Outdoor Furnace: High Limit Safety Switch Basics

Outdoor Furnace: High Limit Safety Switch Basics

All outdoor wood boilers should have a safety feature that protects the boiler from overheating. This device is called a "High Limit Switch" that will cut off air flow to the fire if the boiler gets overheated.

How Does It Work?

A High Limit Switch will 'trip' when it senses the outdoor wood furnace water jacket temp reaches 190 degrees. Normally, the blower fan will turn off at 180 degrees, but if something goes wrong with that device, the high limit switch will trip, which cuts power to the blower fan and solenoid.

The high limit switch also cuts power to the light on the front of the furnace. If you notice that the blower fan and light are not powered, you just need to reset the high limit switch.

Where is it located?

The high limit switch is located on the rear of the boiler. 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.

GH-Model furnaces are different - 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. 

How To Reset The High Limit Switch?

Most high limit switches have a red button on them 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.


High Limit Switch         High Limit Switch


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.

These switches also are sold in an "auto reset" version. However, we don't allow those to be sold on outdoor boilers because you would never know that you have a problem.

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.


We recommend that all outdoor wood burner owners have an extra high limit switch on hand and available. No furnace can be operated without one, and if it fails, your outdoor boiler 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.

See below for our High Limit Switch PROMO and learn how you can get a FREE high limit switch!

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.