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Renewable Energy: Severe downdraught on woodburning stove

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  1.  
    I have recently had an Aarrow Ecoburn 7 installed in my Victorian house. The flue runs up the side wall and exits at the eaves, only about 700mm above, although I expect the stack was much higher in the past. We have installed a twin-wall s/s/ 6" liner at the installer's request, and there is a simple 'hat' terminal. The flue is not insulated round the liner. The flue is about 6m.

    The stove performed fine on the (still) day it was installed, but as soon as a small breeze came up, the room filled with smoke. Yes, it's not quite as bad when the flue is hot, but it's effectively impossible to light the stove except when it's warm and still enough for us not to need it! The front room is like something between a camp-fire and a kipper-factory!

    I whipped the scaff tower up and temporarily stuffed (and sealed ) a 1m length of 6" galv ducting I happened to have lying around (''It might come in useful, Dear''). This helped considerably, but not by any means completely.

    There seems to be a lot of divided opinion re anti-downdraught cowls. Anyone any experience of the problem and a solution? At the moment my favoured option is 2m s/s extension above the stack (I am not convinced that the stack can be safely increased in height due to weetabix brickwork) with an anti-downdraught cowl on the top. I am not heartened by the idea of effectively tossing over £1000-worth of kit away!

    HELP!

    Nick
  2.  
    I had a similar problem with our bungalow. I simply put a rotorvent cowl on and problem solved.
    Cant guarentee it will work but sounds like you will utilise it even if you have to extend the stack.
    • CommentAuthortony
    • CommentTimeDec 8th 2007
     
    Flues should be 1m above roof. Not at all sure about your hat thing?
    • CommentAuthorJohnh
    • CommentTimeDec 8th 2007
     
  5.  
    Thanks all for your comments. Nicky, I was thinking of a rotorvent, although I had heard from some that they had not been effective. It is good to hear your positive experience. John H. Wow, sounds like you have had worse probs than me. How are the 'guys' fixed at the bottom end?

    Will keep you posted on progress. I want a smoke-free Christmas!!

    Nick
    • CommentAuthorskywalker
    • CommentTimeDec 8th 2007
     
    Nick

    What is causing the downdraft? if your chimney used to be much higher then ther may be loads of tubulance in the area and any fancy anti downdraft device might be no good.

    Tall trees/taller buildings/ topography are the usual culprits or it could be your "hat thing'. Is it a DFE terminal or some other device (I have a DIY 'chinamans hat' to stop the rain getting in too much which as far as I can tell if the main reason for having one). You can get very tall 'Marconi' chimney pots which have an anti downdraft function and add height to you chimney (over 1m). There are a number of other patterns that do the same thing, our local reclaim yard has a number of flavours (salt glazed & terracotta) at the moment & all much less than you can end up paying for new ones.

    S.
    • CommentAuthorskywalker
    • CommentTimeDec 8th 2007
     
    Insulating it (pour in vermiculite) will also improve the draw which may help as well.
  8.  
    Skywalker wrote ''What is causing the downdraft? if your chimney used to be much higher then ther may be loads of tubulance in the area and any fancy anti downdraft device might be no good.

    Tall trees/taller buildings/ topography are the usual culprits or it could be your "hat thing'. Is it a DFE terminal or some other device (I have a DIY 'chinamans hat'''

    I think it will be turbulence, but I don't think it's the tall trees/taller buildings/ topography. We are well up a hill, above all surrounding bldgs on that side and clear of taller trees. My ''hat thing'' is a ''chinaman's hat''.

    I had been sceptical about the value of insulating the liner, but suspect that in marginal cases (such as mine) it really does have value. Where do I source vermiculite?

    Thanks for all the help. Keep it coming, all!

    Nick
  9.  
    Newcastle-under-Lyme, Staffs.
    it doesn't weigh much so transport costs should not be high. We use it in the glassworks because it is heatproof and doesn't scratch the glass.

    http://www.hoben.co.uk/vermiculite/insulation.htm
    • CommentAuthorJohnh
    • CommentTimeDec 8th 2007
     
    We get our vermiculite (branded over here as 'Micafil' for our local builders merchant). Having been involved in lining several installations, I would always recommend insulating the flue. It cost us about £16 to do our 7 metre chimney. It does make a difference.

