Outdoor Furnace: High Limit Safety Switch Is Tripping

Outdoor Furnace: High Limit Safety Switch Is Tripping

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

It 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.

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.

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

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.


High Limit Switch         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.

What Causes a High Limit Switch To Trip?

Here are some of the most common causes, and what to do about each case: 

1. Bad Door Seal

The most common cause of a high limit switch tripping is air feeding the fire. If you haven't changed your door seal in 3-4 years, the fire will often suck oxygen through a brittle door seal when the fan shuts off. Simply replacing the door seal solves the problem.

2. Solenoid Stuck Open

If your solenoid is old, it can get "hung" in the open position. This allows air to freely flow into the firebox, even when the fan is off. Remove the wood, coals, and ash from the boiler immediately, and do not use the furnace until you can replace the solenoid.

3. Power Outage

When the power goes out the blower fan stops, and the draft closes, but the pump also stops. When the pump stops circulating the water, there is no way to cool the boiler. Even though the fire is being starved of oxygen, a raging fire will still smolder quite hot for several minutes, and without a pump, the water in the top of the water jacket may boil. 

Just reset your high limit switch and hope the power does not go out often! If this is a common occurrence, then lower the Aquastat temp from 180 to 175, and lower the differential on the Aquastat (white dial inside the grey aquastat box) from 20 to 15.

4. Pump Failure

Your outdoor wood stove will outlast all of its electronic components. This is why we strongly recommend that you have the Emergency Prep Kit on hand at all times. If your pump is not providing the flow volume it should, one symptom will be a tripped high limit switch.

Watch for flow through your filter, or out the filter valve at the bottom of the filter, to diagnose this. Replace your pump as needed.

5. Other Air Leak - Fan Gasket?

If you have ever replaced your fan but did not replace the fan gasket properly, air can get to the fire from this area. Check between the fan, and the fan airbox to make sure the gasket is seated properly. If not, add High Temp caulk to the area to seal any air leaks between the fan and the airbox.

GX boilers have some other areas of possible air leak and those customers should consult with your OutdoorBoiler.com Tech Team

Note to GX10 owners: If your boiler has had service work on the damper opening plate, it must be replaced with a caulk seal on the sides to be completely airtight. 

6. Overfilling Furnace With Wood

This makes a pounding sound. Simply keep the wood, fire, coals, and ash at least 8 inches away from the INSIDE of the front wall of the furnace.

7. Not Removing Ash Sufficiently

For the same reason as (6.) above, be sure to remove the ash weekly and be sure to scrape out the boiler burn pot creosote. Use creosote sticks as needed.

8. Boiler Front Door Left Open

It is fatal for an outdoor boiler to operate with the front door open for more than brief periods of time while loading wood or removing ash. You can cause permanent damage to your steel, even if it does not leak (yet) the water will all boil out and the steel will become fatigued.

If a boiler'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.

For more outdoor wood boiler parts, visit OutdoorBoiler.com or call us Monday thru Friday from 9:00 AM to 8:00 PM at 231-861-8200.