Thats a pretty significant leak. My guess would have been there is a section of pipework underfloor leaking. Have you checked all drops below ground level?
yes I have checked all pipe work, which is ALL run in the downstairs ceiling, You say a significant leak, the four plumbers so far reckon about a tea cup everytime..!
Can you understand the frustration.. 4 plumbers (corgi type) all boards up 4 times, about £120 quid down and new pump, auto vent and expansion vessel.
I'm going to install gate valves on the boiler and see if the leak is in the boiler.
All 4 plumbers say that i would just have a new boiler..! Seeing as this one is 8 yrs old..!
Noooo! please don't use them 'orrible gate valves. Invest in full bore isolation valves, eg screwfix 45277. You could do the same on your downstairs drops to see which part of the system is to blame. Divide the problem up into smaller bits.
My advice is not to use leak sealer unless its your really final resort.
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