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Thread: DHW maybe someone can help

  1. #1 DHW maybe someone can help 
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    Hi,
    I am installing a log burning fire insert and would like to heat water from it, nothing new in that I know But my idea is to use the same idea as solar tube ie a lengths of tube sealed at each end with a small amount of liquid in each one. fFitting into a manifold at one end and fixed to the fire insert at the other end. The manifold would be in an open loop system to a storage tank.
    Can anyone tell me the ideal length the tubes should be? (they are !2mm dia).
    And also what liquid I should use for best heat transfer.
    The reason for doing it this way is so not to remove to much heat from the fire or flue pipe
    And finally how many tubes I will need to heat 80ltr of water in an hour
    Thanks in advance for any help


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  3. #2  
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    The easiest way to do it would be to look on-line for the specifications for a commercial product that does what you're thinking of. There are plenty of those.

    Then think how you can tweak it to work better in your environment for your own purposes.

    As for heating 80 litres of water in an hour.
    From what temperature to what temperature?
    How often does this have to happen each day/ week?
    What happens when the water's hot and the heater's still working? How and where do you store, recirculate and reheat previously heated water?


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  4. #3  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by birdy13 View Post
    The reason for doing it this way is so not to remove to much heat from the fire or flue pipe
    And finally how many tubes I will need to heat 80ltr of water in an hour
    The second sentence defines exactly how much heat needs to be removed from the fire (I have no idea if it is realistic or not).

    I second the idea of using a commercial product. Unless you have the necessary knowledge and skills, something design for the purpose (which is readily available) will be much more efficient (and safer).
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  5. #4  
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    Quote Originally Posted by adelady View Post
    The easiest way to do it would be to look on-line for the specifications for a commercial product that does what you're thinking of. There are plenty of those.

    Then think how you can tweak it to work better in your environment for your own purposes.

    As for heating 80 litres of water in an hour.
    From what temperature to what temperature?
    How often does this have to happen each day/ week?
    What happens when the water's hot and the heater's still working? How and where do you store, recirculate and reheat previously heated water?
    water suply temp averages around 18 deg C, Hot water required will be 50 deg C
    water will be heated once per day.
    hot water will be stored in a thermal store tank, excess hot water will be used for under floor heating with further excess cooled by thermo siphon to a loft radiator
    I have looked online for anything similar but found nothing, most log burners use a water jacket that surrounds the fire or a heat exchanger that fits around the flue, both of these systems work well but the main drawback is the cooling of exhaust gasses which an cause the fire to be less efficient and cause a buildup of creosote in the flue pipe,the main cause for chimney fires. I have the skills to build but not the mental skills lol. thankx again for your help
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  6. #5  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by birdy13 View Post
    I have looked online for anything similar but found nothing, most log burners use a water jacket that surrounds the fire or a heat exchanger that fits around the flue, both of these systems work well but the main drawback is the cooling of exhaust gasses which an cause the fire to be less efficient and cause a buildup of creosote in the flue pipe,the main cause for chimney fires.
    We were advised not to use our wood-burner for hot water (or radiators) for just that reason. I can't really see any way round it: if you want to heat water, you will have to take heat out of the fire. TANSTAAFL (There Ain't No Such Thing As A Free Lunch)
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  7. #6  
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    both of these systems work well but the main drawback is the cooling of exhaust gasses which an cause the fire to be less efficient and cause a buildup of creosote in the flue pipe,
    Last time I looked, which I'll admit was a long time ago, the most recent models of woodburning slow combustion heaters used a "double burn" set up. The firebox was designed so that "exhaust" gases were redirected back through the fire itself to ensure more complete combustion.

    If I were you, I'd have a good look at the design and specifications for commercially produced fires and see if your concerns about the flue pipe are justified. Regardless of what you do, baffle plates and flue pipes have to be maintained and regularly cleaned. Back when we had a not-at-all-sophisticated slow combustion heater, we used a standard product once a year or so to clear this before it built up enough to need professional cleaning.
    "Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill
    "nature is like a game of Jenga; you never know which brick you pull out will cause the whole stack to collapse" Lucy Cooke
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  8. #7  
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    Do be careful and include a relief valve and exercise it occasionally. Unless you are skilled with wood fires, you will find control a bit tricku until you understand the strengths and weaknesses your particular insert.
    If you build up too much pressure, Your pipes may burst spraying boiling water on anyone nearby.
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