    The stays for the twinwall installation on that link Nick, are actually 2 lengths of SS studding fixed to a galvanized corner bracket screwed through the slates and into a joist - not a slate batten.
    • CommentAuthorskywalker
    • CommentTimeDec 8th 2007
     
    I got my vermiculite from Warrens (they are local to me) but any insulation supplier should be able to get it for you (it is the same as the stuff sometimes used for loft insulation rather than the horticultural grade) cheaper than any fire store. I only needed 5 bags for my chimney as it is quite small. If your chimney flue is huge however you may be better off using rockwool sleeves or chimwrap (a sort of flexible jacket) - you will probaly need to pull the liner out to fit these however.

    I'm no expert but did look very carefully at all the options when I diy'ed our flue liner etc a couple of months ago.

    This site was a brilliant source of info: http://www.stovesonline.co.uk/.

    S.
    • CommentAuthorJohnh
    • CommentTimeDec 8th 2007
     
    Another great site for 'diy' flue/stove installations is:-

    http://www.backwoodsman-stoves.co.uk/MAINMENUS/InstallationMenu.html
  13.  
    Nick, I solved my (very severe) downdraught problem with an aeroynamic chimney cowl - check out http://www.fluesystems.com/cowls/info/colt_top.htm
    Approx £40 - £50.

    Also, how many bends do you have - that could also be a contributory factor. The greater the total degrees of bend the slower the air flow and hence more susceptible to downdraught. Are you happy with the quality of the wood - at least a year old, well seasoned?

    HTH - Good luck

    Archie
    • CommentAuthorRod@BHF
    • CommentTimeDec 9th 2007 edited
     
    Hi Nick, we had the problem you described until we were told the hat type chimney cap was designed only for sealing a flue to allow ventilation and not for any gases. We fitted a ridge tile type top cap and the problem dissappeared up the flue!

    Rod

    www.busheyheathfarm.co.uk
  15.  
    Thanks all. I thought I had posted a reply but it does not seem to have 'stuck'.

    Menu seems now to be:

    Vermiculite

    a/d cowl (Archie, the Colt top is shown as fitting a clay pot. Do you know if it'll fit on a s/s flue-pipe too (6")? I should have thought so.

    2m s/s flue extension.

    Archie, not much of a bend, just a slight kink.

    Some of the wood is not as old as it should be.

    Oh, and I seem to have installed my liner upside down!! Oops.

    Cheers.

    Nick
    • CommentAuthorJohnh
    • CommentTimeDec 9th 2007
     
    Nick, the 'liner upside down' thing could cause you big problems in the future, especially if you are burning unseasoned wood. The resultant creosotey gunk that will be deposited on the inside of your liner, will seep out into your chimney void. When your flue heats up again this highly flammable mixture can drip down onto your register plate or whatever and run down the outside of your liner/stovepipe and be potentially very dangerous.

    If you're going to have to attend to the termination of the liner anyway, I would suggest, that however much a pain it will be, you should remove your liner and put it the right way up.

    Good luck,

    John.
  17.  
    Nick, I don't see any reason why it shouldn't fit on your s/s pipe - you may need to drill three samall holes for the screws to locate into, or drill the holes and secure with thin galvanised wire.

    Bends are obviously not a problem in your installation. However, I feel you really want to try some bone dry wood before you get into serious expenditure. Ideal stuff would be old floorboards or joists - any nearby skips you can rake about in?

    Two other thoughts - a bird (now dead!) hasn't got stuck down your chimney? And is there sufficient air coming in to the room? I would have thought there should be, being Victorian, but I've seen old houses practically hermetically sealed with double glazing and laminate flooring.

