A short cycling gas furnace turns itself off before it reaches the desired temperature.
This is an annoying problem that means your furnace consistently turns itself on/off without heating your home properly.
The good news is that it usually has a relatively simple explanation. It could be a malfunctioning thermostat, a clogged air filter or flue, a dirty flame sensor, or a damaged igniter, for instance.
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However, the most common cause is a problem with the furnace’s inbuilt safety system – the limit switch. This helps to prevent dangerous situations such as carbon monoxide leaks, gas leaks, or fires and maintain a comfortable temperature in your home.
A problem with the furnace limit switch can lead to a short cycling furnace. Here’s what you need to know about addressing this situation if it happens in your Calgary home…
How does a furnace produce warm air?
Let’s start with a brief overview of how your furnace produces warm air so that you understand why your furnace may be short cycling.
The burners in your furnace, which produce the heat to be distributed around your home heat the air produced by the air plenum in your furnace. The burners work with the furnace limit switch to ensure that air from the plenum is at least 20 degrees above the room temperature.
Once the air reaches the required temperature, the heat exchanger distributes it to the rooms in your home. This process continues until the temperature in each room reaches the setting specified on the thermostat.
When the desired temperature is reached, the furnace shuts down. However, the blower system continues until the internal furnace temperature is confirmed as safe – by the furnace limit switch.
Where is a furnace’s limit switch?
You can usually locate the limit switch by removing the service cover panel of the furnace.
On most modern furnaces. Look for a long temperature sensor probe attached to a mounting plate and extending through the wall of the furnace.
The plate section of the limit switch has at least two terminals that receive control wires for the blower fan and gas valve and attaches to the outside of the furnace's hot air supply plenum, above the combustion chamber or heat exchanger.
Your furnace may have more than one limit switch so that the temperature can be checked and regulated at different parts of the appliance.
On older furnaces, the limit switch may be mounted on the outside of the furnace rather than inside the service panel.
How does the furnace limit switch work?
The purpose of the furnace limit switch is to prevent the appliance from overheating and damaging the internal components, which can be hazardous for you and your family. It also helps to maintain a controlled and comfortable temperature throughout your home.
When the process starts and the thermostat calls for heat, the furnace burners ignite and start to heat the heat exchanger. The limit switch keeps the blower off at this stage so that cold air is not forced around the home.
However, the switch turns on as soon as the air in the plenum reaches the lower setting on the limit switch. The blower fan starts up and circulates air through the heat exchanger and around your home.
When the air has heated up to the desired temperature in your home, the burners stop but the blower keeps running to extract heat. Once the temperature returns to the lower level on the furnace limit switch, it kills the fan.
If the heat exchanger on the furnace starts to overheat at any stage of the heating process, the limit switch will activate and prevent damage to the internal parts of the furnace.
What can go wrong with a furnace limit switch?
If your limit switch malfunctions, it can potentially ruin your furnace or at least prevent your home from heating up.
Therefore, it is important to make regular checks and troubleshoot/fix any problems with short cycling.
A malfunctioning switch can impact your furnace in three main ways:
Also, if the limit switch hits its top temperature limit and shuts off the burner several times, the furnace’s main control system may prevent the furnace from working at all.
How do you replace a furnace limit switch?
Your furnace limit switch is too important to take any chances with.
Even if you know your way around your home’s furnace system, if it is short cycling, get it checked by a professional technician from Alpha Plumbing.
One of our repair professionals can check the limit switch by disconnecting it and testing for continuity. If the multimeter shows that resistance is infinite, the switch will need to be replaced – which can be done quickly and safely by one of our technicians.
The new limit switch should be of the same voltage ratings and temperature range and can be replaced by attaching the mounting screws and reconnecting the wires.
Need help with maintaining or repairing your furnace?
Your furnace is central to your home and the comfort and health of your family. If you run into problems with short cycling or need to schedule an inspection, call Alpha Plumbing on 403-470-5785 to arrange an honest assessment from a qualified technician.