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I have a question about the thermostat you are using.

Did you happen to try to connect a Smart Thermostat that requires a C terminal along with the R and the W terminal on the thermostat? That may be the reason for your POP

Get a whole new fan center control. The pop you heard was likely the transformer. And the base of the terminal board is lifted off on the right side. Shut the power off, stick your head in there and give it a sniff. Probably pretty rank. Add an in-line fuse like Ed said, because it could be it was the transformers time to pass, or there could be a short somewhere.

That relay is for operating a fan when used on a old gas furnace that has been upgraded with a central AC or with a boiler that uses a circulator to move water around the system. Since this is a steam boiler, you do not need the relay since there is no Fan or Circulator. That is why Weil McLain sells that without the relay for steam boilers 510-312-166 and with the relay for water boilers 510-312-167. I used to be the Weil McLain part guy at the supply house I worked for. So any WM part for boilers from 1996 and before are memorized. The Honeywell R8285A1048 would be a lower price replacement for that part with the relay included. and the AT72D1006 is even cheaper but does not have the G, Y, and W wire connectors on it. You need to use wire nuts in lieu of screw connectors.

Impressive😊

Only difference is the boiler water has been heated and the oxygen has been driven off. I admit that the boiler water is still exposed to oxygen while sitting in the idle boiler.

But I will use an example. We all know that screwing a plug into a steam boiler and no water treatment used the plug will last a long time.

I was call out one Christmas Eve to a post office when a plug on a domestic water tank that was installed 3 years prior had let go and flooded the boiler room putting the boiler and burner that were at a lower elevation under water. Although the burner was completely submerged the lights on the control panel were still on!!

Anyhow my point is that the installer had used a black plug in a DHW storage tank (big no no) and it only lasted 3 years with the oxygenated water and it was a hot water storage tank.

So I think the oxygen has a lot to do with that.

But I agree it looks to me like the use of 8-way is very impressive in maintaining the water quality.