Help with Twinwall Flue Detail Through Timber Frame

[Stratman]

Apologies if this has been concluded elsewhere on the forum - I've trawled and was unable to find a solution.

 

My timber frame is up, windows in, roofing on, cladding commenced.

 

I understand the sentiment surrounding log burning stoves being an expensive folly in a well insulated home, but we are determined nevertheless.  The issue is that I cannot seem to find conclusive advise on the detail to pass a twinwall flue through the external timber frame wall at a 45 degree angle that satisfies building regulations and hopefully prevents the insulation from melting and the house bursting into flames ...which would be a bad thing!  I've found Schiedel products such as the Ignis but the diagrams and discussions never seem to match my build up which is:

 

 

AI helpfully suggests a gap between the flue and combustibles left completely empty with no insulation or airtightness!!!

 

Can anyone help, please?  Preferably with a solution that has actually been installed and signed off by building control.

 

Thanks

[ProDave]

I found an "insulated sleeve" for the purpose when I bought my flue.  It is designed to clamp onto the outside of twinwall flue so must be non combustible to be able to do that, and is 50mm thick which then leaves anything that is combustible >50mm from the flue.

 

 

The reality is my 5Kw stove even burning full tilt, the outside of the twin wall flue where it passes through the roof is barely warm to the touch.  

[JohnMo]

3 minutes ago, Stratman said:

AI helpfully suggests a gap between the flue and combustibles left completely empty with no insulation or airtightness!!!

I framed out where the hole for the flue went through our roof (your wall in this case), using PIR insulation. The size was slightly larger than the distance required from combustible material. This left a closed void. Then filled the void after flue installed with Rockwool as it's a none combustible insulation.

 

Then to get the airtightness back I used a flanged flue rubber diaphragm and taped to the the airtight membrane.

 

 

[Stratman]

Both useful suggestions. Thank you.

 

Does the insulated sleeve get cut back to the external face of the OSB then taped?  What happens as the twinwall passes through the battening void and then the timber cladding?  Presumably there would have to be a 50mm gap from the edge of the flue to the combustible cladding.

[JohnMo]

You can get trim plates to cover gaps

[MPx]

Not helping your question I'm afraid, but I have one of my own for you.  We would have liked a wood burner in our Passive std house.  I never considered the issue of the flue and combustibles...we canned it on the need for airflow to the fire.  Is the MVHR capable of supplying enough air to meet the fire regs?  Last time we fitted a woodburner we had to have an enormous air brick put into the lounge wall to get the installation signed off - not really on in PH!

[Nickfromwales]

5 minutes ago, MPx said:

Not helping your question I'm afraid, but I have one of my own for you.  We would have liked a wood burner in our Passive std house.  I never considered the issue of the flue and combustibles...we canned it on the need for airflow to the fire.  Is the MVHR capable of supplying enough air to meet the fire regs?  Last time we fitted a woodburner we had to have an enormous air brick put into the lounge wall to get the installation signed off - not really on in PH!

You just fit a room sealed WBS that sucks air in from outdoors. 👍. 
 

Sad fact is, folk spend thousands fitting one, suffer silently in protest whilst roasting to death, and then it’ll eventually become an ornament.

 

The further irony is the several hundreds of thousands spent making an airtight, well insulated home, that requires next to no heat at 0° OAT, and then fitting the most fierce heat source that you can lay your hands on…..

 

The heart and the head are two very different things  

 

[JohnMo]

55 minutes ago, Nickfromwales said:

You just fit a room sealed WBS that sucks air in from outdoors. 👍. 
 

Sad fact is, folk spend thousands fitting one, suffer silently in protest whilst roasting to death, and then it’ll eventually become an ornament.

 

The further irony is the several hundreds of thousands spent making an airtight, well insulated home, that requires next to no heat at 0° OAT, and then fitting the most fierce heat source that you can lay your hands on…..

 

The heart and the head are two very different things  

 

You need a stove covered in soapstone and a low capacity to slow everything down. After the first year lighting ours twice and melting, we tried the second time the following year. But now only add one small log at a time, turn the air down to lowest setting to still get a clean burn. Now that log lasts maybe 1 to 2 hours, the heat spreads across the whole house, 2 logs on the coldest day is more than enough.

 

1 hour ago, MPx said:

Is the MVHR capable of supplying enough air to meet the fire regs

NO it leads to depressurisation of the house. You cannot install a WBS in a house with MVHR safety, unless it has primary and secondary air from outside. It is something the OP needs to think about and plan for. Where will they put the air duct, took us an age to find the correct stove 

[ProDave]

Ours works well as long as you open all the downstairs internal doors to let the heat out to the whole house and as above don't put much wood in at a time.

 

Fill it full and keep the living room door shut and you will be cooking.

 

It helps that we have double doors from the room with the stove to the hall from where heat can go up the stairwell and it nicely heats the whole house.

