I was passing column in waiting area of Pearson airport when my sweater cought some kind of device on the wall. Device was without cover and looks like old thermostat. When my sweater caught it, it fell off and started to blow air (I think). Maybe someone knows what is this device and why it blows compressed air? Yes, I informed airport employee.
It might have been a old thermostat. In ancient times, thermostats sometimes worked pneumatically. When the temperature got too low, the thermostat would open a valve that released the pressure on some pressurized air. This would then directly actuate the valve on the radiator.
There was a small bleed system so that when the system was closed, it would pressurize over a minute or so. The valve in the thermostat was significantly larger, so that when that valve was open, the pressure would be near ambient.
This kind of system was common in large buildings where a boiler would be running most of the time, since some zone somewhere always wanted heat. The hot water or steam was always available, so the thermostat and valve combinations controlled whether it would flow thru the radiator for that zone only.