Alexander wrote in message
...
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.

On Sun, 10 Jul 2005 00:47:32 +0100, andrewpreece wrote:


Alexander wrote in message
...
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.


Thus:

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html