These tend to be fitted to the older standard efficiency Ideal 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.
Ideal 172424 Europa Low Water Pressure Switch

Part: 172424

Price: £35.04
Ideal 075176 Response Low Water Pressure Switch

Part: 075176

Price: £22.50
Ideal 175596 Logic Water Pressure Transducer

Part: 175596

Price: £28.82
Ideal 173227 Water Press Switch Kit Response

Part: 173227

Price: £38.61
Ideal 176480 Low Water Pressure Sensor

Part: 176480

Price: £31.01
Ideal 065876 Commercial Viscount Pressure Switch

Part: 065876

Price: £15.86