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General: Confused by Megaflows

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1 to 13 of 13

  1.  
    • CommentAuthordelprado
    • CommentTimeMar 24th 2017 edited
     
    There is a lack of info when googling. What is direct vs indirect, and the fact that they can work with both a heat only boiler and a system boiler?

    Not much discussion about them on here
  2.  
    All a Megaflow is, is a cylinder. I've got one. Its an unvented (mains pressure) unit. Heated via 2 coils - one from the boiler, which is a Worcester Bosch Greenstar system booiler, and one from my solar panels. I think that makes it an indirect Megaflow. i.e its heated via a coil from the boiler, rahter than by direct flow.

    Doubtless a plumber will be along to correct my terminology.
  3.  
    Direct and indirect refers to the method of heating the water in the cylinder (typically a DHW cylinder) from the heat source. With an indirect cylinder the water in the cylinder never mixes with the water supplied by the heat source. A direct cylinder shares the same water with the heat source.

    An indirect cylinder usually has a coil within the cylinder that carries the water from the heat source and will transfer the heat to the cylinder by conduction through the coil. A plate heat exchanger could also be used to create an indirect cylinder.

    With a direct cylinder you need some sort of heat exchanger to get the heat out of the cylinder to the DHW. Thermal stores (TS) are almost universally direct cylinders, from the boilers at least but can have an indirect coil from solar collectors. Where the TS is used for heating the TS can be direct on both the input and output since there is no through flow, just circulation and the boiler, TS and heating share the same water. Where the TS also supplies a DHW cylinder then the DHW cylinder will have to be indirect.
    •  
      CommentAuthordjh
    • CommentTimeMar 24th 2017 edited
     
    Posted By: Peter_in_HungaryDirect and indirect refers to the method of heating the water in the cylinder (typically a DHW cylinder) from the heat source. With an indirect cylinder the water in the cylinder never mixes with the water supplied by the heat source. A direct cylinder shares the same water with the heat source.

    Indeed.

    An indirect cylinder usually has a coil within the cylinder that carries the water from the heat source and will transfer the heat to the cylinder by conduction through the coil. A plate heat exchanger could also be used to create an indirect cylinder.

    Or a tank within tank design as popular in Denmark.

    With a direct cylinder you need some sort of heat exchanger to get the heat out of the cylinder to the DHW.

    Not quite but almost always, since there's yet another distinction there. It could be the potable water flowing through the boiler that goes into the cylinder.

    Thermal stores (TS) are almost universally direct cylinders, from the boilers at least but can have an indirect coil from solar collectors. Where the TS is used for heating the TS can be direct on both the input and output since there is no through flow, just circulation and the boiler, TS and heating share the same water. Where the TS also supplies a DHW cylinder then the DHW cylinder will have to be indirect.

    It's generally a very good idea to have separation between the water in the thermal store and that in solar collectors, since the latter typically need antifreeze whilst the latter doesn't. The return from a central heating flow needs careful management to avoid upsetting the stratification of a thermal store though, so a separate system using a coil or PHE might be preferred.
  5.  
    Posted By: djh
    With a direct cylinder you need some sort of heat exchanger to get the heat out of the cylinder to the DHW.

    Not quite but almost always, since there's yet another distinction there. It could be the potable water flowing through the boiler that goes into the cylinder.

    If you have the potable water flowing through the boiler then
    a) there is no chance to put inhibitor in the boiler
    and
    b) the boiler is likely to scale up very quickly
    so IMO generally not a good idea to have through flow water in a boiler
    • CommentAuthorringi
    • CommentTimeMar 25th 2017
     
    If a DHW tank does is not open vented (header tank in loft), it is call mains pressure. All mains pressure tanks need somewhere for the water to go when it expands as it heats, otherwise they explode and can remove a wall of your home. Most unvented (mains pressure) DHW tanks have a separate expansion vessel, a Megaflows tank has the expansion vessel built into the top of it.

    The makes a metaflow cylinder taller then other makes for the same amount of DHW, but saves you the space of an expansion vessel net to the cylinder.
    •  
      CommentAuthordjh
    • CommentTimeMar 25th 2017
     
    Posted By: Peter_in_Hungary
    Posted By: djh
    With a direct cylinder you need some sort of heat exchanger to get the heat out of the cylinder to the DHW.

    Not quite but almost always, since there's yet another distinction there. It could be the potable water flowing through the boiler that goes into the cylinder.

    If you have the potable water flowing through the boiler then
    a) there is no chance to put inhibitor in the boiler
    and
    b) the boiler is likely to scale up very quickly
    so IMO generally not a good idea to have through flow water in a boiler

    But combi boilers* do it all the time, and so did many traditional boilers. Horses for courses I suppose.

    * And I think combi boilers are the most popular type.
    • CommentAuthordimengineer
    • CommentTimeMar 25th 2017 edited
     
    Posted By: ringiIf a DHW tank does is not open vented (header tank in loft), it is call mains pressure. All mains pressure tanks need somewhere for the water to go when it expands as it heats, otherwise they explode and can remove a wall of your home. .


    No they wont. They will not "explode" There is very little stored energy, the main risk is the thermal expansion of the water which can rupture a presssure vessel, but as its liquid, and not vapour will not "explode"
    • CommentAuthorringi
    • CommentTimeMar 25th 2017
     
    Try a system without a pressure vessel or safety value and a faulty thermostat so the immersion heater does not cut out, then the tank can get to many bars and well over 100c. I have seen photos of walls removed by it.

    However you have to install a system very wrongly for it to happen.
    • CommentAuthorEd Davies
    • CommentTimeMar 26th 2017
     
    Posted By: djh
    If you have the potable water flowing through the boiler

    But combi boilers* do it all the time,
    Do they? I thought they had a heat-exchanger so it's the primary heating water (for the radiators, UFH, etc) which goes through the actual boiler then some of that heat goes via the heat-exchanger to the hot tap water. That's the main thing those plate heat exchangers are made for, AFAIK.
    • CommentAuthorringi
    • CommentTimeMar 26th 2017
     
    Ed, look at the Intergas Rapid for example....
    • CommentAuthordelprado
    • CommentTimeMar 29th 2017
     
    It sounds to me like I want an indirect system then, in fact I can't really understand why anyone would want a direct system unless they had no boiler/gas supply to the house
    •  
      CommentAuthordjh
    • CommentTimeMar 29th 2017
     
    Posted By: delpradoI can't really understand why anyone would want a direct system

    It's partly history - they used to be the only type allowed, I think. Plus the attraction of having a store of water in case the supply is cut. And less worry about the effect of faults if it isn't serviced. But nowadays I agree that direct systems probably cause more grief through corrosion than the benefits they're seen to bring.

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