Why Is My High Limit Safety Switch Tripping?

Before we discuss the "Why" your high limit switch is tripping, let's cover a few of the basics.

All outdoor 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 boiler 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 boiler. 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.


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What Causes a High Limit Switch To Trip?
This is a big question that may not be fully answered here, but 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, even after the fan shuts off. When the fan shuts off, your fire is raging and fires suck oxygen very powerfully. Often, if you have not changed your door seal in 3-4 years, when the fan shuts off, the fire will pull oxygen through a poor door seal that is brittle from age. Simply replace the door seal, and the problem is solved.

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. This is another cause of tripping the high limit switch. This is a dangerous situation and requires immediate attention. 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

Almost every time the power goes out, we get calls about a tripped high limit switch. Yes, the blower fan stops, and the draft closes, but the pump also stopsWhen 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 boiler 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. When a pump fails, it might not go from running like normal to dead. It might die a slow death. 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?

Any other air leak will cause a boiler to overheat also. For example, 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 air box 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 air box.

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 had service work on the damper opening plate, it must be replaced with a caulk seal on the sides to be completely air tight

6. Overfilling Furnace With Wood

If you overfill the furnace with too much wood, or you have wood too close to the front door, your furnace may boil water in the front door area. 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 it with the front door open for more than brief periods of time while loading wood or removing ash. Do not do this. It is tempting, we know this. But do not do it. 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. A boiler door left open with a raging fire inside is the worst possible scenario. Never allow someone to fill your furnace with wood if they are not capable of closing the door. If a boiler 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.

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.
We recommend that all boiler 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!

For more information, please call us Monday thru Friday from 9:00 AM to 8:00 PM at 231-861-8200. 


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