Combi boiler leak?

[peted]

I have a Ferroli F28 combi boiler which is rapidly loosing pressure. Everyone is saying there must be a leak but i have checked all the pipe work, rads, boiler and exhaust pipe isn't dripping outside, no leaks found.
My question really is - if i top up the pressure and then turn off both the taps which lead in and out of the boiler (with the boiler turned off) and the pressure still drops, does this cancel out a leak in the system (pipes and rads)?
A plumber has checked the expansion vessel and said its fine and can't see any leaks within the boiler. Can there be any other way it could be losing pressure? With the heating on it lasts about 30 minutes, the pressure drops to zero, then i only have to open the fill tap for a split second and the pressure shoots instantly back up to 1 bar and the boiler will work again.
The plumber is lost. Any suggestions are very welcome.
Thanks

 

[mcmoby69]

sack the plumber and get a boiler engineer.

could be a leaking inside the boiler somewhere

sounds like an expansion vessel problem tbh.

 

[peted]

Is there a test a plumber can do to rule out a leak in the system? I think closing the inlet and outlet taps and watching the pressure should rule it out but no one can tell me yes or no????

 

[denso13]

Yes, isolate the boiler and see if it still drops. Will prove either boiler or system.

 

[peted]

That's what i did, isolated the pipes going out and coming back into the boiler and the pressure still dropped. I've had 2 plumbers insisting there will still be a leak in the system somewhere, but that doesn't make sense to me, if i've got the system isolated?

 

[sooey]

Are you sure the blow off pipe outside isn't dripping? It sounds like a blocked pipe to the expansion vessel or a problem with the vessel.
You can prove it by closing both valves to one rad, draining half the water out and opening up one valve only.

 

[peted]

i taped a bag onto the pipe outside last night and it is filling with water, but i think that's new thing? Will this be the problem?

 

[martyn88]

i taped a bag onto the pipe outside last night and it is filling with water, but i think that's new thing? Will this be the problem?

Which pipe? Copper or plastic

 

[ukrobj]

If youve isolated the flow and return and the pressure is dropping, it may be the "plumber" you had and explained the expansion vessel is "ok" may be talking out of his ar$e.

If the pipe externally is dripping/running then it means the system is over pressurised and its releasing it as a safety measure.

If the pipe going to the expansion vessel is crudded up then it can give you false readings of system pressure. I would advise draining the boiler, check the expansion vessel static pressure (make sure its just air and not water from the valve)

While its drained, check the pipe to expansion vessel to ensure its clear, also might be worth checking the pressure guage too as its a very fine capillary tube and easily blocked.

Rule them out and see if the problem still exists.

PS - might have to replace the pressure release valve now its opened, as sometimes these do not reseal that well.

 

[peted]

Its the copper pipe which is letting water out.
A funny thing happened tonight though. A different fault flashed up and it was to indicate that the condensed water pipe was blocked (the plastic pipe going out into the garden). This happened last winter, when it had been -10 degrees all week and it froze up, but it hasn't been that cold recently. I released the soft rubber hose, which comes out of the bottom of the boiler and attaches to the pipe that goes through the wall to the garden. Quite a bit of water ran out and i left a cup under it while the heating was on, the cup was full in about 45 minutes, don't know if this is normal?
So i think it sounds like i am safe to say it is more likely to be the boiler that's at fault than a leak in the system?