Flue options - vitreous>twinwall or 100% twinwall?

[markocosic]

Could I pick some brains on flue selection please?

 

 

The previous plan to go out through the wall and run the flue up the gable has been rejected by she who must be obeyed on visual grounds. "OMG that's enormous and fugly"

 

 

Stove

5 kW

500 mm wide / 950 mm tall 

130 mm flue outlet on the top (or out the back)

100 mm external air feed from the back

Flue gas temperature at nominal rating 230C

 

 

 

Flue

I'm probably going to buy from these folks - locally made and can be bought in custom lengths and custom colours etc if desired - but would like to sanity check 25 vs 50 mm insulation.

 

https://www.sauresta.com/en/products

https://www.sauresta.com/static/files/3/54/354/Dumtraukiu_instrukcija_EN_Rasu_2018-10-26_No7.pdf

 

Systems:

- 130 mm single wall stainless (can get black iron to match the stove elsewhere; vitreous enamel isn't really a think here)

 

- 130 mm twinwall with 25 mm insulation (180 mm total;  Wet operation rated; L50 / 316L liner; sootfire rated and 70 mm to combustibles)

T450-N1-W-Vm-L50(050-100) - G70

 

- 130 mm twinwall with 50 mm insulation (230 mm total; Wet operation rated; L50 / 316L liner; sootfire rated and 50 mm to combustibles)

T450-N1-D-Vm-L20(050-100) - G50

 

 

Install location

 

It's going in the corner at a 45 degree angle. It's a convector type so it only needs 200 mm clearance to combustibles. This puts the flue centreline - if mounted directly on top of the stove that's hiding under a jacket in the corner - about 600 mm from the wall (marked with red/blue arrow) and far enough clear of the rafter that is behind the green taped joint for either the 180 mm with 7 cm clearance or the 230 mm with 5 cm clearance to miss the house structure. There would be a hole in the OSB (airtight layer) that is the flue diameter plus 5cm or 7 cm as appropriate.

 

 

Options

 

 

1) Run 50 mm insulated (230 mm outer diameter) twinwall the whole way. 

 

Stove = 975 mm (once it is on some flooring

Adapter = 100 mm (1075)

Damper = 200 mm (1275)

4x 1000 mm sections = 950 x 4 (5075)

 

1x trim detail on the vaulted ceiling

1x airtightness seal at OSB level

1x restraint at rafter level

1x airtightness seal (watertightness seal) at roof membrane level

1x cowl at roof level

 

1x top closer = 115 mm (5190)

1x diffuser = 200 mm (5390)

The ridge is about 5850 mm so we're almost at ridge height here. The roof is between about 3400 and 3900 so there are no joints inside it.

 

Every piece of twinwall is clamped tight. The restraint at rafter level holds the flue near the middle, the stove holes the flue at the bottom, and gravity does the rest.

There's a cowl/clamp on the OUTSIDE of the roof to hold the flue "part way"

 

 

Benefit - simple

Benefit - mechanically robust

Benefit - marginally better insulation when not in use (less area exposed directly to outisde in that you have the thermal bridge from the outer wall of the flue but none of the inner wall is visible

 

Downside - dear lord is 230 mm large compared with the stove and with the curved top rather than a flat top to the stove it will probably look silly and be my fault. e.g.

 

 

 

2) Run the first 2 metres in single wall then 50 mm insulated twinwall the rest of the way

 

Swap the damper and the first two sections of flue for single wall black iron and convert near ceiling level

 

Downside - this is probably wobblier mechanically and might need external supports?

Downside - I can't imagine having much success colour matching black iron, vitreous enamel, and painting twinwall flue ending up the same colour after 5 minutes

Downside - two metres of single wall flue that's as cold as it is outside when the stove isn't in use (heat pump is probably main source of heat - stove is there for the love of burning things / clearing up windfall from the land / fast reheat from cold)

 

Benefit - we think prettier though you still end up with a +100 mm diameter "blob" at ceiling level:

 

 

3) Run the lot in 25 mm insulated twinwall

 

At 180 mm overall this seems to be a good compromise between the visual bleurgh of the 230 mm twinwall and the faff / cold of running single wall then converting to twinwall.