 

We only have it because we have plentiful wood.  I would not have one if I needed to buy wood, and if I did not have one I would be giving away or selling wood for someone else to burn.

[Nickfromwales]

Are we agreed thos could only ever be used in deep winter time, in a “passiv-esque” dwelling? 

[Stratman]

9 hours ago, Nickfromwales said:

could only ever be used in deep winter time, in a “passiv-esque” dwelling?

Yes, I understand it will be an expensive way of adding some heat a few times a year ...not sure why I'm going along with it!

[JohnMo]

9 minutes ago, Stratman said:

Yes, I understand it will be an expensive way of adding some heat a few times a year ...not sure why I'm going along with it!

We like ours, we also have a load of wood to use like @ProDave. On a damp cold day nothing better.

 

Fire spec

External air - primary and secondary air supplies 

Soap stone clad

No bigger than about 5kW peak output. Then you should be able to knock it down to about kW output. Make sure heat can escape the room.

 

Then learn how to manage it, ignore everything your installer says. This is how we do ours. Fire lighter break into two. 3 kindler sticks and couple of pieces about the same volume of a pint glass.  Full air until a good flame then turn down the air to get a small stable flame. Leave it for 20 to 30 mins then add a small log. Your done for the day, if lots below zero maybe add another log.

 

Note:MVHR doesn't help move the heat about very much.

[Stratman]

27 minutes ago, JohnMo said:

Fire spec

Thanks - yes, it'll be 5kW nominal (Ecosy+ Infinity V) and the lounge where it will be is open to the hallway/stairwell.

 

I chose the Infinity V as it has very low distance to combustibles: only 50mm

[Russell griffiths]

11 hours ago, Nickfromwales said:

Are we agreed thos could only ever be used in deep winter time, in a “passiv-esque” dwelling? 

Remember as well both the lads answering on here with a stove are up in Scotland. 

[Nickfromwales]

11 minutes ago, Russell griffiths said:

Remember as well both the lads answering on here with a stove are up in Scotland. 

Yup. 

[Nickfromwales]

1 hour ago, Stratman said:

Yes, I understand it will be an expensive way of adding some heat a few times a year ...not sure why I'm going along with it!

Because you value your testicles. 

[saveasteading]

56 minutes ago, Russell griffiths said:

are up in Scotland

 

As often by those from the South, Scotland is up there and is wet and cold.

The reality is of a very varied and large place.

 3°C colder on average, in the populated areas. Nobody lives on the mountain tops.

The west is generally wet. The east is generally quite dry, except last winter when it reversed.

 

The saved Steading area has rainfall much the same as Kent.

But if up high then of course it gets colder. Proper winters with snow.

 

We put a woodburner in. It is in a very large open space, including stair well. There is a lot of outside wall, including lots of glass and exposed, uninsulated walls. And yet the one log at a time principle applies.

 

So for there it was the right decision as it is:

1. Expected, esp for rentals.*

2. Very pleasing.

3. A backup in the highly likely occurrence of power cuts due to trees v cables.

 

* paying guests include the wbs in their incredibly positive reviews, so it is also economically justified.

 

 

This discusson is very helpul and timely for us.

 

We are (were?) currently planning to fit a wbs in our current family project in the scorching SE.

I think we may leave it out for now, but the air intake and flue routes are ready if we change our minds.

It can be very cold here... I liken it to Dutch winters of a damp cold that eats at your being.... not so much in recent years though.

 

Question. What is a sensible backup if there are long term power cuts? I'm thinking that winter solar might run the basics and a little ufh. Thick socks and jumpers and buy a gas stove or 2?

 

[JohnMo]

1 hour ago, Stratman said:

yes, it'll be 5kW nominal (Ecosy+ Infinity V)

But doesn't have external air from what I could see. It takes room air, so not acceptable with MVHR. unless there is an option kit I didn't see. Then make sure it's primary and secondary air.

[Nickfromwales]

11 minutes ago, saveasteading said:

Question. What is a sensible backup if there are long term power cuts? I'm thinking that winter solar might run the basics and a little ufh. Thick socks and jumpers and buy a gas stove or 2?

Winter solar will give you sub 15% of the installed capacity, if you’re lucky enough to be power-down during a rare sunny spell…

 

Cheap, sizeable battery / hybrid inverter option with a connection point for a generator will work splendidly. Influx of solar energy will add itself in parallel, and the genny (2-3kw) only needs to fire up to charge the batteries for the night, or top up during dark days.

 

If outages are very frequent and problematic then just get an LPG genny set up permanently in an out building or genny shed, so you don’t have to feck about each time, setting up to restore or maintain power. 

 

 

[Nickfromwales]

We digress…..again….. 🙃

[Stratman]

55 minutes ago, JohnMo said:

But doesn't have external air from what I could see

I have downloaded the manual and it says it does have a direct air inlet at the back (also shown on the diagrams - 100mm diameter).  I've had a duct cast through the slab to outside.