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Pressure switch and the way the mercury switch is installed and the location of the spring and the bellows, it will function as a shut off on low pressure — that is, if the water level falls in the system, the switch will open.

Now how low is a very good question!

@109A_5 The knife switch does control the boiler. My doorbell transformer is at the front of the house. I’m not touching anything. As I said, I’m just happy with the vintage showpieces. When my thermostat calls for heat, it fires right up. That’s all I need. As the people who put this in is long gone, I already get the speech from local HVACs how I need to replace it because it’s old. Not because of issues, it’s just old 🙄. So I am not about to mess with something and it stops firing.

OK GW…. there is this thing called a power switch that you can toggle off and on in order to make the chance of letting factory installed smoke out of transformers. They are very easy to operate. For the most part if the electrician did his job properly, you just toggle the handle to the down position before you put your screwdriver inside the control. here is an example of what the switch might look like.

Hope this helps you in the future.

Sincerely

Mr. Ed.

@EdTheHeaterMan

When I read the post title, I had visions of what your comment would be😊