Gas pressure and boiler problem

Nominal 21 +/1 mb at the meter outlet; 19 to 23 mb, depending on flow rate.

I'd suspect the gas pressure is a red herring. It may be the cause of the fault; possibly the supply pressure drops at times of peak demand. The explosive ignition can be caused by water in the combustion chamber, either a leak or condensation from the flue running back. I wouldn't think it likely that it is caused by a slightly low gas pressure. Did the fault coincide with the cold weather?

Also spark generator wiring had been changed and makes a difference, allaegedly.

That is the regs. It is probably the case that all have never been on, but the test is with all on.

What type of burners is it? pre-mix? If so, theses are susceptible to poor pressure and fluctuations.

With respect, this is wooly thinking. A system will always be designed with a safety margin so if, for example, it's specified to run down to 18mb it will probably run happily at 16mb *when every part of the system is new*. As the system ages, and various components deteriorate, this safety margin will be eaten into. Eventually it may only work properly at the specified 18mb.

You can't then turn around and say "but it worked before, so it can't be the pressure". Just because the boiler used to work at a lower pressure than it was specified to, it doesn't mean you can expect that always to be the case.

Richard.

So what might -vary- in that would require more pressure?..

Burners wear - the holes can get larger in time, affecting performance and efficiency.. A burner's fan can increase or reduce speed affecting performance as well.