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Reverse convection flow in wood-burner

Discussion in 'Plumbing and Central Heating' started by cbell, 14 Feb 2009.

  1. doitall

    doitall

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    Yes Tony, it's true there should be an anti gravity loop on the return, whereas the pipe drops out of the boiler before rising to the cylinder.

    The point that everyone seems to be missing, is the banging and bumping where the boiler is boiling.

    That signifies NO CIRCULATION, heat cannot go up either pipe unless it is displaced with cooler water.

    I would like to see more information on the stove and store.
     
  2. Agile

    Agile

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    An uncontrolled wood boiler will boil if there is no circulation and without the best pipework layout thats just what I would expect.

    Most stoves actually create most heat low down and so the vincinity of the return may well be hotter and contribute or even be whats causing the reverse circulation.

    Correcting the return pipework will prevent reverse circulation and the boiling.

    Tony
     
  3. doitall

    doitall

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    Any boiler will boil if there's no circulation, an irrelevant statement I think.

    Altering the return could make the situation worse if there's a problem with the flow, I don't like not seeing an open vent, and that could have a lot to do with the problem.

    Hot water will always rise, whether its in the boiler jacket, the store, or the pipes, unless there's an outside influence preventing it.
     
  4. cbell

    cbell

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    Tony ("Agile") said:
    The swept tee is now my equal favourite with another idea I've had, which is to add a zone valve to where the CH flow meets the heat store. This zone valve would normally draw from the heat store, but in the situation I've described it could switch to suck direct from the flow from the stove thus circulating around it under pressure. What appeals to me about this is that I don't need a second pump, but I'd have to think carefully about where to attach the pipe.

    The diagram is correct: all the same water - why not? Remember that the heat store is not domestic hot water, but just a reservoir in the system.


    "doitall" said:

    Bog standard 210 litre direct hot water cylinder off the shelf. Cost about £240. So-called "heat stores" tend to be £1000 upwards - silly money! It came with 25mm foam lagging, so I've since wrapped loft insulation round it to raise this to about 250mm thickness. You are looking at the side from which the CH pipes come in and out, those from the stove are 180 deg around the other side out of sight. You can also see the expansion (lagged) and fill (not yet lagged) pipes going off to the header tank above and to the right out of shot.


    The diagram isn't perfect: there is an expansion pipe from the top of the heat store to above the header tank, and also another expansion pipe from near the pump to ditto. There are also two separate fill pipes, and lever valves dotted around in strategic places to isolate bits of the system.

    Just outside Tiverton.


    "Doitall" said
    The store is shown above, here is the stove. It's a "Fireview" 20kW model from Woodwarm, Cullompton, Devon. It is fully boilered: sides, back and roof, plumbed in series as the manufacturers recommend from side/back boilers into roof. Sorry, I don't currently have a photo of the plumbing round the back.


    I take your, and others', point that an anti-syphon loop should dip down below the back. You can't see the piping here because it is neatly hidden behind the chimney, but it doesn't do this - going instead straight into the side/back boilers at their base. However you can see that it is a solid hearth (with rock below) so it could only dip down about 5 inches. (BTW it was plumbed in by a qualified plumber well used to dealing with solid fuel devices and mixed CH systems.)

    I can imagine that this might contribute to the problem, but in normal usage it is OK.
     
  5. oilman

    oilman

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    What is the distance between the flow and return pipes on the stove?
     
  6. cbell

    cbell

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    I take it you mean vertically. Roughly the height of the fire-box of the stove itself, ie about 500mm.

    The difference in height at the heat store end is as you see in the photo, probably about 1m between flow and return tappings.

    Otherwise all pipe runs are pretty much parallel and either vertical or slightly sloping upwards where they have to be horizontal. The overall rise from stove to heat store is about 5 metres.

    The pipes are lagged individually in the roof space and in their runs inside the walls, and then covered by a layer of loft insulation. There will be some heat transfer between flow and return, but it will be negligible over a short period of time.
     
  7. oilman

    oilman

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    If the stove connections to the cylinder are the same heights as the CH connections, it gives quite some potential to drive the reverse flow, though with 500mm spacing on the stove there's possibly not quite enough there to stop it.
     
  8. cider

    cider

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    Fairplay to your controller, wish I had the time to make things ike that!
    Look at the Dunsley web site for ideas to join your injector tee in.
    They have some good circuit diagrams to copy.
    Last multi fuel I did used an injector tee to boost gravity ran from the heating pump.
    P.S. Hope that heatstores base is bridging more then 2 trusses :eek:
     
  9. doitall

    doitall

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    How :eek: There must be at least 1000mm between the flow and return, which must give you at least 10c differential in favour of the flow.
     
