What exactly happens when you flip the emergency boiler switch off?

bipbap

I know it cuts the power to the boiler but what exactly happens to the gas flow? How does it powering off cut the gas from flowing and make it safe?

bburd

Turning off the power to the boiler also turns off the power to the gas valve,  which will close if it is working properly.

JakeCK

120v from your breaker panel runs through that switch and then to the boiler where it is sent to the circulator, transformer, and occasionally other line voltage devices. The transformer puts out low voltage that powers the gas valve, and other equiped devices like thermostats, zone valves, and auto dampers. Many of the safeties are wired in series with the gas valve so if they trip they cut power to the gas valve, shutting off the flow. The auto damper is also wired to the gas valve circuit that closes when the damper is fully open to prevent firing with the flu closed.

This is true for at least conventional ci boilers. Newer computer controlled ones maybe different? 

HVACNUT

If there's a standing pilot on the boiler, furnace, or water heater, there will always be some gas flow regardless of the switch. Shutting off the gas valve is the only way to stop flow in that situation.

retiredguy

All the emergency shut off switches that I saw were in a school or commercial building that was required by the boiler code in that area. Most of the time they were "break glass" stations similar to those of a fire alarms. They had to be tied the 120volt input control power to insure that the boiler or boilers would shut down when the glass lens was broken. They also had to energize a light or some kind of visible signal so everyone would know what had shut down the boilers. Break the glass with the little hammer that was attached to the break glass station and all control power was shut down to all boilers in that room.

A little funny; we had to test these when starting up a new job or when performing a yearly service. On one occasion, I broke the glass but the flashing light did not come on and the boilers did not shut down. When I heard the sirens from the fire trucks I instantly knew there was a problem. Someone had wired the fire alarm into the break glass station instead of the boiler control station. We found out that the fire alarms worked. The fire company was not impressed and someone got a bill for the mistake.

Jamie Hall

I rather like the idea of testing these things. Probably should be part of the annual checkup. Why? Because there's always a chance that someone has decided to protect their equipment with an uninterruptible power supply, which won't shut down.

Someone also asked abovt newer computerized equipment. Hopefully it is programmed to restart gracefully when power is restored. However, it is worth noting that pulling the emergency switch will defeat any post-purge or orderly shutdown procedures. Not usually a problem, I would think, but...