What Is a Pneumatic Thermostat?

Andrew Kirmayer

A pneumatic thermostat is a device that uses a tube filled with air to control the temperature of an air conditioning or heating system. When the temperature is set, the thermostat reacts to changes in temperature by reading the level of air pressure in the tube. If it gets warmer than the setting specified, the pressure level will trigger the system to activate until the air is cooled to the desired temperature. A heating system will activate in the same way if the temperature is cooler than the setting on the thermostat. When using a pneumatic thermostat, a mechanical or digital dial on the outside is turned to set the range.

Thermostats can control heating and air conditioning of a home or building.
Thermostats can control heating and air conditioning of a home or building.

The two main kinds of pneumatic thermostat include direct and reverse acting systems. Direct acting types of thermostats increase the pressure in the tube in proportion to the rise in temperature. In a reverse acting pneumatic thermostat, the pressure lowers when the temperature increases, while rises in pressure occur as it gets cooler. A controller serves as the connection between the pressure tube, the thermostat, and the rest of the system. This interface can be an electronic circuit, an actuator, or a pneumatic-electric switch that has a direct or reverse configuration.

Problems with the thermostat include blockages in the restrictor or nozzle, or a need for calibration. The supply or exhaust valve can be clogged, or the connections for the control and supply pressure reversed during installation. Repairing a pneumatic thermostat, if the control pressure stays at zero, typically requires nozzle cleaning or recalibration of the device. The restrictor might also need to be replaced. If the control and supply pressure remain basically equal, dirt can be clogging a nozzle or valve, so the blockage needs to be cleared and the thermostat re-calibrated.

When air leaks from the exhaust port of a pneumatic thermostat, then oftentimes the supply and return connections are wrong and need to be re-connected. The air going into the thermostat should be clean. Pressure levels should also be constant at around 20 to 30 psi, or pounds per square inch (about 1.7 to 2.1 bar). This can be measured using a test gauge along with a needle adapter.

When maintained and calibrated correctly, the pneumatic thermostat will respond accordingly to changes in temperature. It will also provide the desired control to keep the thermometer within the necessary range. The device can be used in the summer or winter, and its pressure levels can be set to activate the heating or cooling systems as needed.

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