UN-VENTED SYSTEMS AND COMBINATION BOILERS
You cannot pump showers on these systems but depending on the system and the quality of the shower you should get a good shower. On Un-vented Systems it is the water mains pressure in your area that is important. The system stores your hot water at mains pressure or regulates it down to about 3 Bar, obviously you have to bear in mind that the mains pressure in your area will fluctuate during the day. Depending on the mains pressure many power showers will work with these systems and most gravity showers. On Combination boilers it is the mains pressure and the Kilowatt output of your boiler that is important. The system heats your water on demand, but the variation of mains pressure and the temperature of the cold water will affect the pressure delivered by the boiler and therefore your shower. Most power showers require at least 24Kw boiler and preferably a 27 or 29Kw one, but most gravity showers can be used on these systems.