Boiler positioning — flue proximity to wooden truss

I've got the fitting templates for my two boilers — a Vaillant 618 system boiler and a Rinnai 26i water heater. I'm trying to work out the exact positions on the loft gable wall to mount the boilers, and I could do with some advice.

The diagonal webs of the (not very tall) roof trusses are turning out to be a real pain. Ideally I'd modify them, but to do that legally I'd have to involve a structural engineer and Building Control approval, and that's just too complicated.

The drawing below shows the best compromise I've been able to find.


The pink area shows the area that the building regs say the flue terminals can't encroach upon due to the minimum 200mm proximity with the eaves on the outside.

The boilers can't really go anywhere other than the central triangular area. The areas further out (the other sides of the diagonal webs) are too short once you take out the pink bit. I've also read somewhere that two boilers shouldn't be less than 100mm apart.

The only trouble is, this position puts the flues only about 80mm from the diagonal wooden trusses. Is that OK? I can't find any firm data about it on the web. Admittedly it's more than the minimum 75mm they're allowed to be away from things like guttering and drain pipes.

Any advice offered will be gratefully appreciated. Cheers.

Can you turn the bend from the top of each boiler 90 degrees towards the middle and then add another bend in each case to take the flues out through the wall closer together?

jobloggs said:

Can you turn the bend from the top of each boiler 90 degrees towards the middle and then add another bend in each case to take the flues out through the wall closer together?

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Thanks for the suggestion, but it would only gain me a few millimetres. Also the building regs say that no two boiler terminals can be less than 300mm apart.

why cant you employ verticle flues?

WDIK said:

why cant you employ verticle flues?

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I could, but it would be more complicated and expensive to have vertical flues going up through the tiled roof than it would to drill a couple of big holes in the wall. It would only reduce the proximity to the wooden truss if I brought the boilers forward on some sort of mounting frame, instead of hanging them on the gable wall.

I've spoken to the CORGI registered installer who will be doing the rest of the job, and he's happy with it. We agreed it would be sensible to 'box in' the diagonal truss webs with some sort of fire resistant material.

Anyone got any recommendations on what is the most suitable material to box in the webs?

Distance is perfectly ok .
Heat on outer duct which is where the fresh air from outside is drawn in is minimal .

When all else fail read the MI's