For all cold-mains-fed combi boilers, it is important to ensure (before/at the time of installation) that the water main is large enough to deliver adequate water to the boiler whilst other cold outlets are in use (eg flushing toilets, washing machine filling or cold taps being run).
If the water main is insufficient, cold water may be ‘pinched’ from the boiler when more than one outlet is used.
The chances of this happening could be reduced by giving the boiler priority over all the other outlets.
This could be done by piping it up as the first draw off from the mains.
The performance of any mains fed hot water system depends on the mains water supply offering an adequate dynamic pressure and flow rate.
This is because the flow rate must be sufficient to supply hot and cold water simultaneously.
Whilst there maybe sufficient mains pressure, the flow rate is dependant on the size, type and condition of the incoming main.
It is important not to confuse pressure with flow. Also, dynamic pressure will be less than the static pressure.
A water pressure reducing valve may also be required to protect the appliance from excessive incoming mains pressure.
Maximum incoming water pressure is normally 10 bar.
Source: Worcester Bosch