  10. cbell

    cbell

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    True, although most of the time the temperature differential in the heat store is quite large - typically > 20 deg C - and it's not a problem.


    I think I have three possible solutions:

    (1) an injection tee + pump on the flow circuit from the stove, triggered either manually or by a high return pipe temperature.

    (2) a 3 way valve on the CH flow, triggered as in (1) above, that switches from drawing water from the heat store to drawing it directly from the flow pipe from the stove, thus forcing flow to be in the right direction.

    (3) add remote sensing and readout of temperature at base of heat store, so that I can avoid the "hot store, cold stove" problem in the first place.

    The last of these is by far the cheapest, has the merit of not needing any plumbing, and would give useful info during normal running; so I think I'll do this anyway and see if prevention can be made to work - if not I'll have to adopt cures (1) or (2).


    Once again my thanks to all contributors, even those who reckon I'm in cloud cuckoo land.
     
  11. cbell

    cbell

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    I'll have a look at the Dunsley site for inspiration - thanks.

    The controller took 3 weekends to design and make, and another w/e to wire into the existing rats-nest of wiring. I tend to go rather slowly and carefully when dealing with 240v AC! It cost about £70 for the bits of which the box itself was the most expensive, followed by the length of multi-core cable to get to the pump/valve area. It's all switches, relays and neons - no fancy electronics.

    I also added another two channel timer: one channel for the wood-burner and one for the boiler, so that I can limit when the boiler will fire up if I haven't bothered to light a fire. However I'm meeting a certain amount of "consumer confusion" on the part of my non-technical wife now that there are 4 timers and a control box to think about!

    The heat store *is* definitely sitting on two trusses, with spreader beams underneath, and in fact it is also partially supported by the chimney breast and a stud wall. I wanted a bigger store, to hold more heat, but I got cold feet about adding more than 1/4 ton up there!
     
  12. doitall

    doitall

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    The relationship between the CH and primary connections is not a problem.
     
  13. Onetap

    Onetap

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    Where did he say the pipe drops on leaving the boiler? I missed that.

    I think it's probably boiling and air-locking (or steam-locking) the flow pipe and this is stopping the gravity circulation (NO CIRCULATION). The steam pressure is then displacing the water in the stove downwards and up the return pipe.

    The manufacturer's website say the stoves have top and saddle boilers; we don't know how these are piped up, so that may be affecting the gravity circulation. I'd like to know whether the return connects to the bottom of both side boilers and the flow to the top boiler.

    I think my pump theory is scuppered; it is possible that there is a jet of water from the CH return causing a high pressure at the opposite gravity return. The simple test would be to turn off the CH pump and see if normal gravity circulation is restored. It's unlikely since it doesn't affect it during 'normal' operation.
     
  14. Parkymike

    Parkymike

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    You will never stop what is happening as long as you have a hole in your @ss.
    You have made the whole system far too complicated for solid fuel with extra timers etc and it will never stop the store trying to heat the boiler if it is hotter.Hopefully you will realise that it should be pumped upto a neutral point in the system from either boiler and then to have the flow and return to the heating on tappings from that.
    In effect the neutral point is doing what you want the alternatley switching motorised valves to do.Even if you pump the pipes from the solid fuel boiler you will still cool the heatstore as the temperature will even out regardless.Nice as your control system is admit that you have wasted time and money by expecting a solid fuel boiler to work like a gas or oil one,they take too long to warm up and the variable heat will always even out with being cooler at various times in the burning and heating cycle.
    What you should have done is fit a dunsley nuetralizer as it works,and if your qualified installer had anything about him he should of suggested it to you.When the wood runs out the oil takes over,no timer is needed except on the oil boiler and the stats on the woodburner only need power.The woodburner should also of had at least somewhere for the excess heat to go i.e. heat leak circuit or a stat on the gravity return to dump heat into the radiators which should have been fed direct from the woodburner as an extra little cicuit.Nice as your control panel is it is unnessesary and will never work as you intend.
    Please take a step back,cut your losses and fit a system that will work easily and will not leave your wife scratching her head and accept that solid fuel boilers are not supposed to work like regular boilers.
    If you don't agree that's your perogative but I have seen all sorts of weird and wonderful systems over the last twenty years of working on solid fuel and people expect too much from them.
    It's the main reson why I only fit dry systems now as manufacturers will tell you anything to get a sale but the reality doesn't live up to their claims,and you have to explain it to customers who don't believe you.
    I have no arrogance in what I say but am just being an experienced realist .
    Best regards
    Parkymike...
     
  15. oilman

    oilman

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    A dry stove man? I agree wet systems are frequently problematic. Did YOU build that tracked Landy? Really impressive kit. ;)
     
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