I have the central heating system pressure of my combi boiler set at 1 bar but about once a month it drops to zero and I have to top it up. There does not appear to be a leak so does anyone know why this happens?
An old chestnut this: it comes up about once a week. See Ed Sirret's FAQ if you can track down a post by him. Otherwise, it's a 95% chance that the overpressure relief valve is opening when the system pressure reaches 2.5/3 bar, and a fair bit of hot water is blasted outside via the overpressure vent pipe thingy. The system pressure rises to such levels because your air reservoir ( which cushions the CH pressure when the combi heats up ) is either empty or the diaphragm inside has split. If it has split the little bicycle valve on the air reservoir will leak water if you unscrew it. If it is the reservoir, eithe pump it back up with air if the diaphragm is OK, or replace with a new one, either in the original position, or somewhere along the CH pipes if that is more convenient.
The second possibility is that you have a 'drying leak', which only leaks when the combi fires up, and the heat of the CH loop evaporates it as it leaks, leaving little evidence.
Andy.
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