 

Aside from the 50 vs 70 mm clearance to combustibles - which is neither here nor there - what is the benefit to running with 50 mm insulation vs 25 mm insulation on twinwall? 

 

Part of me thinks salesman wants to sell 50 mm twinwall because slightly spendier than 25 mm. Part of me thinks that this is just what people use around these parts when the similar diameter flues are in use on larger appliances and larger buildings. (150 and 200 mm internal diameter are standard in the DIY sheds) Part of me thinks I'm missing something obvious but can't for the life of me work out what!

 

 

4) Hide it in the corner and DIY shed the lot?

 

150 mm ID / 250 mm OD twinwall is stupid cheap if you're ok with galv mild steel outside (fine if being primed and painted to match the roof) and 304 stainless inside (fine if only ever burning dry) - €42 per metre 

 

https://online.depo-diy.lt/products/6252

 

We could...run a 150/250 twinwall flue right in the corner of the room. (to the left of the green tape showing where the rafter is) Rest the flue stack on the floor. Use a tee to take either the rear-exit outlet from the stove. False wall to hide the flue entirely. Pain the bit you see on the roof to match the roof.

 

Condensate drain and fresh air inlet could be done the same way (150 mm plastic duct feeding 150/250 twinwall flue that joins the outer wall of the main flue but has a condensate drain pain to separate the two sections).

 

I've not found a good picture of this to show she who must be obeyed though!

 

 

Thoughts? I'm leaning towards 3 or 4 unless a daft idea.

 

[ProDave]

Option 2 is what i did.

 

The single wall flue does not seem "cold" when the stove is not in use, certainly not cold enough to cause concern at heat loss.

 

I bought from flue-pipes.com and their black single wall and twin wall all matched in colour and were a pretty good match to the matt black of my stove.

 

For the roof penetration, they sold an insulated sleeve.  This is a hard insulating tube that is a snug fit around the twin wall and can be made a snug fit and sealed in a hole in the roof.  It was obviously designed to be in direct contact with the twin wall flue so was a neat solution.

 

It is not at all "wobbly"

 

 

P.S that overhanging kitchen worktop was temporary

[Iceverge]

 

 

Option 1 for sure.

 

It's the poorest part of your envelope thermally. So when it's not lighting you want the best insulation possible. 

 

It'll light quicker and burn cleaner as less energy needs to be diverted from the firebox to heat the flue.

 

Looking at the quality of your build I suspect your heat load will be pretty small. You want the option to throttle the fire with minimal taring.  The last thing you need is an extra kW or two coming from an uninsulated flue. 

 

I was going to suggest getting a smaller stove but it looks like you've already purchased. Maybe even one with some soapstone to buffer the heat release. 

 

When we lived in a cottage with terrible insulation etc we had a 6kW stove. Unless it was running super hot it got filthy. (Yes the timber was suitable!) The smallish room still got up to near 30 Deg if you left it on all evening. Stone walls, 2 drafty windows and 3 doors. 

 

I would go out the back and straight up to the roof. It'll look fine. I'm not a fan of hiding a flue unless completely necessary. Nice to be able to keep an eye on them. 

 

[saveasteading]

We have just been told by the BCO that a boxed-in section of flue requires an access hatch for inspection (in case the flue is suspected of leaking.

This make sense but I think is intended for service ducts rather than complete and insulated boxings. Costs £30 so we are simply agreeing.

 

For an exit through the roof I will be contemplating fitting one of these above the vapour control.

It is always difficult to seal around a round flue without these. The red ones are made for hot pipes.

Then the roofer can do whatever he always does as well.

[saveasteading]

That stove is very cheap. 

