Three years ago our CH system was completely renewed by a reputable gas-safe registered local company. The job included a new Vaillant combi boiler and new gas supply pipework. When installed, the working gas pressure during HW operation was recorded as 20mb (CH operation) and 19mb (HW operation). The boiler has been working fine for he last three years, apart from a couple of minor problems which were sorted by Vaillant. Recently the Vaillant engineer who came out to do the third annual service said that the working at pressure at the boiler was only 13.5mb and, if it had been slightly lower, he would have had to switch it off and declare it unsafe. He also said the pressure at the meter was about 0.1mb below what it should be (i.e. not helping the situation, but not enough to indicate a regulator problem). My question is can a new gas pipe become partially blocked so quickly, and what are the chances of unblocking it without having to replace the pipes (which would involve lifting floors etc)?