I’m working on connecting the resistor/heating element of my boiler, and I’m using the thermostat shown in the photo below. From what I understand, this is a 4-pin thermostat.
I measured continuity with an ohm-meter and found the following:

• The two terminals on the left have continuity with each other.
• The two terminals on the right also have continuity with each other.
• Left and right sides are not connected to each other.

On the boiler itself, there are L and N coming from the electrical panel, and of course ground.
I connected one terminal from the left pair and one terminal from the right pair with the heating element. When I switch ON the circuit breaker on the electrical panel, the resistor heats up and the water starts warming. When I switch the breaker OFF, everything stops. So far, it behaves exactly as expected.
My question is:
What are the other two unused terminals on this 4-pin thermostat supposed to be connected to?
Any explanation about the typical wiring and function of these 4-pin boiler thermostats would be very helpful.
Thank you!

 

Good morning, @ChrisTsall...

I think these are duplicate terminals; they allow the connection, via an additional pair of wires, of a pilot indicator (lamp) on the panel, identifying, for example, the state (ON/OFF) of the thermostat.

 

I suspect that thermostat is designed to be used on 120 volt or 240 volt boiler. When using 240 volt both line one and line two need need to be broken for safety.

 

Any explanation about the typical wiring and function of these 4-pin boiler thermostats would be very helpful.

2 ways to wire the thermostat.
Terminals 3 and 4 are the NC safety contacts that open if the thermostat overheats disconnecting both lines going to the heating element.

 

2 ways to wire the thermostat.
Terminals 3 and 4 are the NC safety contacts that open if the thermostat overheats disconnecting both lines going to the heating element.
View attachment 360373

perfect

 

manual

 

IF the device in post #1 is the same as the ones pictured in post #4, then there is an internal circuit diagram molded on the outside, and an adequate resolution photo will allow us to see the inside circuit. ONLY THEN can you possibly be sure of a correct explanation.
OR, read the manual that came with the thermostat. As others have suggested.

A year ago I did emergency repairs on a domestic water heater. Those thermostats both had their contacts welded closed, and so there were 4500 watts boiling water and spewing steam.
Thus the calls for emergency service.
Those thermostats did not look like this one. Not at all,

On all installed heating systems there should be circuit diagrams to show both the connections and the controls.