I would like to bring up the reason why it is so important to correctly pipe size with today’s boilers
Since combination boilers hit the UK market it as always been required to take 22mm pipe work to within 1m of the boiler, this was to give the required heat output for the hot water
However if the gas pipe work was undersized because of the burner set up (normal atmospheric) as long as the boiler had the minimum gas pressure for it to operate the low flame it was safe ,all that would happen, is you would have tepid water or fill a bath in 2 hours if you wanted hot,
When the gas valve opened from low flame to give high flame the flame would increase to whatever maximum gas pressure was available giving you a lucky dip max water flow rate. The heating would work OK
As time went on boilers became more efficient and delivered better hot water with lower gas pressures/small pipe size to the point were most 24kw normal atmospheric burner boilers could deliver adequate hot water requirements (which suited a lot of builders, stick it on the wall, even comes with power flex, plug on end and away you go but I digress) again quite safe as long as gas pressure high enough to provide low flame all ok.
However now we are fitting a new kind of boiler, efficient and to achieve that efficiency we have a new burner lay out, the gas valve is not operated with modulating solenoids but by the fan speed. For low flame the fan runs slower giving lower pull on gas valve releasing less gas and for high flame the fan speed increases giving bigger pull on gas valve releasing more gas
This is why it is so important to have to correct gas pressures, because if not, the fan go’es to high speed and opens the gas valve fully and as the fan is designed to remove the burnt gases for a high flame at this point, if gas pressure/flow to low and only small flame available, the small flame will be pulled from the burner by the fan at high speed and extinguished or partly extinguished ,resulting in burner pulsating, incomplete computation causing unburt gas to be expelled through flue possible mini explosions in heat exchanger from unburnt gases and damage to the burner itself.
Of cause the safety devises on the boiler will kick in and put the boiler to lock out or into another circle but these devises are not there to be used in the normal operating boiler mode and the boiler should be able to operate normally without them
I have explained the above simply I hope, to explain why pipe sizing has now become a big issue, and why people have got away with undersizing in the past. It really is important to the safe running of the new era of appliance or infact the running of them at all that pipe sizes are correct.It is not just about gas blokes hitting the reg book