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Renewable Energy: Conventional vs mains pressure dhw

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    • CommentAuthorLeo
    • CommentTimeApr 20th 2007 edited
     
    All the diagrams I've seen of solar/ back boiler mixed DHW systems have a header tank. Is there any reason why the system shouldn't be of the newer mains pressure with expansion vessel variety. Likewise is there any reason radiators from a back boiler/ woodburner shouldn't be on an unvented system? I'd like to reclaim some loft space.
    • CommentAuthordragonbc
    • CommentTimeApr 21st 2007
     
    If you look at buidling regulations and the HETAS web site all will be clear as to why you should not use unvented system with wood fuelled back boilers. It is nnot permitted
    • CommentAuthorGuest
    • CommentTimeApr 21st 2007
     
    Thanks, but I can't find any ref to unvented central heating on the HETAS site - can you give me a link? I'll try and get a look at the building regs also, but can you tell me where to look? In the meantime could you briefly explain what the problem is?
    • CommentAuthorLeo
    • CommentTimeApr 21st 2007
     
    sorry that was me
    • CommentAuthorTuna
    • CommentTimeApr 23rd 2007
     
    I should imagine that a woodburner in particular has far less control, therefore much higher chance of boiling the water, therefore in an unvented system, things could go very wrong indeed.
    • CommentAuthorGuest
    • CommentTimeApr 23rd 2007
     
    The header tank (feed and expansion cistern) is for the primary system – that is the water in the back boiler, radiators and heat exchanger coil in the cylinder. On modern gas-fired boiler systems the primary water can be in a sealed system fed from the mains, so no cistern is required. However, I’m pretty sure this is not possible for solid fuel appliances.

    In a conventional system there would also be a (much bigger) cold water storage cistern, which would provide water for the secondary system – that is the hot water that comes out your taps. An unvented cylinder does not require a cold water storage cistern and has provision for expansion in an expansion vessel.

    I can't see why you couldn't use your back boiler to heat an unvented cylinder, but I haven’t found anything that says it’s ok either! I can't see anything in G3 (building regs covering unvented cylinders) that prohibits it. The back boiler would heat the primary water and you would have a 2-port motorised valve wired into the cylinder thermal cut-out (compulsory in-built safety device for unvented cylinders) and user controls to control the supply to one of the two coils in your unvented cylinder. The other coil would be heated by the solar panels. Check that the cylinder is compatible with an open vented primary system with the manufacturer. I’m pretty sure that Viessmann have a system that combines solid fuel (albeit a wood pellet boiler) and solar.
    • CommentAuthorLeo
    • CommentTimeApr 24th 2007
     
    Thanks Guest, but why don't you think it's OK to have the primary system unvented? Surely in the worst case scenario (runaway heating) it's only under the same stresses as a gas system? Although of course thinking about it, and reflecting on Tuna's point the lack of control makes it far more likely for a solid fuel system to boil - perhaps that's it, but I'd love to know for sure what the implications are.

    I'll be seeking advice from a specialist installer before going ahead but just wanted to know what the possibilities were. My current gas system has an unvented primary system with an expansion vessel and 2 x 50 gallon cold water tanks in the loft feeding the dhw (which seems kind of excessive - perhaps the previous owners were expecting the apocalypse or something). The Boiler will need replacing soon and I want to make sure that any upgrade will be compatible with the future addition of solar and/ or woodburner. I also don't like the idea of all that water hovering over my bed.

    Perhaps the answer is to swap them round and replace the swimming pool with a small header tank for the rads etc.
    • CommentAuthorJohn11668
    • CommentTimeApr 27th 2007
     
    A thermal store would be permitted and there are models with the FV tank incorporated.

    The secondary hot water side can then be mains fed so no Cold storage needed in the loft. You may have to watch for an adequate head over first floor radiators if the Cylinder is on that floor
    • CommentAuthorGuest
    • CommentTimeMay 4th 2007
     
    A Gledhill thermal store (www.gledhill.net) or similar alternative might provide a suitable solution.
    • CommentAuthorLeo
    • CommentTimeMay 5th 2007
     
    Thanks for those suggestions, I'll certainly follow them up. Currently cylinder is in cellar and boiler is between 1st and 2nd floors.:confused:
    • CommentAuthorJohn11668
    • CommentTimeMay 11th 2007
     
    Regs say that there should be adequate gravity circulation from a solid fuel appliance to dissipate the slumbering heat output . It would normally be targetted to the cylinder and a heat leak radiator . In the event that the cylinder is below the boiler I trust you have an alternative heat leak circuit

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