The problem is. my central heating system(combi boiler) loses pressure, albeit slowly over a few months . at first i thought it just some air in the system taking a long time to escape, but i've noticed there is a lot of vapour coming out of the outside outlet pipe when the boiler is in operation. is this right?, the system is supposed to be a closed off pressurised sytem, so i guess it must lose water in the form of vapour . am i supposed to be repressurising it so often?