Flue efficiency

[ConcreteCastle]

Hello
As per my other thread I'm currently opening up an old chimney to install a woodburner.

The problem is the old clay flue now comes out underneath the ridge of the roof, so I need about a 500mm step (i.e. centre of the existing chimney to where I want the centre of the flue to go through the roof) to clear this and get it through the roof. See the pic below. The flue will exit to your right as you are looking at the picture. If you look closely you will see that I have marked 30 and 45 degrees on the wall where this would need to come off the old clay flue to end up where I want it.

I understand this sort of step will reduce the efficiency of the chimney somewhat, but not sure how much.

A couple of questions:

Is this set up likely to produce a reasonably efficient chimney?

Will the 45 degree bends be significantly less efficient than the 30 degree bends?

Thanks in advance.

 

[lostinthelight]

what flue pipe did you intend using, it will need to be insulated to go past the roof timbers

 

[ConcreteCastle]

Planning on using 6"/150mm twin wall flue, with an outside diameter of 210mm.

Provided I can get the pipe vertical below the timbers there will be over 100mm clearance each side between flue and timber. I understand the minimum is 50mm.