A pneumatic thermostat is a device that is hooked up to a heating system in a home or business setting according to the Dr. Fix It website. The thermostat can be set to react to a specific temperature, either causing an air conditioning system to turn on or a heating system to turn on. The thermostat does this because it is connected to a series of tubes, and can tell the temperature of the environment based on how much pressure is contained in them.
Using an attached dial, a pneumatic thermostat can be set to keep a room as close to a specified temperature as possible. When a room is getting too hot, the air pressure in the tubes connected to the thermostat will be greater. This will cause the air conditioning system to kick on, cooling the room back down to the specified temperature. At this point, the air pressure in the tubes will return to normal and the air conditioner will automatically kick off.
In the winter months, a pneumatic thermostat can control a home or businesses heating system in much the same way it would control an air conditioner in the summer months. After specifying a temperature, the pneumatic thermostat will sense when a room gets too cold as the air pressure in the connected tubes will dissipate. It will then turn on the heating system to return the room as close to the specified original temperature as possible.