Your combi boiler need be at the accurate pressure for the hot water to flow correctly between the radiators, particularly if they are on different floors. Usually, the perfect pressure is between 1 and 2 bars, but this may vary from boiler to boiler. Ask your heating engineer.
Combi boilers are both a high-competence water heater, and a central heating boiler, united in one compact unit. No separate hot water cylinder is essential, which saves a substantial amount of space, while supplying the comforts of central heating. With combi boilers installed, heated water is provided rapidly, and unlike conventional systems, no hot water is heated and stored, but is delivered as needed to taps and showers, and the same heat is distributed to the air circulating system. The water is fed from the mains, so the water pressure is generally greater.
Another advantage is a great saving on the price related with installation. Since there is no tank to be installed on the roof, and less piping is needed, there is important reduction in the time and cost of installation.