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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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    Maybe, but NOT when the store is hotter than the stove. :rolleyes:
     
  2. Norcon

    Norcon

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    cbell wrote

    Can I ask what type of valve you used here?
     
  3. Norcon

    Norcon

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    Absolute rubbish!
     
  4. Parkymike

    Parkymike

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    Hi Oilman,
    I don't wish to hijack the thread but I got fed up of doing surveys for people who wanted weird and wonderful systems only get someone in to do as they wish against my advice and relieve them of a large sum of cash for a system that doesn't work and never will.So I said stuff it to wet solid fuel and now no more problem.
    The Land rover was one I spotted at Lix toll filling station near Killin in Scotland while on holiday, it just reminds me of the new logo.
     
  5. Parkymike

    Parkymike

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    My word Norcom,
    That's two strong words on a freindly discussion.
     
  6. cider

    cider

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    I dont quite agree he should srap the lot because it isnt right. As for neutral zone to run everything from that doesnt have to be a Dunsley. The heatstore can become the neutral zone by quite simply running the flow and return from heating directly to it using flanges.
    A dunsley neutraliser is nothing more than a small cylinder with multi tappings.
     
  7. doitall

    doitall

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    On that note I will agree cider :rolleyes: I may not on anything else, but scraping the lot is ludicrous.

    If push comes to shove you can always fit a pump on a bypass arrangement as per someones drawing although I would fit it in the long leg. Perhaps a better idea is to fit a pump in the primary return and use it as a shunt punt, that way the store and stove will always be the same temperature.

    It still doesn't answer why the flow is locking, perhaps a pipe is dipping, or there's even too much resistance where the pipe goes back into the wall before it rises.
     
  8. doitall

    doitall

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    You are suggesting it's stagnating, to do that the water would need to be 320c. ;)

    As I said elsewhere, I think, the op should measure the flow at the store and stove, and then the return, that way you can tell what is happening.
     
  9. cbell

    cbell

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    Well, short of a colostomy it seems I'm in trouble then...

    The control system - obviously - has no effect on the running of the wood-burner itself - how could it? It is there to *respond* to how the wood-burner behaves, and to mix'n'match its output to the demands from the house and the alternative of using the oil-fired boiler. In my opinion (biased perhaps) it works very well.

    I looked carefully at the Dunsley neutraliser, and did in fact design and cost a system which used that, but I rejected it for two reasons.

    Reason 1: A neutraliser acts as a common pressure point in the system, allowing flow from oil-fired boiler and wood-burner to mix.

    However, as others have pointed out, it has only a small volume and therefore it cannot (and is not designed to) act as a reservoir of heat, so it is not really suitable for a heat source such as wood-burner which fluctuates over time.

    Consider this: if the wood-burner were running flat out, but the CH system decided that everything was nice and warm and shut down, where would that 16kW of heat input go? I'd need a pretty fast responding control system to deal with that situation, wouldn't I?

    And at the opposite end of the spectrum when the stove died down either the oil-fired boiler would cut in (which I want to avoid, to save money) or I would be pumping colder and colder water around the system.

    Finally: my heat store gets warmed up last thing at night when when the stove dies down as we're tucked up in bed. Therefore it is ready to start heating the house the following morning - a Dunsley Neutraliser wouldn't do that, would it?


    Reason 2: A Dunsley Neutraliser is more expensive than a direct HW cylinder and, given the layout of our house, would have been an absolute pig to plumb in, requiring loads of floors to be pulled up, extra pumps, and a whole load of bother.



    Of course I'm biased, but I think that my heat store, with its ability to absorb and store 16kWh of heat, is simpler, safer, better and cheaper.

    It acts both as a safety buffer and as a way of smoothing out the fact that both demand for and supply of heat vary considerably over time.

    In addition a wood-burner needs to be running hot in order both to consume its smoke and to be efficient, so some method of decoupling its refuelling cycle (typically 1.5 to 2 hours) from the heat demand is really necessary to get the best out of it. A heat store is the obvious solution, and in fact the stove manufacturers recommend this as the best solution.


    The control system is in fact extremely simple and, moreover, works *exactly* as I intended. My wife is also intelligent and has proved herself able to work it.



    Sorry, no. It should be fairly obvious from the above that I have never expected a wood-burner to perform like a regular boiler, and that I have designed a system to cope with the differences.

    Also, conclusive point, it works very well and I see little point in ripping it out on that basis as we've been warm this winter in temps of -9 deg C with snow outside for a negligible fuel cost. In fact I would recommend this solution highly to anyone who is interested in fitting such a system.


    The original topic of this thread was the unexpected problem that gravity convection could go into reverse in the (rare) situation of very hot store and cold stove.

