every now and again the pressure drops on my boiler and i have to top it up. where is the water pressure going as i don't have a leak?thanks
Well I would have to argue with you there then. A combi boiler is a sealed system if you have to top it up with a connection so it cannot evaporate. It may be weeping through your safety valve or automatic air vent or at any one or more of numerous connections in your system. Try pressurising to 1 bar or 14lbs UK/US leave off and check under the radiator valve caps and around the connections and bleed valves. If none of those then suspect pressure vessel is failing or needs to be pumped up. You only need to lose a cup full to go flat and with a hot system you will not see a leak.
