Replacing a boiler's thermostat and overheat safety thermostat

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  • OVERALL DIFFICULTY INDEX: 3/10
  • IF YOU HAVE NO TOOLS, YOU PAY $10 TO GET THEM FOR THIS JOB
  • IF YOU HAVE NO EXPERIENCE YOU SPEND 1.5 HOURS TO CARRY OUT THIS JOB
  • YOU SHOULD OBSERVE SAFETY RULES AGAINST ELECTROCUTION
BEFORE YOU START
  • It is very important to make sure the boiler is cold and that its power is cut during all the operation to avoid risk of burns and electrocution. 
  • The below intervention can be done with the only use of  a screwdriver and multipurpose pliers available at any home. You also need glycerin.
DISCLAIMER:
Our pictorials are of informative nature and do illustrate works carried out by ourselves at our own premises for our own use and satisfaction.
Readers who envisage to carry out similar works inspired by the below method shall do this at their own risks and liability.

 

How to replace a boiler's safety thermostat:

1- Voltage reaches this boiler, the burner light is off and the main thermostat "clicks" when rotated. All seem fine except it doesn't start. The safety thermal switch shown to the right might have tripped or is broken.
2- Unscrew the cover with your hand and press the button. if the boiler starts, the problem could be in the main thermostat (not covered here). If not, then proceed to the next step.  
 3- Take out the top cover of the boiler (usually no tools required) and test the safety switch with an ohmmeter to confirm the diagnostic. Or, if you have no meter, go to step five, directly connect the wires and power up your boiler to test.  
4- The switch is either fixed to the panel with a single nut as shown here or with an additional couple of screws. 
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​5- Unplug the wires that connect to it. There are usually two wires (like for any switch) but a third wire for ground can be found. 
6- Take out the bulb from its seat. Other bulbs do share the same tube so be careful. Remove any previous sealant like the red silicone shown here.
7-You can now buy a similar thermostat. Do not worry about little differences just make sure it fits easily to the panel and that the electrical connectors are well oriented.
8- Start first by inserting the new bulb in the intended tube.


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 9- Extend the copper capillary tube taking care not to bend it at all and while keeping it in spiral / spring shape. Fix the thermostat to the front panel.
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10-  You need a bit of glycerin. Use the one available in pharmacies. It works.  
11- Fill the tube where the bulbs are fitted in order to cover these completely.
12- Seal the tube with any RTV silicone gasket maker of any color.   
13- If needed, rearrange your electrical wires, insert heat resistant sleeves, change the connectors, etc.   
14- Fit the new cover on the new safety thermostat and refit the boiler panel. Set the main thermostat to the desired temperature.  
15- Restart the boiler and test it. Keep an eye on the temperature gauge. The boiler should reach the preset temperature, stop and restart after a while and so on without having the safety switch tripping.

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