On 2008-04-24 20:39:05 +0100, John said:

Hello folks,

we have a Vaillant Turbomax at the Scout Hut which has not been the most
reliable, however the current issue is that the water pressure drops on
an irregular basis! There are no obvious leaks but some evenings it is
down to 0 - when we fill it up the pressure seems to hold (and the
boiler works), it may then be OK for a week before suddenly dropping to
0.

I suspect the Pressure Relief Valve since the relief pipe seems to
always be wet and there is now moss growing on the path underneath the
outlet, but I would value expert opinions.

This can be because the pressure vessel is on its way out. You fill
the system, it gets hot, but instead of the water going into the vessel
it is expelled through the valve.

It may also be that the valve is crudded up.

If this is a boiler with integral pressure vessel, then it should be
possible to effectively "abandon" it unless it has corroded through and
is leaking. Replacing these is usually more expensive than an
external pressure vessel.

Often the least expensive solution is to install a sealed system kit
consisting of a pressure vessel, manifold, pressure gauge and relief
valve. These are sold as a package for quite a bit less than the sum
of the parts. One of these could be fitted in place of the existing
pressure relief valve.