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I have had a few issues recently with an electrical boiler and I have replaced a few electrical parts (relays, time delay switches,...) to keep it running.

Doing so, I noticed that the internal wiring was quite different from the wiring diagram on the panel cover. The diagram includes both a 120 VAC relay and a 24 VAC relay, while the boiler as actually wired runs only with the 24 VAC relay.

The question that came to my mind is: Does the actual wiring include all the safety interlocks required for the safe operation of this 20 kW electrical boiler?

To answer that I started studying the wiring diagram, but there is something I cannot quite understand: the 24 VAC relay is energized when the 120 VAC relay is activated, AND the 120 VAC is energized when the 24 VAC relay is activated.

Take a look at the wiring diagram for this 240 VAC, 4 elements, 20 kW boiler.

wiring diagram

I cannot make sense of the operation of both relays. My question is "Why is it like that and how does it work?"

Thank you very much for your insights on this.

Note: I first posted this question on diy.stackexchange.com, but did not get any help.

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    \$\begingroup\$ You will probably find it a big help if you convert the wiring diagram into a schematic diagram which, if you lay it out well, will reveal the purpose of each part of the circuit. The relays have no part numbers so it's hard to know whether the contacts are normally closed or normally open. There's a schematic button on the editor toolbar and it is very easy to use. \$\endgroup\$ – Transistor Dec 23 '18 at 9:08
  • \$\begingroup\$ To add some thoughts to @Transistor 's comment: The schematic depicted is intended to resemble every single physical wire inside the device. It enables service personnel to follow the wiring to track down malfunctions. It is not very useful to understand the design of the circuit itself. \$\endgroup\$ – Ariser Dec 23 '18 at 14:18

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