch /Pressure switch (furnace keeps turning off and on)

Once again depending if your furnace is older or newer it may turn off and on differently. This is where it’s best to have an expert come take a look.

When gas gets built up in a new furnace it may shut down for extended periods of time until it can be reset to try again. Some older furnaces may not have this and could cycle off and on as gas is built up.

This switch turns your furnace off if gases are not properly ventilated out of the exhaust flue. The exhaust flue is where all the excess exhausts get carried through to the outside of your home.  If this is being triggered it means gases are built up inside your furnace, preventing performance and possibly being dangerous for you and your family.

This gas build up  can cause the flue limit switch to trigger and if it starts to heat up your furnace the high limit switch will be fired. This is simply a secondary precaution in case one of the features doesn’t work.

What to do?

Check the flue if possible.

On newer furnaces the flue is generally on the side of the house. Check to see that warm air is coming out from the flue. Typically you can see the steam or hot air and be able to tell if it’s running properly.

Usually on older units this is located on the roof of your home so in the winter this can be dangerous to access. If it’s in eye sight take a look while your furnace is running and see if there is gas coming out. This is better when it’s cold out because the air coming out will be very hot and cause visible steam in the winter.

Sometimes small animals or branches and debris could be clogged inside the flue and lead to the flue limit switch to go off.

In this case it may be best to contact your local Calgary Furnace Repair Experts.

Other causes of furnace turning off and on.

Cause #4

Furnace Blower Motor Not Working

It is possible for your furnace to run with blower motor off. The blower motor is what blows air over the heat exchanger. This is the part of your Furance that heats the air. If the motor is off or not working the the heat builds up inside the furnace and trigger all the safety features such as the high limit switch.

Check your vents

When your furnace comes on check your vents and see if there is any air coming out. If there is no air pressure coming from your vents this could be the culprit.

Cause #5

Furnace Is The Wrong Size

This will only be an issue for those who have recently had their furnace replaced. An oversized furnace can use more energy as it will turn off and on more often. The reason is because it warms your home up so quick that it’s run time is extremely. It might sound great in theory but it typically leads to cold spots in your home as the heat isn’t able to fully distribute. After it turns off and the temperature drops the furnace will come on again quickly warming your home. This leads to overuse and short cycling.

Cause #6

Thermostat is malfunctioning

Sometimes thermostats like other smart devices can malfunction. This can lead to false reading or other issues. Double check your thermostat batteries and that you have proper power supply to the thermostat.

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