    Cheers

    Archie

    Persevere, I know how disheartening it can be - especially when you'd told SWMBO that you'd all be going around in shorts and T shirts once this you'd got this thing going!
  18.  
    John,

    I think you will be right, but the weather and my inertia dictate a 'watching brief' till the spring, I think. I have a 'hatch' in the register plate so that, if I am careful with my rockwool 'bung', I can inspect the liner for 'fall-out' without losing all my vermiculite!

    Archie,

    A really good friend is a furniture-maker, so I can get lots of really dry s/w, but it's hardwood I really need. When I turn down the draught I get blow-bk even with really dry wood. No birds, sufficient ventilation, and I have just worked out who SWMBO is!

    Thanks again, all.

    Nick
    • CommentAuthorJohn11668
    • CommentTimeDec 10th 2007
     
    As someone said a general rule of thumb is " at least 1 metre above the slope".
    A more effective rule is that it should go high enough so that distance measured horizontally to the slope of the roof exceeds 2.3 metres.

    Aerodynamic cowls such as the colt range are often effective for eddying or swirling winds but if your chimney is blowing down cos it terminates in a pressure zone on a windy day, the only solution is to extend beyond that zone!
  20.  
    John 11668 said:

    ''A more effective rule is that it should go high enough so that distance measured horizontally to the slope of the roof exceeds 2.3 metres.''

    Yes, it doesn't! I'll see what the meas. is with my heath-rob 1m ext'n, and with my proposed 2m.

    Thanks.

    Nick
    • CommentAuthormikebs
    • CommentTimeJan 7th 2008
     
    A very useful pdf which explains chimneys from Euroheat http://www.euroheat.co.uk/docandpdfs.php?pdf_real_id=1033&show1=&prod=&ftype=pdfs
    •  
      CommentAuthorScarlett
    • CommentTimeJan 8th 2008
     
    I have been a chimney engineer for 18 years, in that time I have solved many downdraught problems caused by many different reasons. I have tried most of the anti downdraught cowls available with mixed results. cowls may solve some problems but are mainly designed to cover up a basic problem with the fireplace, stove or chimney design.

    An anti downdraught cowl or rainhood (chinamans hat) can in fact cause unwanted turbulance at the flue terminal!

    I have removed many seized aspirators, constantly blowing tar over any mechanism is asking for trouble. In one case the aspirator stopped, the house filled with smoke and the elderely gentleman is lucky to stil be here. The main worry though is that your liner, if fitted correctly ,should last 20 years plus, any metal terminal fitted will only last upto half that time. When the terminal rots enough from exposure it will probably fall and cover your flue.The hope is that you will not be snoozing in front of your fire at the time!

    With your particular problem I would suggest throwing your liner away and starting again as it is not long enough.

    The top of your liner should have a minimum clearance horizontally of 2.3 meters away from any structure, if you add 2 meters of insulated chimney system to a weak stack, a good wind will have the lot down. A liner should also be in one complete length with no joins especially at the top.

    I would suggest removing a couple of courses of the stack, add a new liner (7.5 meters, arrows pointing up) ,backfill with loosefill vermiculite to 600mm below then top up with rammed vermiculite concrete 6:1 opc. Fit the plate and clamp, add 6-8 courses of brick and top up concrete insulation, add a 900mm pot set into the top two courses(corbeled). Stuff up the liner and carefully pour micafill to 100mm from top of pot. Finish around the liner with 4:1 vermiculite concrete. Cut the liner 25mm above pot.

    Assuming this will raise the flue to the desired height, you have adequate ventilation, there's no cooker hood extractor near by and your arrow stove isnt like their old models which smoked with the door open I can guarantee it will work.

    I do have an issue with the installer who should have picked up on the chimney height,Is he Hetas registered?
  23.  
    Scarlett,

    Thanks very much for this. I really cannot afford at this stage to buy another liner, but I do take your point. It is ironic that I had over-ordered (as I thought) and have hearly 2m lying in the garden, that was cut off! I will try to make a compromise which incorporates as many of your suggestions as poss, ensures stability but doesn't involve replacing the entire lliner. Yes, the installer is HETAS reg'd..
    • CommentAuthorJohn11668
    • CommentTimeJan 9th 2008
     
    " is he Hetas registered"

    I suspect not , In do not know of a single Hetas registered installer ,
    Rocking horse poo springs to mind!

