if you do as steve said you cant go wrong.
Bear in mind that if your means of topping up the boiler pressure is by way of an external filling loop
(usually located under the boiler in the form of a flexible braided hose and a valve at either end) then you must remember that once the pressure gauge reads 1 to 1.5 bar (cold) you should close the valves and then disconnect the flexible braided hose as this is a requirement of the current water regulations.
Reason for this is that the filling loop incorporates a double check valve to ensure your heating water does not mix with your drinking water and in the event that the water main pressure droped for any reason i.e a burst main in the street, then the pressure from the heating water which is independant of mains pressure would force its way back through the dcv or as is more commonly known "upstream" of the dcv and thus contaminate your drinking water, so that is the reason why you should always disconnect the filling loop after topping up the boiler pressure.
Hope that explanation hepled
kenny g, July 2008
