Furnace heating turned off

Why Does My Furnace Keep Turning Off?

Furnaces are a central element of most Calgary homes and they mainly just sit there and do their jobs. We don’t need to think about them too much.

However, when something goes wrong with your furnace, you need a quick diagnosis and fix to prevent an ice-cold home.

Troubleshooting the issue can help identify where the problem lies. If your furnace keeps turning off, this is known as “short cycling” and there are several possible reasons why this may be happening: from a faulty thermostat to a dirty flame sensor.

We take a look at short cycling and the main reasons why a furnace cycles off and on…

Top Article ⟶ What to do if your furnace stops working


Is my furnace short cycling?

Generally speaking, a properly functioning furnace will turn itself off and on once or twice per hour.

In very cold weather, it usually takes your furnace longer to heat your home to the desired temperature and it needs to work harder. Your home’s level of insulation will also affect how hard it needs to work.

If your furnace is short cycling, it will keep turning off and on repeatedly every few seconds (or up to two-minute intervals) regardless of the temperature outside.

However, different furnaces behave differently according to their efficiency level, so this needs to be considered too.



High-efficiency furnaces vs older furnaces

When the temperature in your home drops below the level set on the thermostat, it turns the furnace on.

High-efficiency furnaces (variable speed or modulating furnaces) adjust their speeds and energy consumption to maintain efficiency without repeatedly turning off and on.

They adjust the power level required automatically and generally run longer than older, lower efficiency models because they don’t have to turn off to maintain efficiency.

Therefore, if you have a new, high-efficiency furnace that turns on and off repeatedly, something is wrong.

Lower-efficiency furnaces are known as “single-stage” models. These furnaces must either run at 100 percent power or zero percent power. This means that they turn on and off more often to maintain a decent efficiency level.

What causes a furnace to repeatedly stop and start?

Regardless of the type of furnace, sometimes they develop problems that cause them to turn off more often than is necessary. Such problems are usually related to inbuilt safety mechanisms in your furnace and most need investigating by a professional furnace technician.

However, you can run some troubleshooting tests if you know what to look for. Typically, there are six main causes of short cycling…



1. Overheating

A furnace can overheat due to the flow of air becoming restricted. Often, this is because of dirty air filters, which get clogged with dust and particles, impacting the normal flow of air. This can lead to furnace parts wearing out and repairs or replacements being required — not to mention the poor quality of air being circulated in your home.

If your furnace overheats, the limit switch will be activated and cause your furnace to turn off for safety reasons (i.e., to prevent fires).


2. A dirty flame sensor

Short cycling furnaces are often caused by dirty flame sensors too. These sensors detect how much gas the furnace needs. If they become dirty or clogged with dust or soot over time, they cannot detect gas levels so readily.

For safety reasons, the sensor will tell the gas valve to stop supplying gas to the burner and the furnace will turn off to prevent excess gas buildup.

With regular maintenance from Alpha Plumbing, you will prevent this problem from occurring as a flame sensor check/clean is a standard element of our furnace service.


3. Poor gas ventilation

Another safety element that may kick in with your furnace is if the normal ventilation of gases to the outside of your home s impacted in some way.

Poor ventilation from the furnace’s exhaust can be a fire hazard. The flue limit switch/pressure switch will automatically engage if this problem is detected, turning the furnace off.

Older furnaces may cycle on and off but newer furnaces will usually shut down for a few hours and reset.

4. A problem with the furnace blower motor

The blower motor’s job is to blow air over the heat exchanger, which heats the air before it circulates around your home.

If the blower motor develops a problem, the furnace may still operate without it, which can cause a buildup of heat inside the furnace and the limit switch to be triggered, shutting off the furnace as a safety precaution.

To identify this as the issue, you should check the furnace’s vents. If air pressure is escaping, this could be the reason for your furnace turning off.


5. Your furnace is too big for your home

If your furnace warms up your house extra quickly, it might sound like a bonus. However, an oversized furnace uses more energy than necessary and will turn on and off more often, as well as possibly leading to “cold spots” in the home.

If you get your furnace professionally fitted, the technicians can advise you on getting just the right size and efficiency model for your home so that it doesn’t keep short cycling.

6. Something is wrong with the thermostat

A malfunctioning thermostat will send the wrong information to your furnace and may cause it to short cycle.

The thermostat should be located out of direct sunlight and set to heating mode and the correct temperature indicated. If your furnace is short cycling and you’ve already checked the air filters, the next check is that the thermostat has not been accidentally adjusted or the batteries don’t need changing.

Beyond that, you will need to call a professional.

What should you do with a short-cycling furnace?

If you find that your furnace is short cycling and the cause is not clogged air filters or a simple thermostat issue, it’s most likely one of the furnace’s inbuilt safety mechanisms kicking in — which is ultimately a good thing. This prevents fires, overheating, and gas buildup in your home.

However, it’s not good for your energy bills or the longevity of your furnace —because turning on and off more than necessary wears the parts out.

It also means that the issue is not a DIY job and will need to be checked by a professional furnace technician.

Need help with maintaining or repairing your home furnace?

If you run into problems with your furnace, don’t attempt fixes without the proper experience, tools and knowledge.

To keep your family safe, schedule an inspection with Alpha Plumbing by calling 403-470-5785 for an honest assessment from a qualified technician.

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