Gas pipe sizing question

Razor900 said:

But it doesn't matter what you call it John with all due respect you aren't competent to advise the OP are you?

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Last I checked I don't need to be competent to advise them that plasterboard fitters are not gas qualified

DP said:

Which number is correct- boiler model or the gas consumption ( in which case will not be 28kw)

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I’m so sorry I am such a numpty! The boiler is a 38cdi not 28 so that probably changes things completely. The gas flow rate given is 4.2 IIRC.

Since my original posting I have spoken to Gas Safe and they said it is correct that the gas pipe can go behind the dot n dab wall as long as there is a continuous length of adhesive both sides of the pipe. Problem is the dab thickness ( I suspect) will be less than the depth of the pipe.
The Gas Safe guy also said that I could batten the joists and drop the ceiling. He used a fancy name for the airflow within the floor void and said it would be considered adequately ventilated.
Thanks to all who took the time to reply and sorry for the confusion regarding boiler model.

John D v2.0 said:

..... certainly does through the vertical joints of a blockwork wall.

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Not in this blockwork. The father in law did it spot on. Had comments about how good the blockwork was from the BCO who exclaimed “did he think you were painting it”!

Bodgitnscarper said:

Not in this blockwork. The father in law did it spot on. Had comments about how good the blockwork was from the BCO who exclaimed “did he think you were painting it”!

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It's not an indication of it being done badly. mortar shrinks when it dries. Horizontal joints would be ok but the verticals something has to give.
If you used mortar with no water maybe it's be ok, but you have to render ou use a membrane of you want non leaky blockwork.

Bodgitnscarper said:

I have this ridiculously large pipe up the wall and under my ceiling that, if run in 22mm would have fitted behind my dot & dab.

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Take out the 28mm pipe and fit in two parallel 22mm pipes.

If the installer has oversized the boiler supply pipe the then get him back to fit in 22mm. Show him the calcs for the gas sizing. If 22mm can do it, then have one 22mm pipe, but all other pipes (15mm to fire) must go right back to the meter.