Combi boiler,have need of to fit external expansion vessel.8 litre or 12 litre? ...
Answers: Depends on how big the system is
expansion vessel sizes depend on how much water is contained by the system , the charge pressure of the vessel and the pressure releif valves operating pressure , folks in the know use a chart to work out the volume of hose in the system, consequently after deciding some other factor a vessel and charge pressure is decided . if you own a combi it should have its own expansion vessel so i cart it that your combi boilers is either not big plenty or it has messed up if its failed due to age only get a resembling for like vessel classification same size your old 1 will hold it printed on it .if its not big enough a 12 litre should suffice along near your existing one . if its failed and to much hassel replacing the one contained by the casing later 18 litre will do . no harm will be done if you step to big , but to small and the system will not be able to accomodate the expansion of the heated sea