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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.
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  1.  
    • CommentAuthorSodesu
    • CommentTimeMay 1st 2012
     
    Hello to all! I've been set in self-build project for the last few years now, a personal emboided trip down enduring street!
    Have been involved in many building projects for at least 20yrs.looking to upgame my green mo. And innovate my approach.
    So very pleased to join your forum-Would also like to add anything of relevance that I can.
    Sodesu.

    We have a 30KW stove with back boiler. A 120L dual core HWC is linked to the stove gravity feed and return pipe - 28mm copper, and an offset loop with 2 small leak rads.
    The radiator loop is run straight from the WB and extends approx.12m from aside HWC upstairs.

    Questions arise:

    How large would the (metal)feed and expansion tank need be to cope with the thermal expansion created from the 25KW boiler output?
    I'm thinking it would be necessary to have a Pressure relief valve over HWC, due to its relatively small 120L capacity? Would a larger leak rad help?


    The intention is to run four double rads to 2 large bedrooms with their en/suite using heatleak rads.
    However 12m run on the rad loop, as we would like to run pump free, seems open to airlock problems.
    Could Large bore pipes on inclines cope?

    Any help would be appreciated.
  2.  
    <blockquote><cite>Posted By: Sodesu</cite>How large would the (metal)feed and expansion tank need be to cope with the thermal expansion created from the 25KW boiler output?</blockquote>
    The size of the f&e tank is not so much a question of the stove size but of the volume of water within the system. That said the standard answer to this question is 10% of system volume, although you can get away with less on an open vented system.

    <blockquote><cite>Posted By: Sodesu</cite>I'm thinking it would be necessary to have a Pressure relief valve over HWC, due to its relatively small 120L capacity? Would a larger leak rad help?
    </blockquote>
    I think pressure relief valves are a good idea even on open vented systems and cost v. little. The need or otherwise for a pressure relief valve would not be negated by a larger heat leak rad. - its not really one or the other.

    <blockquote><cite>Posted By: Sodesu</cite>The intention is to run four double rads to 2 large bedrooms with their en/suite using heatleak rads.
    However 12m run on the rad loop, as we would like to run pump free, seems open to airlock problems.
    Could Large bore pipes on inclines cope?</blockquote>
    It's difficult to design from an armchair some distance from the action - but I have designed and installed a gravity system with a similar amount of horizontal distance and it worked well. The pipework was a combination of 28mm, 22mm and 15mm for the rads and 22mm for the DHW. As it was a gravity system all of the rads acted as the heat leak although 2 of the largest rads had no valves so could not be turned off. The pipework should be sized to match the size of the rads, designed to divide the flow according to heat load so a mixture of 28, 22, 18, and 15 would typically be used. The question of air locks is usually one of attention to inclines on installation rather than the size of the pipe although it is easier to get an air lock in a 15mm pipe than a 28mm pipe.
    • CommentAuthorSodesu
    • CommentTimeMay 2nd 2012
     
    Peter, thanks for your recent comments, clear enough, will utilise the 10% calcs for the F&E tank.
    Appreciate it is hard to use my general assumptions as a plan, it's hard to explain, SBT;
    still, might try and avail of your consideration a little further,if you would be so generous.

    I was thinking of installing one PRV 30 psi off the stove gravity flow pipe, or would this better suited to the rad loop? which would come directly out of the stove, having 2 outs an 2 returns, out back.
    The gravity rads were going to come off the gravity flow pipe from the stove to the f&E tank, otherside of HWC pipe link which lies directly above stove.
    Maybe there's better siting for this and concerned the 30psi might be a little limiting to gravity flow.
  4.  
    My current system (17 rads and 5m head to the F&E tank) has a 2.5 bar pressure relief valve and is located on the output as the first thing next to the boiler. The idea is that it protects the system from over pressure. IMO it does not matter which output it goes in providing it is close to the boiler and any and each circuit connected to the boiler can not be isolated from the PRV.

    As the PRV is tee'd off a flow pipe and not involved in the circulation I do not understand why you would be concerned that it might be a little limiting to gravity flow.
    • CommentAuthorSodesu
    • CommentTimeMay 4th 2012
     
    Peter, thanks again for your recent comments, got me thinking again.

    Firstly, that the 'Tee' is vital to the continuos flow of expanded water in the event of valve failure.
    However, as I understand it: When a valve is relieving water in volume, it is generally due to excessive temperature.
    Should excessive temperature have built beyond the capacity of the open vent (possible? due to uncontrollable heat source) to distrubute heat quickly enough.
    Then there would be less of a temperature difference in the system.
    Hence, with less system thermal flow there would be a need for greater pressure relief than a volume dispersable dependent prv could offer.
    No pump, fluid dispersion aid here.
    Sorry if I'm missing the clearer picture, just trying to build one into my thinking. Hope theirs no stupid questions in the interest of safety.


    Secondly, perhaps there is a need for a second failsafe Temparature\pressure relief valve on the boiler with a lower psi of 2.5.
    In the belief that the copper cylinder would be the first to go from over pressure.
    Perhaps thermostatic cylinder temperature relief is overkill if most cylinders are able to cope with 3 bar as standard.

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