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Reverse convection flow in wood-burner

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

  1. oilman

    oilman

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    I think it might be possible.

    If you consider the pipes both rise vertically from the stove.
    The tank has been heated, and the stove then cools, the tank will be the driver.
    This could start the flow in reverse, and there may be enough flow to maintain it even with the stove heating again.
    There may be very little difference in height of the flow and return pipes on the stove.

    One possible cure may be to drop the return pipe at the stove right down to ground level then bring it back up to the return connection.
     
  2. Onetap

    Onetap

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    How?

    There are two vertical pipes, cross connected at top and bottom. The water in one is hotter than the water in the other. The water in the hotter pipe is less dense and will rise to the top of the system.

    Are you suggesting that the hotter, lighter fluid will sink? It is the same as whether things float or sink; less dense floats on more dense. .

    The whole system works by having a heat source at low level heating the water and a heat sink at high level cooling the water. Have I missed something? Please explain.

    However the pressure difference produced by gravity is tiny compared to that produced by a pump. I think he has the connections to the heating system in the wrong place and the reverse circulation (if any) is being caused by the heating system's pump..
     
  3. oilman

    oilman

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  4. Onetap

    Onetap

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    I read your post carefully several times.

    The tank is hotter than the stove and the tank will be the driver. The tank is hotter than the stove, so water convects upwards from the tank?

    Sorry, but that still makes no sense.
     
  5. JohnD

    JohnD

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    I have known a gravity HW system that under some circumstances would flow up through the "return" pipe and return downwards through the "flow" pipe.

    Once it had started circulating this way it would not reverse and go properly unless you turned it off and let it go cold.

    It was a problem because the Return pipe had a Cyltrol valve on it (which I later wound fully open)

    The installer thought it might be due to a long horizontal pipe run. Many years later I took the floor up and found he had propped one of them up on a half brick as an attempted fix, which might have helped. It did not happen after a new boiler was fitted.

    Boiler was an old sectional cast iron Glow Worm commercial.

    If you tell me that in theory I can't happen, I will believe you. In theory there is no difference between theory and practice. In practice there is.
     
  6. cider

    cider

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    ;)
     
  7. cbell

    cbell

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    Well guys, it may or may not be possible but I assure you that it happens: the return flow to the stove is hotter than the flow from the stove to the heat store. There are also horrible bumping and rumbling noises from the boilers as the water boils at their base and can't escape upwards. These are indisputable facts. (The boiling noises during over-heating with normal convection are completely different, more hissing/roaring such as one gets from an electric kettle, and they come from the top of the stove.)

    As for the CH pump driving the problem: possible, but I don't think so. This tends to happen when the CH is not drawing water from the store (ie pump not running) since that is when the water temperature at the base of the store is highest because their is no cold return from the radiators coming into it.

    I suppose there must be a slight pressure difference generated by the CH pump when it is running, but it must be small (28mm inlet and outlet pipe into a tank 450mm diameter, vented at its top) and - more to the point - 99% of the time this causes no problems. Also surely any pressure difference would be relative suction at the top and hence more likely to want to draw water from the flow pipe from the stove, which also goes into the top.
     
  8. JohnD

    JohnD

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    I am just a householder but I think that sounds like a good idea.

    It used to be called a thermosyphon effect. The water in the stove itself will rise upwards as it heats. I used to have an iron multifuel stove with gravity to indirect cylinder. The Flow came out of the top of the stove and the return went in the bottom, so it would be difficult to make it flow backwards.

    I was thinking you need to alter the pipework so that the gravity flow from the boiler, as it starts to heat up, must go up the Flow pipe. There will be a certain amount of cool water in both pipes until the flow gets going.

    I don't know if there is any advantage in lagging one or both pipes.
     
  9. doitall

    doitall

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    if the return is hotter back to the boiler than the flow, then it is being forced.

    How and why.

    No idea other than the Ch pump.
     
  10. cider

    cider

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    The day you stop believing is the day you stop learning.

    Can you divert the primary flow away from the heat store once its temp is satisfied therefore continuing gravity circulation from stove ?

    28mm diverter valve on inlet to heatstore dropping back into return controlled via cyl stat.
     
  11. doitall

    doitall

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    I spy a clue :LOL:

    That can't happen, unless there's a restriction on the flow, I think the water is boiling and going up the return as it's the only path.
     
  12. Onetap

    Onetap

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    You had also said that the CH would draw water directly from the wood-burner if the store was at less than 55 degC. There is , I'd guess, a 2-port zone valve on the flow on this circuit; so, where does the return connect into the gravity circuit and can this provide an alternative return path from the CH system when the store is hot? Something similar happens with tedious regularity on domestic heating systems where the upstairs CH return connects to the HWS primary return to save 4m of pipe.

    A diagram would help.

    I do not say that it is impossible for the water to flow up the return and down the flow. I still say that you will not get hotter water going down and displacing colder water upwards by gravity alone. This isn't theory, it is indisputable, unchangeable, concrete fact. If this is happening there's something else driving it, either the pump (my theory) or the steam (doitall's) or something completely different.
     
  13. doitall

    doitall

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    Whatever Onetap, there cannot be an open circuit for it happen, unless it's being pumped.

    I think there's a valve or something on the flow which is why it is boiling, and the only escape is the return.
     
  14. cider

    cider

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    Why would water boil up the return and back down the flow :?:

    Do you not know how a standard gravity circuit operates in overheat :?:

    Why do you think there is a tank in the loft with an open vent off the flow discharging over the tank and cold feed going back into the return :?:
     
  15. doitall

    doitall

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    Explain your theory in simply terms for a DIYer then.

    Why or how can a gravity circuit boil if it's installed correctly, unless there's a restriction in the circuit.

    Read also, and explain in simple terms how he controls the store at 50c if there's no controls.

    I would think any DIYer with half a brain could soon work it out.
     
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