Our Friend the Pressurestat

By Hylan Joseph

All steam boilers need some mechanism to regulate boiler pressure to maintain a safe machine. The most common way of regulating steam boiler pressure is with a pressurestat.

A pressurestat is a mechanical device that operates a switch, or set of switches, to control voltage to the elements as the boiler heats. Think of them as a switch that is connected to a balloon that inflates as the steam pressure increases.

When the device is cold, the balloon is not inflated, and the switch is held “on” by a spring. As the pressure increases, the balloon inflates until it presses the switch and turns the switch “off.” If that switch is connected to the steam boiler heating element, then the element will have power when the pressurestat closes the heating circuit (during heating).

When enough steam pressure has been generated, the balloon inflates and turns the switch “off.” When the steam is used, or the boiler cools, the spring pulls the switch back on and the cycle continues.

In practice, the balloon is a metal diaphragm, and the switch may be a single switch that activates a larger switch called a contactor (to be covered later), or it may have a set of heavy switches activated by a larger diaphragm. Either way, the principle is the same, using steam pressure to open and close the heating circuit.

Next newsletter we will review how a contactor works, how these two components work together, how to test them, and how they fail.