    Thanks to various contributors I have two plumbing solutions and one control/monitoring system one. Also my understanding of the situation has improved considerably.
     
  10. cbell

    cbell

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    In the end we didn't.

    When designing the system I was worried that I didn't have a significant heat leak in the system, so I decided that I wanted a heat dump radiator on the 1st floor of the house working by gravity since the real danger condition is during a power cut when the (electrically pumped) overheat protection won't work.

    However I didn't want it to be running continuously but rather only when the heat store showed signs of getting too hot, so I wanted a thermostatic valve that would *open* at about 80 deg C.

    So I did some intensive Googling and eventually discovered "temperature and pressure relief valves". These are safety valves intended for unvented (pressurised) HW cylinders, and they are designed to open either when the pressure increases to some number or when the temperature reaches some value. (Sorry if you already know this).

    Obviously my system is vented, and running at atmospheric pressure, so I don't care about the pressure aspect, but I was planning to exploit the "open on high temperature" to leak heat to that radiator.


    In the end we didn't do it for a variety of reasons:

    (1) The stove has proved to be highly controllable, and shutting down the air supply results in it dying down to a very low level in seconds. Therefore it is easy to stop any impending overheat.

    (2) We weren't sure without testing that the T & P relief valve would in fact let through enough water to be useful. It's designed for high pressure / low flow whereas we wanted a fairly high flow rate.

    (3) As I've mentioned in an earlier post today a 210l heat store can absorb a lot of heat. It would take 16kWh of heat input to raise it from 20 to 90 deg C (ignoring losses), and that gives a fair old safety margin.

    Realistically it would take about an hour of fairly hard burning during a power cut for the water to get anywhere near boiling, and it would boil first of all in the stove giving lots of audible warning. So you'd have to be both dumb and deaf to get into that situation.

    And if the cut lasted *that* long I'd probably fire up our stand-by generator.


    So I'm sorry, we never ended up buying or testing one.

    In fact I don't think it would have worked anyway because they come preset to work at 90+ deg C, and boiling in the stove can occur when the heat store temp is a lot less than that.


    I now believe that a stand-by power source to run the conventional CH system is a better and cheaper solution. I concede that not everyone will have a stand-by generator, but a 150 watt static inverter (£30) plus a 12v battery (£40) will be cheaper than the radiator + plumbing, and will keep the whole house warm in the bargain.
     
  11. Onetap

    Onetap

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    I would (and do) still fit a safety valve to the store, it's the last stop before explosive burst mode and they're cheap. It is possible that the open vent pipe could freeze or the cold feed could block with limescale.

    The T&P relief valves open full bore on temperature activation and are meant to allow cold water into the unvented cylinder to cool it rapidly.

    How could a heat dump radiator on the first floor disspiate heat by gravity from a heat store on the second floor?
     
  12. cbell

    cbell

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    But if the feed/expansion tank is frozen (a near certainty in that scenario) where's that cold water going to come from??? (And the displaced hot go to?)

    I suppose the simplest solution would be to make it a total loss failure mode and vent to the outside of the house, making sure there are no low points in the vent pipe in which water can collect and freeze.

    I discussed this with the plumber, but he felt it wasn't necessary because we have two vent pipes and two feed ditto, both heavily lagged; this is a soft water area (we have a borehole); and the system has lots of inhibitor.

    Actually, now I think about it, an ordinary oil or gas-fired CH system could just as easily explode in the "blocked vent and feed" situation, so I don't think we are in much more danger than before!

    There were no problems in the recent cold weather, which got down to -10 deg C overnight here. I can assure you that I was up there checking. Also with a fire every day, and a cut-off temp of 50 deg C at its top for heat extraction, the store only drops to 30 - 35 deg C, and this heat will be conducted up the vent pipe to keep it warm.

    However I have to admit that you are right, and it would be protection against the ultimate worst case. If we ever drain down to replumb something I think I'll probably fit one with a simple pipe to the outside world. I suppose that ideally it ought to go near the stove, since the pipe runs to/from the store might freeze, but this is an old house and we are talking about getting through 4 feet of stone wall to reach the outside at floor level, so I think it will end up in the roof.

    My proposed method was to plumb from base of store to radiator, and to take the return from the radiator directly back to the stove's return pipe on the ground floor, so the circuit through the radiator would become an alternative (and colder) return path.

    My hope was that the colder and denser water would create flow, however whether or not it would have worked I don't know. Theory says it should have, but the shape of the house, the realities of the pipe runs and the constrictions of the radiator valves that would be involved made us wonder a bit.
     
  13. Parkymike

    Parkymike

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    Quote:
    Nice as your control panel is it is unnessesary and will never work as you intend.


