These tend to be fitted to the older standard efficiency Saunier Duval boilers and monitor the pressure of the water present within the boiler, they are designed to switch the boiler off if the pressure in the boiler falls below a pre-set pressure normally 0.5 bar or 0.25 bar of pressure. This prevents the boiler form running at low pressures which prevents damage to the boiler.
Saunier Duval Water pressure switch 2000801181

Part: 2000801181

Price: £37.71
Saunier Duval 0020023216 Pressure sensor HW

Part: 0020023216

Price: £44.80