Chimneys for wood burner

[MrFluffy]

Another chimney question sorry, but Im in a bit of a pickle and I know theres some knowledgeable peeps on here. Id ask on diynot, but it would end up with loads of people just quoting from regulations which arent applicable, I could do with a dose of common sense and experience instead

Ok, Ive got a 50Kw hs tarn gassifying wood/oil furnace and got it inside the house into the utility room without demolishing anything (a tale in itself...), but need to fit a chimney. Ive been told by lots of people to keep the chimney inside, or it wont draw properly so Im attempting to do that plus the exit to outside involves a fragile cob wall which Id rather not carve a 120mm hole in and weaken any more.
I need to penetrate the next floor's joists which are at 500mm centres and by a lucky chance it looks like I can hit the 250mm dead centre with the centre of the chimney to do so. But Ive got two bits of flue, one 180mm stainless single skinned one, and a elbow of 150mm single skinned. The furnace has a 150mm outlet.
If I go out and get a 7m length of 150mm stainless flexy flue, can I cut and jam or tig in small steel spacers between its exterior and the 180mm single skinned tube and create my own cool wall flue and say leave a 50mm airgap between the exterior and combustibles ? Maybe fasten some reflective steel plates on the joists etc and skin them with fireboard to be sure. Belt and braces. Downstairs the flue will be 300mm from normal plasterboard fastened onto metal richer rail system, does that need a skin of fireboard too for security?
Ive just been the local suppliers and theyre predicting mid jan before they can get me a real dual skin coolwall flue in the diameters I need, and right now its -2, and we're predicted to hit -8 by the weekend and theres no heating / ufh system has water in it already and a 5kw electric space heater isnt keeping pace, so getting the heat source hooked up is quite urgent, but so is not burning down the house in the attempt...
We'll be running on wood alone until jan if thats relevant.

 

[Sam]

I think they look for 25mm between the inner and outer wall you will have to blank the bottom and then fill the cavity between the two pipes with fermiculite granuals ( I think that is how it is spelt )tapping the sides as you fill it can be bought from a builders merchants once filled seal the top.You could tack 10mm tags at an angle to the inner flue this will help keep the flue central to the outer wall. Make sure the flue is 50mm away from any combustable material when passing through the floor or roof space.

 

[the artfull-codger]

I think they look for 25mm between the inner and outer wall you will have to blank the bottom and then fill the cavity between the two pipes with fermiculite granuals ( I think that is how it is spelt )tapping the sides as you fill it can be bought from a builders merchants once filled seal the top.You could tack 10mm tags at an angle to the inner flue this will help keep the flue central to the outer wall. Make sure the flue is 50mm away from any combustable material when passing through the floor or roof space.

I wouldn't fancy making your own flue,you'll never keep the flexy inner flue central & if you put lugs on to do so they will transfer the heat from inner to outer,your insurance might be invalid in the unfortunate event of a fire, as sam says 50mm is the recomended distance from combustible materials,better to get ''selkirk'' twinwall designed for the job. putting it outside & not getting a proper draw is allso not true,selkirk is designed for inside or outside, obviously inside is warmer & better but draw depends on height & surrounding conditions eg trees/buildings & being above the ridge to name a few, I have both a multifuel range cooker & a stanley roomheater both with selkirk flues(6"bore) apart from the 1st6ft which is 6"single enamel hope this helps.ps since2002 you come under building regs.

 

[Sam]

You do not necessaraly have to get the flue above the ridge if you are exiting below the ridge the top of the flue has to be 2.6mtr from the roof on the horizontal.
Simple way to understand is draw the pitch of your roof on paper draw a line for the flue and from the top measure on the horizontal 2.6mtr to the pitch this will keep it out of the negative pressure zone. If you exit closer to the ridge than the stated 2.6 then it has to be 600mm above as the wind creates lots of turbulance coming over the ridge of the roof. The reason for the twin wall is to keep the gases hot and stop them from condensing when it hits a cold area and solidifying then forming tar on the inside of the liner.

 

[MrFluffy]

Sam, I think we're going with penetrating the celing/joist area with a preformed chimney block for extra isolation and run the flue angled up the inside of it around a doublewalled section made as indicated (assuming I can find non asbestos vermiculite granules) where it passes the flooring beams, aiming it to exit the outside wall upstairs where its single skin blockwork instead of 3ft of cob and run up the exterior save worrying about isolating from the trusses and cross beams etc, as mrs f has indicated that even with real twinwall she would be nervous about it passing a wood framed structure after her friends have filled her with tales of plasterboard igniting etc.

For now we'll use the 9m of snap together single skin 316 stuff Ive got kicking round on the exterior and wait for some twinwall to appear. Between now and when we buy some real twinwall creasote of the innards wont be a real issue, but we'll still suffer from lack of proper draw/backdraft down to the cooling effect you mentioned. I can always prime it with the electric space heater first in the meantime

Ive seen the calculations on woodheat.org for the horizontal distances you mentioned but havent had a good think about the implications before now. I think the roof pitch is so severe it'll have to peak past the ridge, but need to measure to be sure. It would save two expensive bends. Diy kitchen roll tube inclinometer here we come

Artful, there are no building regs here hence my "not applicable" comment above, Im not in the uk. Ive probably been messing with my profile again but it should read nomadic in europe

All part of lifes rich tapestry eh, another few weeks and Id have been sorted for it, but cant grumble too much, its december after all.

 

[Sam]

P.S. if you have to use bends go for 45deg or less as the smoke exits better on a straight run with less restriction.