Contributed by Patrick Gordon of Calafrica (

Patrick Gordon takes a look at pressure control valves.

Nominal rated system pressure Pressure control closing pressure
(below system pressure)
Pressure control closing pressure
(below system pressure)
100kPa (Blue) 70 – 85kPa 95 – 100kPa
200kPa (Black) 140 – 170kPa 190 – 200kPa
300kPa (Brown) 210 – 255kPa 285 – 300kPa
400kPa (Red) 280 – 340kPa 380 – 400kPa
600kPa (Green) 420 – 510kPa 570 – 600kPa

Q: Why does my Pressure Control Valve not deliver the pressure that it is rated at?

Patrick Gordon

A: Before we answer that question let us consider the functions of the valve. The valve has two main functions:

The valve will reduce the mains pressure to the desired system pressure. Normally the lowest maximum acceptable pressure of the components used in a system.

The valve will also relieve any excess build-up of pressure for whatever reason. Typically, this would be due to the heating of the Hot Water Cylinder.

When you first measure the set pressure of the valve, you will see that the pressure will always be lower than what it is rated at. A 100kPa valve will only deliver pressure to the value of 70-85kPa. This allows for a certain amount of build-up of pressure before the valve comes to the second function, the relieving of excess pressure when it reaches 95–100kPa. If this were not the case, the valve would constantly be opening for expansion. These mentioned values can be used as a percentage for the other pressure ratings of the valves., for instance, a 400kPa valve will have a set pressure of 280–340kPa and a relief pressure of 380–400kPa.

All the control valves in the system are related in the differences of their pressures so that there is a uniformity in the level of safety that they provide. Do not tamper with the settings of a valve by placing coins etc. on the spring as this will destroy the engineered safety that is built into every unit. When presented with a problem in an installation, contact the manufacturer before you make any modifications.

Remember it is not knowledge that is power, but how we apply that knowledge that makes us strong.