The Correct Method for Testing Thermal Run-Away

What is Thermal Runaway?

Thermal runaway is a safety feature that stops the printer from reaching extremely dangerous temperatures when the thermistor becomes loosened or damaged. Most popular firmware’s like Marlin, RepRap and Repetier have this feature enabled by default.  

A printer is constantly monitoring the temperature of the heat block and compares it against the desired temperature. When it is too hot, the heater cools down, when it is too cold, the heater heats up, much like a modern day oven. When the thermistor is loose, has fallen out or is damaged, it is quite possible that the temperature the printer is reading is well below the desired temperature. This causes the printer to keep trying to heat up the element with no change in temperature being read. This is where thermal runaway would be triggered if it is enabled as it detects the temperature dropping below a specified value for a certain amount of time, however, if it weren’t enabled, things can get worse. The heater block would continue to heat up and there are two main things that could happen: the heater block starts to melt and your heating element possibly starts a fire or your control board becoming overloaded and blowing up components.   

Don’t believe the heater block will melt? Take a look at this!


At Phaser3D, we’ve had multiple Prusa i3 Mk3’s trigger with thermal runaway before and after a quick diagnosis, we’ve found the cause to be damaged thermistors. If we ran cheap stock printers like the Ender 3 and Anet A8 thing’s could have been much worse as they’re advertised to have thermal runaway but our in house testing has found that this is not the case, which many others online have found as well. 

Before you leave your printer alone whilst its printing, its best to test it yourself.

 Image result for 3d printer fire 

Don’t let your printer turn into this! – Credit: Hackaday

How to Test Thermal Runaway

Step 1: Heat the nozzle

To first test the printer for these safety features, the nozzle will need to be pre-heated to a regular printing temperature like 215°. 

Step 2: Blow, Blow, Blow

In order to reduce the temperature of the hotend, you need to use either a source of fast moving air, like a hair drier on its blower setting or an air compressor, or a source of cool air. At Phaser3D, we used a can of compressed air turned upside down. It provides both freezing cold air and fast moving air. Blow on the nozzle and watch the temperature drop slowly.

Step 3: Watch for a Trigger

The stock settings in the Marlin firmware with thermal protection enabled  will trigger usually within 30-60 seconds of testing using a hair drier or air compressor. For the Prusa i3 Mk3, its a drop of 15° for 45 seconds. It will pop up with a “THERMAL RUNAWAY” error on the display of your printer and stop the connections to any peripherals such as a Raspberry Pi’s. If this doesn’t happen and you notice the temperature still dropping, thermal runaway isn’t enabled.

This is what the Thermal Runaway error should look like. 

Step 4: Enable Thermal Protection

If your printer didn’t trigger, to ensure the safety of your house and printer, it is best to update the firmware currently on the printer with the latest one and check that it has it enabled. If you’re unsure how to modify firmware such as Marlin, it is as easy as a simple google search to try and find a pre-compiled version. Searches like “Ender 3 Marlin Firmware Update” and “Anet A8 Thermal Runaway Firmware Update” gave many resources to update your printer. If you’re up for a bit of a challenge or want to learn how to modify some firmware check out this guide from Thomas Sanladerer where he shows how to enable thermal runaway and also min and max temp settings. If you’re after a more detailed look at the inner workings of Marlin or would like to learn how to compile your own firmware, check out this configuration guide  that covers all the features within Marlin and how to use them. 

We’d love to chat to you about your 3D Printing needs. Get in touch!

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