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    Hi all,
    Is the temp of a double skinned flue supposed to be too hot to touch. A friend of mine has a wood burner in a ground floor brick built fireplace to ceiling, then stainless flue goes thru bedroom to roof cowl etc. He was under impression that it would not be dangerously hot as implied by installer. As such he was thinking of enclosing it in fire board which goes against the whole stainless flue pipe choice. I have a brick chimney so can`t help.
    Any advice/comments please.
    • CommentAuthorJeff B
    • CommentTimeFeb 23rd 2010
    I have a double-walled S/S flue on my wood pellet boiler. Even immediately above the boiler it is only just warm when the boiler is running, so your friend's set-up does not sound right to me. I admit that probably a wood burning stove would burn much hotter than my pellet boiler. There are plenty of people who are more expert than me who contribute to this forum, so doubtless you will get further advice!
    Thanks jeff,
    I thought it sounded a bit hot. My thinking was that the insulation was missing between the two stainless liners but he assures me that is not the case.
    • CommentAuthorchuckey
    • CommentTimeFeb 23rd 2010
    Are you sure that the temperature is not due to conduction from the top of the stove to the outer of your SS liner. FWIW From the small amount of browsing I have done any sort of liner requires a 24" border all round it as it pierces floors etc of fire-proof material.
    • CommentAuthorCWatters
    • CommentTimeFeb 23rd 2010
    If I remember correctly the human hand can only tollerate about 60C absolute maximium. So "too hot to touch" can be very missleading. Measure the temperature properly and give these people a call. Ask if they think it's normal...


    They were helpful when I needed advice.
    • CommentAuthorJohnh
    • CommentTimeFeb 23rd 2010
    Most double wall S/S liners usually have to have clearance of 50mm or more from combustable materials.
    We've got a 2 S/S double wall insulated flues at our house - a Selkirk one and a Negarra. I seem to remember that the Negarra requires a 70mm (?) clearance but the Selkirk only requires 50mm from combustable materials. The Selkirk does get 'hand hot', this is attached to a stove but has 750mm of vitreous single skin pipe that then leads into the S/S insulated. The Negarra flues an Esse Woodfired cooker and operates at much lower temperatures.
    • CommentAuthorRobinB
    • CommentTimeFeb 23rd 2010
    If you need 50mm clearance how do you get a flue pipe through the ceiling and roof, please?
    My 40kW wood gassification boiler has 1.5m of single walled flue (usually at 75 degrees 1 m from boiler but can be as much as 120), then 8m of double walled flue - outside wall is always cool or warm, never anywhere near hot to touch.

    Robin - roof 'clearance' is in-filled with HT expanding foam and covered with copper flashing. For ceiling Google 'register plate for wood burner' it’s all there!
    • CommentAuthorowlman
    • CommentTimeFeb 24th 2010
    <blockquote><cite>Posted By: RobinB</cite>If you need 50mm clearance how do you get a flue pipe through the ceiling and roof, please?</blockquote>
    With traditional joists and rafters, by cutting and inserting a trimmer joist. I also line the insides of the trimmer and joists with FR material. If you have a trussed roof and therefore also the ceiling, you've got a problem, best to go to the truss manufacturer, for obvious reasons they don't like you cutting trusses, but they MAY have a solution, but I doubt it. Don't cut a truss without professional advice.
    Thanks for the comments,
    To give more info, the fireplace is brick built in sections floor to ceiling(on gnd floor) , then in bedroom (first floor) it converts to the stainless liner from floor to ceiling ( aesthetically pleasing? ) and then through roof. He basically asked me if i knew why his flue was so hot.
    He has had it professionally installed so i`m sure all barriers are there, but being 3m from source, he was concerned of the temp.
    Hi all
    Had a chat to the flue owner today and he said that the flue melted a power cable , which in turn set off the co2 alarm! He mentioned that the hetas installer is adamant that the installation is correct, so would probably charge for further investigation.
    Can anyone advise of any solution other than box the flue in with FR boarding as that seems his next step.

    Thanks TB.
    • CommentAuthorJohnh
    • CommentTimeFeb 25th 2010
    How close was the power cable?
    • CommentAuthorRobinB
    • CommentTimeFeb 25th 2010
    Rather than box it in could he make a mesh guard around it - like a fireguard? Presumably the heat coming off it is of some value to the room.
    • CommentAuthoradwindrum
    • CommentTimeFeb 26th 2010
    My twin walled flue is darn hot when running my log boiler. I got second hand flue which had been stored outside so maybe water ingress is affecting the insulation (although these things are designed for external conditions?). I have also seen 2 or 3 other boilers running and their flues very hot to touch as well. My flue stays clear though and in 2 years of running it I havent needed to clean it. Is there a problem with hot flues (other than danger to adjacent materials...mine goes through tin roof so no problems)? Presumably colder flues are only shooting the heat up and out, while my flue is letting some into the shed. I understand soot build up can be a problem if the flue gasses are condensing.
    JohnH, the power cable was a temporary one i think and has now been moved.

    The mesh sounds like a good idea,cheers robin.

    I agree with adwindrum, that a hot flue is an efficient flue. But maybe in this instance he underestimated the surface temp.

    Thanks again.
    An update, the flue was `boxed in` with FR boarding, but there seems to be another problem, backdraft!

    Certain wind conditions caused downstairs to be `filled` with fumes! He asked me about the `swirly` type of stainless cowls,(the ones that rotate).

    Can anyone recommend these, eg do they actually work? and do they ever `clog` up? Or would an anti-downdraughtcowl be the solution?

    Thanks in advance.
    Apparently the swirly ones have as bearing assembly that inevitably gives out eventually, or not so eventually depending on the quality of the workmanship and, of course, luck - same problem with the turn to point the right way vane type - in the end I had the choice between one that was the best (optimo) for wind or one rather less ugly but still 'good' - I went for the good one.
    Thanks gotanewlife.
    The `turn to point the right way` type, is that with a bearing for rotation as well ?
    Have you any info on these types of cowls?
    Cheers TB.
    • CommentAuthorCWatters
    • CommentTimeApr 8th 2010
    Chimneys should extend 1m above the highest part of the house or you are almost guaranteed to get down draughts with the wind from some directions. If you can't raise it then look at a cowl. Perhaps...

    Truffleboar, sorry I wasn't clear, we decided not to use a movable one at all and chose between one that looked just like the stainless steel anti-downdraft chimney on CWatters link or one a little less ugly but not quite as effective.
    • CommentAuthorcrusoe
    • CommentTimeApr 8th 2010
    Flue temps will vary on BS 4543 twinwall depending on the appliance. Too hot to touch is quite usual but would indicate a hearty fire. If it's always too hot to touch on older flues, this could be settlement of the F/R powder some flues (esp Selkirk Metalbestos) employed. Would tend to be hottest near the top of sections though.

    Fire resistance ensured by air gap, usually 50mm from combustibles, and using proprietary firestop spacer, which has the gaps built in.

    Downdraught a much bigger subject. If you can find a copy of the old SFAS' 'Guide to Curing Chimney Problems' or similar wording, the principles were well, if briefly explained. Raising flue often cures the trouble, but not always easy or practical, as unsupported lengths over about 1.5m will have to be guyed. When it next happens, try opening a window on the wall facing the oncoming wind, ideally in the stove room itself, thus pressurising the room and often neutralising the downdraught. If that works, fit a vent facing the problem wind. Cowls are often a waste of time until you get those basics right, and often don't work well in every condition.
    Thanks for your comments, sorry for delay, will try out your suggestions and keep you informed.

    Cheers TB.
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