Go to Forum Home Building Design Air Tightness at Flue Penetration

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    • #31607

      We have a situation where a wood burning stove in a first floor living room exits through a flat green roof.

      The stove is room sealed and will have a 150/204mm Selkirk twin wall insulated flue, the Hwam 3610 stove is quoted as having flue gas temeratures of 292 deg C. We have a timber joisted roof zone of 400mm mineral wool insulation and a bauder green roof over and are using a Decktite DF107 weather flashing & Selkirk cone flashing on top of the roof.

      We have a suitably Proclima Rolflex air tighness gromet on site that could readily be fitted inside and taped before the flue is passed through inside to outside. The EPDM is said to be stable between -40 and +120 deg C.

      I attach a section showing the situation. If anyone has any practical knowledge of the likely temperature of the outisde of the flue and whether the Rolflex will be up to it or an alternative method, high temperature tape? advice would be appreciated.

    • #38317
      Mark Siddall

      What is the thermal resistance of the flue? If you know that it should be possible for you (or me… if I fnd a spare moment) to calculate the temperature surface temperature of the face of the material in contact with the Rolflex grommet. (It should also be possible to determine the required thermal resistance of the flue in order to achieve a surface temperature less than +120C when the flue is +292C.)


    • #38318

      Hi Mark
      The flue is 150mm i.d. and 204mm o.d. with 316 grade stainless internal and 304 grade external skins, the 25mm cavity us filled with Rockwool so I would assume 0.04 conductivity. I suspect the skins are approximately 1mm thick.
      With it being a point heat source the gases will be cooling as they rise but by how much is the question. I suppose if you could work out the difference at the start of the flue using +292 and 20 deg C that would give an idea of the relative temepratures.
      I would appreciate it if you could have a look as I wouldn't know where to start.

    • #38319

      I have now had some technical input from the UK distributor who advises that as a rule of thumb, the temperature at the outside surface is 1/4 the inside temperature. So around 75 deg in normal use at the stove end.
      The flues are tested for integrity to 1000 degC in case of chimney fire but these reallly shouldn't happen as long as the stove is burning decent fuel and the chimney is swept once a year.
      On this basis I am going to recommend the use of the Rolflex gromet and just remind the client to have the chimney swept once a year.

    • #38320

      Final update… Stove supplier/installer advised detail should be able to cope with the worst case scenario i.e. chimney fire situation.

      Proposed solution is now to install another Decktite (red) flashing capable of withstanding 250 deg C from underside and tape membrane to it. Decktite flashings above and below should result in very good air tightness.

      I know it looks like I am talking to myself in public (never a good thing) but this may be useful to someone else so I thought I would update it.

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