How Does A Pneumatic Thermostat Work?

Function

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.

Air Conditioning

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.

Heating

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.

Older thermostats were controlled by a device called a thermometer coil. Thermometer coils are made out of bimetallic strips--two strips of metal attached together. When the temperature goes up, metals expand but different metals expand at different rates. That makes the coil expand as it heats up and contract when it cools. When it contracts enough, the coil bumps into a switch which turns on the heater. As it expands, it moves away from the switch, turning off the heating again.

Bimetallic thermostats are large, bulky and not very accurate, and have been largely replaced by digital thermostats. Digital thermostats use thermistors, special resistors which are sensitive to temperature changes.

Most resistors resist the flow of electricity at a constant rate, but thermistors change as it gets hotter or cooler, slowing or speeding the flow of any electric current fed through it. In a digital pneumatic thermostat, a current flows through the thermistor to a meter, which calculates the temperature based on how quickly the resistor lets the current flow.