Low gas pressure at boiler 15.5mb

We had a new boiler Valliant ecotec II Plus 637 fitted approx 2 years ago. This was fitted in attic so gas supply pipe goes up the side of house and then enters attic and goes across the width of house and then to boiler which down the other side wall. Basically run from meter is approx 17m. There are about 6 turns on the journey.

Last year I called someone to have boiler serviced and he noted that the pressure was just under 18mb. He said it was borderline but was ok. At meter point it is 20mb.

This year he has found that the pressure has dropped to 15.5mb at boiler.

The only change I can think of is that in interim we extended the kitchen and gas hob was moved to approx 6m further away than original position. The gas hob pipe is a T off the pipe leading to boiler. Could this have caused the pressure drop? Service guy claims that not.

Bottom line is that the service guy is suggesting I replace the boiler supply pipe from 22mm to 28mm pipe for around £500. Is this the correct thing to do?

Any advice appreciated.

Yes, and should have been done on installation. The last two engineers didn't measure things properly.

Assuming this engineer is measuring from the correct place.

Sounds like the service guy has learnt how to set these boilers to maximum rate since their last visit.

If the Boiler will operate at max and Gas rate correctly then you can leave it. Vaillant won’t mind as long as it’s above 13mb and it doesn’t affect the operation of other gas appliances

^^^That's until there's a fault that they can attribute to low working pressure, such as intermittent ignition lockout.