**Pressure is used to circulate liquid in a heating or cooling system and is induced by means of pumps. **

**The pressure in a system is positive in relation to atmospheric pressure.**

Pressure is used to circulate liquid in a heating or cooling system. Pressure is increased by means of pumps. The pressure in a system is positive in relation to atmospheric pressure. Pressure has numerous units of measurement, though those most commonly used are: Pa, kPA, bar and mVC.

The ratios of the units are:

** Bar and kPa **

1 bar = 100,000 Pa = 100 kPa.

Other units include lbf/in² = psi (pounds per square inch) and mm mercury column (mmHG). These units are specified as:

1 psi = 6895 Pa

1 mmHG = 133 Pa

** mVC**

The unit mVC (meter water column) is dependent on gravitational acceleration, or g, which varies according to the distance from the equator.

When using mVC, the approximate value 1 mVC = 10 kPa is often used, which equates to a gravitational acceleration value of 10 m/s^{2}.