    The control system is in fact extremely simple and, moreover, works *exactly* as I intended. My wife is also intelligent and has proved herself able to work it.

    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!


    Did I imagine these words then?



    I appear to be shot down in flames for my post but I know that a dunsley sytem is designed that way for a reason and it works.Your system works intermittently and I can see various things that should be done differently,If you persist which you will because pride won't let you fail(which is a good thing)I wish you the best of luck,but I have dealt with systems that have been added onto and altered to try and make them work and unfortunately they never give full satisfaction.I learned experience the hard way and the long way.
    One other thing to consider is that now you have made your own control system box to work it all,Did you get it tested and approved to a suitable standard because if something ever goes wrong with the wiring like a fire your house insurance will walk away.Using parts already in the market your o.k.

    Quote

    In addition a wood-burner needs to be running hot in order both to consume its smoke and to be efficient, so some method of decoupling its refuelling cycle (typically 1.5 to 2 hours) from the heat demand is really necessary to get the best out of it. A heat store is the obvious solution, and in fact the stove manufacturers recommend this as the best solution.

    Ask yourself if a heat store of 210litres is big enough for a house with five zones to it.Also ask yourself if you need one in the first place?
    Comparing a nuetralizer to your heatstore as a for and against discussion is another waste of time as it isn't intended to work that way.
    They are wired up so the stats will switch on pumps and safely dump excess heat rather than holding it in a store (hmmm I wonder why)could it be to prevent the very problem your having?
    Also you called the nuetraliser a common pressure point.It is in fact a common neutral point which is there for a reason.By having your heating flow and return off the oil Primaries you have made it harder for the prevention of backflow,hence the need for the motorized valves that could have been avoided.
    Where you fit the cold feed and expansion into a system has an effect too.

    The key word to think about is 'Neutral'

    You seem like a Knowledgeable sort of a chap and should be able to sort it out as you seem to have an answer for everything,which makes me wonder why you posted in the first place.
    best regards Parky.
     
  14. cbell

    cbell

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    No shooting on my part. I think the Dunsley neutraliser is a good solution for boosting a CH system from a relatively small stove, but I believe it would be trickier to use it for a fully pumped system taking heat from a large wood-burner for all the reasons I gave.

    Remember that my wood-burner can kick out more heat than the average CH boiler, and that the only way to shut it down is manually to close the air intakes. Where would that heat go when the CH pump shut off? Either you'd need a "pump on overheat" system - which I have got - or you'd need to divert it to something like a HW cylinder - which I do.

    If you think about it I have, as others have pointed out, effectively got a neutraliser in the system, only mine is bigger and acts as a heat store as well. In many ways we are actually saying the same thing!

    Don't forget that it actually works very well 99% of the time, I'd hardly call that "intermittent".

    Of course I won't give up, and I will solve the problem. And of course I've made mistakes: the man who never made a mistake never made anything! The trick is to learn from them, share one's experience with and learn from others, and do better next time.


    Good point. They were going to test it, but in the end they didn't - it should at least be insulation tested. The reason I made it was because there is simply no standard solution on the market that you can buy off the shelf. Nor, in fact, is there a standard plumbing solution for mixing a wood-burner with an oil or gas-fired CH system, which is why I have the problem in the first place. (Yes there are solutions, of which the Dunsley neutraliser is one, but there is no "text book" solution.)

    It's a difficult and dangerous world we live in, isn't it? I shall have to rely on the quality of my own handiwork, our smoke detectors (of which we have 6, all linked together with battery backup), and our 2 dogs.
     
  15. Onetap

    Onetap

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    It won't, that's what it does on an unvented HWS store and they're very effective. But a pressure relief valve would relieve the pressure on the store and prevent a burst.

    If it has got to 95 degC, you want it to go to 'steam & boiling water down the drain fast' mode and not be fannying about with dubious circulation through a heat dump rad. Besides that, the T&PR is not intended to be piped to a circulation system where the discharge is under pressure. The pressure on the discharge will be the same as that in the store and will slow, or prevent, the valve opening to relieve high pressure.

    The control thermostat would have to fail closed (it happens) and the open vent to freeze or block. With a sealed system you'd need the control thermostat and the high-limit thermostat to fail closed and the pressure relief valve to fail closed.

    It could happen; for example, if the house were unoccupied during a freeze and the stove were fired up with CF & OV still frozen. There would be no convection and the frozen pipes would not thaw out.

    PS The Dunsley neutraliser is a simply low loss header or a small buffer vessel. Low loss headers have been fabricated out of vessels or pipe for decades for chilled water or heating primary/secondary circuits. I think your store is, if anything, better than the Dunsley offering.
     
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