This type of pressure switch functions using the build up of steam pressure within the main boiler. A feed is taken from the top of the boiler and fed to the pressure switch. As the steam builds up in pressure from the heating of the water inside the boiler, a diaphragm inside the switch then expands, at the operation point set by the pressure adjustment spring, the contacts open and power is removed from the element. The power cycle above is then repeated as steam or hot water is used from the boiler.

For: Cheap to implement and simple operation.
Against: Wide hysteresis of around 0.15 bar due to the type of components used in the construction.

Here we use a temperature sensor mounted towards the top of the boiler, the electrical signal from the sensor is then fed back to the machines control circuit which then supplies either a DC or an AC feed to operate a solid state relay that supplies the power going to the element.

The operating principles are in line with the diaphragm system above, but due to full electronic control we have a much tighter control of when power is supplied to the element and generally longer life for the element due to power only being supplied when the mains supply is at its zero point in the phase.

For: Accurate heat control, no moving parts, reduced running cost.
Against: Greater skill level in fault finding