pre cast flue help

[buckley plumb]

I need advise on the old precast flue blocks.the current pre cast flues have a finishing block in the loft cut at 45 degrees and the hole in it is stepped to take the inner and outer twin wall sectionsof a 45 degree bend.The ones i need help on are mainly on 1970s houses and the flue block in the loft does not have a 45 degree on it ,it looks like a breeze block with a 6" hole cored into it no step for twin wall as its normaly asbestose single wall pipe that fits into it.my question is the hole in this block is 90 degrees to the rectangle flue section so can i just fit a 45 bend into it as in effect the block is 90 degrees and cuurent regs state no 90 bends.also the hole is not stepped so the inner to outer twin wall flue would have to be filled to stop the fumes going between the inner and outer sections.i have come accross a few of these now . i spoke to gas safe and they said i could renew the asbestos flue with twin wall as its classed as maintenance of the flue but were not very helpful on the question above as the person on the phone had never seen what i was describing. there must be someone out there who has come accross these flues . the block looks like the one on page 171 of the BES catalouge what part/no 11973 goes into could i use this part to connect twin wall to it and would this meet current regs - would the connection be classed as a 90 bend.

thanks ant

 

[GPSNewcastle]

I think it would mate, and therefore class as a restriction

 

[kirkgas]

the regs do not say you CANNOT use a 90 bend, it says you SHOULDN'T which isnt the same thing, best working practice is to have 2 x 45 bends to assist the POC's leaving the appliance, however IF the spillage test is OK then it is acceptable, and in this case you are working on an existing system therefore you can change like for like componebts to effect a repair.