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If it matters, I live in Ireland. We have an oil boiler powering a two zone hot water radiator heating system and a separate electric hot water tank for tap water.

Outside our house in the corner of our property we have a fenced in area. Inside that is a small metal box. I can remove one side of that box to access the boiler. When I look inside the box, up in one of the corners is this set of controls.

I assume the dial controls the temperature the boiler heats the water to, but what is the push button for? Please note, it is not the "reset" button, which is located on the boiler itself and glows red when the boiler pilot light is unlit.

Any ideas?

boiler control

  • Is that the button you push to open the gas valve for the pilot so you can light it? – longneck Jan 10 '18 at 16:10
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This is a thermostat (or aquastat) that regulates the temperature in your boiler. Appears to be an IMIT TLSC model. Here's a similar product on amazon.

According to the instructions below, that button is a manual overheat reset. This thermostat has redundant controls - a dial you can turn (set 0-90C), but also a safety shutoff, designed to turn off at a temp just above the dial's maximum (90-110C according the instruction sheet below). That way if the dial-operated controls get stuck on somehow, there is an extra check to avoid overheating your boiler. (Overheating the boiler is bad because it creates pressure, leading to leaks or explosion.)

English Instructions

The button is usually covered by a removable plastic cover, which appears to have been removed here.

It's possible you also have a reset control located elsewhere on the boiler, e.g. on your oil burner. A reset control on the burner would cut off fuel to prevent overheating. The pictured thermostat's control would cut off the electrical signal to run the burner. These systems are not entirely redundant - you're protected against a wider range of failures by having both.

  • So if I understand correctly: The boiler should respect the dial and heat water to that temperature. If something goes wrong, and it heats the water toward dangerous temperatures, there is a safety that will shut the boiler off. This switch is to reset that safety? – raydowe Jan 10 '18 at 16:21
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    Yes. To be extra precise, the way this thermostat works is that it has a probe into the boiler, and when that probe senses a temperature below the set point, it provides an electrical signal to your burner that causes it to burn fuel to heat the water. There is also a safety cutoff feature here that would cut the electrical signal even if the dial-based cutoff fails, like a circuit breaker. This button resets that safety cutoff. – Shimon Rura Jan 10 '18 at 16:27

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