All outdoor wood furnaces should have a safety feature that protects the boiler from overheating. This device is called a "High Limit Switch" that will cut off airflow to the fire if the boiler gets overheated. Not all furnaces have a high limit switch but the majority do. This kind of switch is also called a snap disk or thermodisc.
How Does It Work?
The high-limit switch is just a small switch but it has a very important role on an outdoor wood boiler.
The basic but important role of the high-limit switch is that it is a safety sensor that if something is wrong with your outdoor boiler then cuts to power to prevent damage.
High-limit switches are normally closed. When the temperature of the boiler reaches a certain point or degree, this high-limit switch opens and will trip and will cut power to the blower fan to prevent the boiler from overheating.
Two Types of High-Limit Switch
There are two different types of high-limit switches that are used for outdoor wood furnaces - the manual reset and the auto-reset.
Is the Auto-reset Better?
The answer is NO. Why?
Remember that high-limit switch trips when it reaches a certain temperature. And that it functions when it senses that something is not right with your outdoor wood stove. That means, something caused the stove to overheat, and you need to know that.
The thing is, if you use an auto-reset high-limit switch, you would never know that you have a problem because it automatically resets when the boiler cools down. That is why we highly recommend using the manual-reset type because you will have the chance to address and solve the cause of why your boiler overheated.
This type of high-limit switch has a red reset button in the front middle. You MUST WAIT until the water jacket temp drops to at least 150 degrees or less before you can reset the high limit switch by PRESSING the RED BUTTON. If the boiler is too hot, it will not stay in.
Where is it located?
The high-limit switch is located on the rear of the boiler. It is commonly mounted near your Aquastat and blower fan. It is mounted so that the back of the switch is flush with the water jacket steel so that it properly senses the water temperature.
But on some brands and models of outdoor wood-burning boilers, the high limit switch is mounted on the front inside the control box. A screw must be removed to access the switch to reset it.
What Causes a High Limit Switch To Trip?
So, what causes a high-limit switch to trip?
This is a much bigger question that cannot be fully answered here, but basically whatever is causing an overheating situation, will trip the switch. These reasons include the following:
- Bad door seal
- Overfilling furnace with wood
- Not removing ash sufficiently
- Pump failure
- Power outage
- Solenoid stuck open
- Boiler front door left open
Please note that if an outdoor wood stove's front door is left open, then the furnace will burn out of control and this is one of the worst things for a furnace. Please ensure that the door is always closed and securely latched.
The common high-limit switch recommended for an outdoor boiler is the one that is set at 190F because the ideal set point of your Aquastat is 175-180 degrees F. So if you notice that your outdoor wood boiler blower fan stops working and it doesn’t turn on, chances are, the high-limit switch cut its power at 190 degrees to prevent the temperature from increasing, which could cause further damage. So you need to manually reset it.
Read our blog article “Has Your Outdoor Furnace Overheated?” to know more about the cause of overheating.
How Long Will High Limit Switches Work?
Some last a long time. Some die after only their first trip. This all depends on how hot the high limit switch is heated to. If the switch is never overheated, naturally it will last a long time.
If your switch has died after only just the first trip or only a few trips, your furnace is overheating at extremely high temps that are killing the switch.
We recommend that all outdoor wood burner owners have an extra high-limit switch on hand and available. No outdoor furnace can be operated without one, and if it fails, your furnace is useless. Under normal conditions, your furnace will operate correctly, and will be able to SAVE LOTS OF MONEY on your heating bills! You can order a high-limit switch HERE.
To browse for more outdoor wood boiler parts visit our online store or call all our offices Monday-Friday 9 am - 8 pm EST at (231) 861-8200.