    Most wood stoves are DIY anyway
    • CommentAuthorJohnh
    • CommentTimeJan 10th 2008
     
    (John 11668) "In do not know of a single Hetas registered installer"

    There are at least 2 posting on this forum :-

    http://www.itsnoteasybeinggreen.org/forum/viewforum.php?f=34
  26.  
    Nick,

    Can reconmend our HETAS registered stove installer-come-sweep, 'Border Sweep' http://www.chimneyworks.co.uk/south_yorkshire.html

    ...naturally there's a chance that this could be the same bloke...

    When we had people round to price we noticed a difference between the plumbers who install stoves and sweeps who install stoves the latter being cheaper and more helpful...

    J
    •  
      CommentAuthorScarlett
    • CommentTimeJan 10th 2008
     
    "Most wood stoves are DIY anyway"

    That kind of talk kills!

    Yes there is a shortage of good stove and chimney specialists.Many HETAS engineers come from being plumbers, gas engineers, scaffolders etc., most have only become registered over the past few years. With confusion in the market over good working practices there are large variations in quality of workmanship and advice from company to company.

    the reason for low numbers of good engineers who offer the complete stove and chimney service (the two go hand in hand) is because it is a specialised profession, in many ways a lost trade.

    chimney sweeps are a good source of information in finding decent installers as they are the ones who have to deal with the aftermath of a badly fitted stove. A good installation, where the end user has been given the correct instructions will be a pleasure to sweep.
    • CommentAuthorsune
    • CommentTimeJan 22nd 2008
     
    I pretty much agree with Scarlett.

    We have found that rotorvent cowls (the spinny ones) CAN be dangerous if not properly maintained as they can end up as balls of tar.
    Having said that we have had very few problems with the MAD type antidowndraught cowls - non moving cowls.

    You really do not want to be messing about with adding a botched 2m length of single skin pipe / liner to the top of your chimney.
    remember tar pours down the inside of a chimney a bit like water and soot and deposits build up especially when the flue is cold - like that short section.
    Adding a twin wall support plate to a crumbling brick surface - what is it going to fix to??

    First check that you do not have a ventilation problem - open a window when the stove is smoking - does it improve? if so you need to add fixed ventilation to the room.

    I'd rebuild the stack and make it higher.

    You can get (you are not meant to use it though) a liner to liner joiner to join your extra section of liner on INSIDE your rebuilt stack.
    You would need a support plate and clamp below the joiner and another at the top of the stack - ie you'd need one extra. You are not meant to do this really.

    If you insulate (v v good idea) I would use leca not vermiculite. vermiculite is great until it gets wet - if any water gets into the chimney then it gets all soggy and compresses, which leca does not.

    There are lots of v good hetas reg installers - we deal with them every day. There are also lots of v good no hetas installers - again we deal with them every day.
    • CommentAuthormagnolia
    • CommentTimeJun 3rd 2008
     
    Probably a bit late with the comments but....
    Hello All,
    I have found that the best anti-downdraft cowl is one they have been using in Germany for years. Its called the Windkat cowl (www.windkatcowls.co.uk or www.windkat.com) and you can get it from www.fluesystems.com
    A friend recently installed a big expensive woodburning stove and tried every solution on the market with no success. Her last option was to get a massively expensive electric fan system but her installers turned up with a windkat cowl and its worked perfectly so far.........
    I agree with the above comments that ideally a bit of a rebuild is necessary, but the cheapest quick fix could be installing a windkat. They do a couple of configurations one for installing on top of a chimney stack and the other that plugs into the top of a flue pipe.
    • CommentAuthorCWatters
    • CommentTimeJun 3rd 2008
     
    My architect said the rule for a chimney was to go above the ridge or you will get a down draught in some wind directions. I know a bit about aerodynamics and that makes perfect sense to me. It's not hard to see how anywhere else below ridge height could experience high or low pressure or just turbulent air flow. Just had a look at the Windkat cowl web site and it does look like an interesting design worth trying.

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