Your furnace is a complicated piece of equipment. There are various components working in unison at all times and when one of them goes bad, it is imperative to identify the problem in short order. The last thing that you want is for your furnace to continue failing, leading to bigger and more expensive repairs.
One such issue that you may be experiencing is that the high limit switch keeps tripping. When the high limit switch keeps tripping, there are two likely causes. The first is overheating. When the unit overheats, one of the results is that the high limit switch keeps tripping. The other cause of this problem is that the switch itself is going bad.
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What is a High Limit Switch?
There are some more commonly known components within the average furnace, but the high limit switch can go unnoticed in most cases. This switch is quite important because it serves two functions.
- Fan motor. The first and most important responsibility of the high limit switch is to turn the fan motor on and off. This happens in every single cycle of normal operation for your furnace.
- Prevents overheating. The other main role of the high limit switch is that it is meant to protect from overheating. When the furnace overheats, there is the potential for damage to the other components and can even lead to fire in the most extreme cases.
The high limit switch has a long, temperature-sensing probe that is attached to a separate component known as a metal mounting plate. The switch is then wired into the blower fan motor as well as the gas valve. You can typically find the switch just past where the heat exchanger is located.
The purpose of the switch is to detect the temperature of the air blowing into your home. Regulating it and keeping it at safe levels for both you and the other components is what it does best.
The First Job of the High Limit Switch
Before getting to the problems with the high limit switch, it is important to know the overall role that the high limit switch plays in your furnace. When operating normally, the high limit switch is meant to control when the blower fan turns off and on.
The blower fan needs to be controlled by the limit switch, not the thermostat (which is what many think). You don’t want the blower fan turning off and on at the same time that the gas burner does.
- Heat exchanger. When the burner kicks on, it will take a little bit to warm the heat exchanger. It takes a little more for the heat to then transfer into the air supply that is already waiting within the furnace.
- Blower fan. If the blower fan turned on at the exact same time as the burner, it would basically be blowing cold air into your home. During the coldest months of the year, the last thing that you want is for more cold air to blow into your home.
- High limit switch. This is where the high limit switch comes into play. When the air gets hot enough, the high limit switch then tells the blower that it’s okay to come on. This is why there can be a couple of minute delay between the thermostat switching the heat on and the warm air actually coming out.
The Second Job of the High Limit Switch
While that first job – making sure that the air is warm before blowing it out – is important, it isn’t the only job of your furnace’s high limit switch. The second job of the high limit switch is to act as a safety device within the furnace.
When the furnace runs, as we touched on above, the primary role is to switch on the blower when the air is hot enough. But if that supply of air becomes too hot, the job of the high limit switch is to shut off the gas valve to stop the heating.
Overheating. The high limit switch is also there to act as an intermediary with the gas valve. When the temperature gets to be too high, the high limit switch stops the gas valve from heating up the air any further.
Lockout mode. Should the high limit switch get tripped due to overheating several times in a short period of time, the furnace will go into lockout mode. That is where the furnace will not turn back on in any manner until it has been serviced and reset.
The lockout mode is extremely important. If the furnace overheats, the heat exchanger can crack and when that happens, poisonous gas (carbon monoxide) can get into your home. By shutting down the gas valve, the high limit switch is actually keeping your family safe and preventing potentially costly repairs or a complete replacement of your HVAC system.
Signs That the High Limit Switch is Bad
Recognizing the signs that your high limit switch is going bad can help prevent major issues with your furnace. And like any other component in your HVAC system, the high limit switch can and will wear out over time.
- Not signaling the blower. Perhaps the number one sign that your high limit switch is going bad is that it is failing to signal the blower fan, primarily when turning off. If the fan keeps on running and you’ve made sure that the thermostat is set to AUTO instead of ON, then it could mean that your high limit switch is going bad.
- Lower temperatures. Time can weaken your high limit switch, wearing it down. When that happens, your switch may start tripping at lower temperatures than normal. Normally, a high limit switch might start to trip around 155-degrees Fahrenheit. Instead, you may notice that it trips at around 130 degrees. This is what can lead to the furnace going into lockout mode over time even without overheating.
- Replacing your switch. If either of these cases become a regular occurrence, the high limit switch will need replacing. The tricky part is determining whether or not the switch is actually bad or not. Overheating can still occur, leading to the switch tripping far more regularly.
Protect Your High Limit Switch
Protecting your high limit switch is important due to the safety role that it plays. The more that it gets tripped due to overheating, the more that it wears down. Thankfully, there is a relatively easy solution when it comes to protecting your high limit switch.
Clogged filter. A clogged or dirty filter can cause major issues within a furnace. Overheating is one of those common causes and can wear down your high limit switch far faster than normal if not addressed accordingly.
Make sure to check, clean, and replace your filter on a regular basis. With a clean filter, your furnace will be able to run more efficiently and will also improve the overall quality of air in your home. It only stands to reason that less dust and dirt circulating through the air is a good thing.
Regular maintenance. As is the case with any major appliance or system in your home, regular maintenance is never a bad thing. In addition to replacing the filter, having the unit serviced yearly can help you stay ahead of costly repairs or replacements.
Can You Bypass the Furnace Limit Switch?
Should the temperature be below the setpoint, the resistance that is across the switch should be zero. But there may be a time where the high limit switch still will not kick the blower on. When that happens, warm air won’t be distributed through the home.
When that happens, you can bypass the switch. It is advised that you do not simply bypass the switch and let your furnace run as it normally would. Bypassing the switch is meant to simply see if the furnace is still otherwise working.
Should you bypass the high limit switch and notice that the furnace is operating otherwise normally, it is time to replace the switch. It is always best to call in a certified technician to handle these sorts of repairs and your warranty may even cover the replacement.