Can anyone offer me some advice. I have a microbore heating system that doesn't heat hot water or one radiator, and have been advised that the system has a negative pressure. I require to replace the boiler to a combi and replace all radiators with single inlet and outlet valves as opposed to the current single valve. The pipes for this would be visible.I've been quoted £3700 but am concerned that if after all of this work, that the original underfloor pipework will clog up and also need replacing Does this sound right? Should this solve my problem? I've previously tried power flushing but to no avail.Thanks for any feedback or advice.