>I have a combi boiler feeding two small holiday cottages. Total of two
>large radiators, two small radiators, two showers and kitchen sinks. There
>had been a history of loss of pressure, so when the cottages were renovated
>and the floors taken up (suspended floor) every joint was checked and there
>was no evidence of leakage or staining so the floor was re-laid and tiled.
>Everything pointed to the Worcester 20-25 boiler which had other problems
>(it had already had it's expansion vessel, safety pressure valve and auto
>air vent changed).
>Symptoms were that if the boiler stayed warm it was ok but if allowed to go
>cold (power cut for example) the pressure would go to zero, no sign of any
>pressure on the gauge what so ever.
>
>So we had the boiler replaced (Grant Vortex 21 combi). Then we had a power
>cut, and ARGH !!!!!! the pressure fell to zero. Re-pressurised and it was
>ok for a few days - then the oil control solenoid failed (infant
>mortality?), the boiler went cold, and again the pressure went to zero
>indicated on the gauge. Last night again a repeat - system not working as
>no pressure this morning - (it was fine when I checked it at 20:00 last >evening). >
>So logically the leak is most unlikely to be the new boiler as the symptoms
>are pretty well identical to the old one, which begs the question, how can
>a leak suddenly develop, allow total loss of pressure, yet seal fine when
>the system is topped up and show no evidence?
>
>Andrew
After a huge amount of faffing about I think we've found the leak :)
We put isolator valves in the two 'flows' and common 'return', blanked off the riser pipes to each end of all the radiators, and pressurised each run having a gauge on to see pressure drop. Nothing, zilch, no leaks !
So when everything else has been eliminated, Sherlock tells us what remains MUST be the problem. So a very close look at the brand new boiler eventually reveals that the flexible pipe to the integral expansion vessel is leaking at it's 'tap connector' Dismantling shows the rubber washer to be graunched in two, presumably in the factory, and is leaking when cold, but sealing when hot. Exactly the same symptoms as the previous boiler showed, but in this case the leak seems to have been into the combustion chamber from the water jacket. Early days yet, but it does seem we've bottomed this one after a lot of heartache :)
Andrew