9.8.5. Excessive available pressure at source<br>

### 9.8.5. Excessive available pressure at source

Often this is caused by pressure drop on control valves, which are not matched to the flow rates. Depending on the values in the catalogue, permissible pressure drop on these valves may also occur – then finding out the cause is easy. Some valve types, however, can (in theory) carry significant flow rates, and their permissible pressure drop is set at a high level. The use of such valves on terminal units with high flow rates (resulting not only from high thermal output, but also from low temperature drop) will not cause errors associated with the exceeding of valve operation conditions, but will exaggerate the required available pressure. In such case, if possible, the type of valve should be changed to one with wider Kv range.

Another cause of this may be the use of pressure differential valves with high minimum value of stabilised pressure differential. For instance, when the pressure drop on the stabilised portion of a system is 11 kPa, and the valve used has a stabilisation range of 25 to 50 kPa, then the pressure drop of the system will increase by about 14 kPa plus pressure drop on the pressure differential valve and associated valve (to which the impulse tube is connected). In such case other type of pressure differential valve should be used and/or it should moved towards the source. Of some help may also be the specification of lower DPmin values for pressure differential valves. However, one should also take into account the valve manufacturer’s recommendations.