When I have poked at different sources the quality to price relationship has been fairly obvious, until the very expensive ones.

I assume you have done research and comparisons and this is ok.

[Temp]

Can't add much, just that there are two different grades of stainless steel used to make flues. One is more resistant to corrosion than the other. Can't recall which is which but check it out.

 

[markocosic]

Thanks folks; more food for thought there.

 

 

Heat loss:

 

1) From the *OUTER* wall. A 230 mm OD flue will lose more than a 180 mm OD flue. There's more cold wall punched through the house. This is where I don't see the benefit from more insulation on the flue.

 

2) From the *INNER* wall. That's 130 mm no matter what. Having it exposes loses more directly to the air when not in use (if it's warm that means you're losing heat through it through convection currents within that pipe?) There probably won't be losses from the outer wall through the insulation to the inner wall. As in you lose enough through the outer wall conducting directly to the outside that you don't see any benefit from 25 vs 50 mm insulation between this and the inner wall. Are there any passive house types who have analysed this 25 vs 50? Is there a historic reason that 50 exists and 25 never used to be an option?

 

3) From the length of the flue when not in use. Short flue out the back always loses less than the large flue out the top.

 

 

As such option 2 should be out and so should option 4. Option 3 should win over option 1 and it looks a lot ore reasonable "in person" than "in the trolley" at the DIY shed. Actually no, 13 cm is tiny, 18 cm isn't that big after all, though 23 cm is starting to push it. (roll of roofing membrane is 18 cm)

 

 

 

Short and out the back with an external flue still wins though. The big going through the wall it hotter now but the only bit likely to ever get a soot fire is outside so meh. Again smaller in real life than it looks in the shop. She says it's ok after all. Winner due to no roof penetrations in an important space.

 

 

 

 

Material - decide to go with 316L (the better one) for robustness against low-output potentially condensing operation. 

 

 

Stove - it's rated for 2.5 to 7 kW operation; which I think is fine on this basis:

 

Gypsum is 1000 J/kgK which means each m2 of double layer plasterboard needs 24 kJ to raise it by 1 degC. 325 m2 of wall/ceiling plus 170 m2 of floor (which for arguments sake lets say has the same heat capacity) gives 500 m2 and 12,000 kJ to raise it by 1C. 6 kW. does that in 30 minutes. 3 kW in 60 minutes. If you're down at 5C (because it's been left at setback - this is a cabin - then you're talking a 7.5 hour burn at 6 kW to before the walls/floors/ceilings stop sucking out heat...so I think it'll be fine for reheat from cold or for an evening burn in a cool building. Winters are not tropical here and even with 30+ cm of mineral wool everywhere and MVHR you're still averaging >1 kW from Nov>Feb if wanting to keep the place at comfort temperature.

 

https://weatherspark.com/h/y/148554/2021/Historical-Weather-during-2021-at-Vilnius-International-Airport-Lithuania

 

It isn't the best. It's ok - external air feed, well built but no other frills, 80% efficiency rating. Nothing else on offer was materially better in terms of 'useful' features. Prettier yes. Functionally no; so I picked it up whilst there was still one on the shelf at that price!

 

I'll give it an additional flue damper to reduce the draw once lit. Beyond this I think you're looking at adding a catalyst to do materially better. (on the bucket list)

 

https://www.youtube.com/watch?v=96kA_Cwye04

https://www.youtube.com/watch?v=JHIJ9vbfF_0

https://www.youtube.com/watch?v=LjQemZThSv4

 

etc

 

Edited April 18, 2022 by markocosic
swap youtube to links

[ProDave]

With the rear option, watch out for the "distance to combustible materials" particularly for the single wall flue. That could push the whole lot out into the room.

 

I really don't get much heat loss from the single wall section of flue.  I think the crucial thing is ducted air into the stove in my case from under the suspended floor, means cold air top and bottom of the flue so little convection.  IF the stove was taking warm air in from the room, you could get more convection and heat loss.