HELP: insulated Twin wall flue, connection with ceiling

[Adsibob]

An issue has arisen on site in that the twin wall insulated flue we have had installed has not been installed with one of these ventilated fire stop plates (see attached spec sheet) creating an air gap between the ceiling plaster board and the flue. Hetas installer says it’s not required because we are using an insulated twin wall flue, and so the flue will never exceed 50C or possibly 60C to the touch and is happy to sign off the installation as is. This didn’t sound right to my project manager so he called the manufacturer, Schiedel, who is saying that it is required.

The ceiling has already been plastered and finished with clay based plaster, and I don’t believe the plasterboard is of the fire rated type. This is about 2.2m from the stove itself which is rated 5kw to 9kw, to the extent that is relevant. 

I’ve had a look at the cross section of the flue:

 

 

and there do appear to be several layers, including a thick layer of insulation, so maybe the installer is right. Not helped however by his admission that he sacked one of the engineers involved in our install because his work was unreliable!

What would you do?

 

Edited April 12, 2022 by Adsibob

[MJNewton]

I'd go with what the manufacturer says. Their instructions tend to trump all generic regs and rules etc and so can be regarded as gospel. 

Edited April 12, 2022 by MJNewton

[Adsibob]

1 hour ago, MJNewton said:

I'd go with what the manufacturer says. Their instructions tend to trump all generic regs and rules etc and so can be regarded as gospel. 

good advice. This is what I will do. Bloody stressful when a supposed authorised contractor tries to bend the rules. Para 1.45 of Approved Doc J says this:

 

"Factory-made metal chimneys should be kept a suitable distance away from combustible materials. Ways of meeting the requirement for chimneys designated to BS EN 1856-1:2003 comprise: a. locating the chimney not less than distance ‘xx’ from combustible material, where ‘xx’ is defined in BS EN 1856-1:2003 as shown in Diagram 13; b. where a chimney passes through a cupboard, storage space or roof space, providing a guard placed no closer to the outer wall of the chimney than the distance in a) above."

 

It's clear from DIagram 13 that the manufacturer is right: see PDF page 798 of this: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1010058/Single_stitched_together_pdf_of_all_ADs__Jul21_.pdf

 

 

 

[dpmiller]

Is it not 1.46 that applies in this situation?

[Adsibob]

9 hours ago, dpmiller said:

Is it not 1.46 that applies in this situation?

That's a good question. That rule provides:

 

Where a factory-made metal chimney penetrates a fire compartment wall or floor, it must not breach the fire separation requirements of Part B. See Approved Document B for more guidance but the requirements may be met by: a. using a factory-made metal chimney of the appropriate level of fire resistance installed in accordance with BS EN 1856-1:2003 Annex NA; or b. casing the chimney in non-combustible material giving at least half the fire resistance recommended for the fire compartment wall or floor.

 

According to this, the BS EN 1856-1:2003 standard was superseded by BS EN 1856-1:2009

 

BS EN 1856-1:2009 appears to be available for purchase here: https://www.en-standard.eu/bs-en-1856-1-2009-chimneys-requirements-for-metal-chimneys-system-chimney-products/

 

Bizarre that one has to pay for these things. I guess I could check with the manufacturer if its twin wall flue pipe complies with that standard, but when we checked with them they directed us to the firestop plate and said we had to install it.

 

[MNN]

There has been confusion between HETAS requirements and  AD B and J for years. We've struggled with woodburner flues and regulations several times.

We've got much the same problem as ADSIBOB.

The house is four storey. Twin wall flue from first floor level has to pass through a 30min fire resistant floor on the next floor. HETAS installer has put in the usual vented firestop at the floor.

My concern is that the 'vented fire stop' will allow smoke (from an accidental fire in the room with the woodburner) through and smoke is the critical thing here from the means of escape aspect.

I'm after some suggestions as to possible solutions before going toback to building control. For instance:-

If the opening in the floor has an intumescent band around it at the floor level, is that likely to expand and stop smoke or is it likely that the temperature will not be hot enough to activate it before smoke becomes an issue?. Is that likely to be accepted as a solution.

Using 1.46 would mean constructing a fire resistant enclosure up the next two floors which rather negates the reason for having a visible flue.

[Adsibob]

Why are you worrying about this? If there is an accidental fire, the alarms will go off before smoke leaking through any gaps becomes a concern. I wouldn’t be messing around with intumescent seals.

[markocosic]

Twinwall can go through a certain amount of non combustible insulation before it risks getting dangerously hot and you need to make a counterbore in the top side of the insulation to cool it.

 

Mineral wool = insulation and smoke barrier around flue

 

Stretchy silicone flashing = airtightness layer

 

50 mm gap to plasterboard or joists or floorboards

 

Stainless trim plate to hide that 50 mm gap.

 

Nice details in this are essentially from the EN standard:

 

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

 

evs.ee for EN standards cheaper than elsewhere fwiw.

 

https://www.sauresta.com/en/double-wall-chimney-system-npnp

 

 

[markocosic]

To might also like triple flue fwiw.

 

Makes air fed stoves easy and further reduces clearances in some cases.

 

https://www.sauresta.com/en/triple-wall-chimneys-system-npnpnp

 

 

[MNN]

Thank you adsibob and markocosic. Smoke is the issue as the compartment floor is to make a safe escape from the upper compartment. As I understand it, protection from smoke is critical as the first concern, to enable people to escape. Later when fire penetrates people should be out.

Markocosic shows presumably a European solution which shows filling the compartment floor void where the flue passes through. This makes common sense to me. (It also gives the flue termination dimensions that UK Building Regs use). Our HETAS installer claims filling the floor with insulation is against their code.

Armed with the Sauresta info I'll contact Building Control.

Many thanks markocosic

[markocosic]

You may need the euro standard (in en for English) that the twinwall is made to.

 

That's best from evs.ee in my experience. (the Estonians sell the euro standards for less than most standards bodies including bsi)