Our expert responds #29

Question:
- Is mixed water supply of a hydrant network allowed?

The mixed water supply of the hydrant network is allowed in compliance with what is contained within UNI 10779.

Mixed water supply is accepted:

• Only for activities falling within a hazard level 1
• If only the internal protection is installed (absence of UNI 70 above-ground or underground hydrants).

For such cases it will therefore be possible to make the water supply as a derivation from the general water supply system of the building.

The following additional conditions must be met:

• The flow and pressure performance of the system will have to consider the joint needs of the fire-fighting and sanitary water systems
• The aqueduct reliability document must be requested
• A non-return valve or other equivalent device must be inserted in order to prevent the return of water from the internal network to the external water network  
• There must be complete independence of the fire-fighting system starting from the water supply point – see fig. A1 reported in Appendix A of UNI 10779

In the event that the value of the pressure required from the utilities is not reached, it will be possible (necessary) to install a pumping unit.

The pumping unit, in this case, must not have to meet the requirements of UNI EN 12845, as can be clearly read in point A2 of UNI 10779 which states:

• “In the case of mixed water supply where the above requirements are met, the provisions of point A.1 shall not apply.”
  Point A1 in turn reads "For the construction of water supplies, the corresponding requirements of UNI EN 12845 are applied with the following additions".
   

So in this case nothing in UNI EN 12845 for the pumps, and consequently also the room does not have to follow the UNI 11292 standard