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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorTimSmall
    • CommentTimeApr 17th 2013
     
    Hi,

    I was wondering if anyone had used PEX pipe for boiler pressure relief valve discharge pipework? Sticking a bit of copper through my thermal envelope is a bit annoying...

    K values:

    Copper: 400 W·m-1·K-1
    Stainless steel: 16 W·m-1·K-1
    Brick: 1 W·m-1·K-1
    Polythene: 0.45 W·m-1·K-1

    Stainless steel is obviously fine, but 15mm stainless is difficult to get hold of (and a bit pricey), whereas I've got loads of PEX knocking about. JG say not to use PEX within 1m of the boiler in the FAQ - no problem since I can run the pipework down the wall in copper first, then just put the PEX bit inside the wall (could even stick the compression fitting inside the brick).

    I found this in a US PEX supplier's FAQ: "Zurn PEX tubing may be used on the discharge side of a typical
    water heater temperature-pressure relief valve."

    I think the max temperature for the discharge pipework is circa 135 degrees (i.e. just above the boiling point of water at 3 bar). Melting point of PEX is quoted as being between 114 and 140, but the pressure should be pretty much zero, as there will just be circa 400mm of pipe, followed by ~100mm of copper on the end (i.e. the bit that's outside the thermal envelope - PEX doesn't like UV light - I could even make the outside bit of copper 22mm) which is then open to atmosphere.

    ... so I reckon it's probably OK, but I wondered if anyone else had any thoughts / experiences on this one?
    • CommentAuthorTimSmall
    • CommentTimeApr 17th 2013
     
    Relevant links:

    http://content.zurn.com/web_documents/pdfs/installation/zmktg370-04%20PEX%20Installation.pdf

    (page 23)

    and:

    http://www.speedfit.co.uk/Home/Technical-Support/FAQ-s.aspx

    "Can I connect Speedfit pipe and fittings directly to a hydronic / central heating boiler?"

    BTW, I was thinking all metal to PEX fittings to be brass compressions (copper olives, with copper or stainless steel PEX pipe inserts).
    • CommentAuthorEd Davies
    • CommentTimeApr 17th 2013
     
    If the PEX is barrier pipe:

    Aluminium: 237 W·m−1·K−1

    OK, pretty thin (0.2 mm?) but still probably 1/10th or so of the conductance of copper pipe.

    Obviously barrier pipe is not needed in this application (except for pressure/temperature resistance?) but it might be what you have to hand so be tempted to use.
    • CommentAuthorTriassic
    • CommentTimeApr 17th 2013 edited
     
    "Unless the boiler manufacturer states otherwise, a minimum of 1 metre run of pipe from the boiler casing should be copper. "

    So once you're 1m away from the boiler there is nothing to stop you using a compression fitting to connect the Speedfit to the copper. Remember to use the correct insert in the plastic pipe when using a compression fitting.
    • CommentAuthorcrusoe
    • CommentTimeApr 18th 2013
     
    "I was wondering if anyone had used PEX pipe for boiler pressure relief valve discharge pipework? Sticking a bit of copper through my thermal envelope is a bit annoying..."

    Tim, you need to get out more. Had this one before. As Triassic says for F+R, but 100% metal for discharge. I phoned Hepworth re Hep2 0 once as I had been asked to look at just such an installation. Horse's mouth advice was, as per textbook, 95C max for plastics. As a discharge can be at v high temps if boiler cooks for any reason that excludes plastic plumbing.

    In practice PEX (not PB) would likely be fine, especially MLCP as it has great structural stability. I did an energy job for a GBF member/visitor in Mallorca and the local plumber had used Unipipe (PEX/MLCP) direct on to the solar panels, a mimimum stagnation of 180C, and that was three years old. BUT no telling what the lifespan of the pipe is, and if you want to get BCO approval, stick with copper.
    • CommentAuthorEd Davies
    • CommentTimeApr 18th 2013
     
    Say it's 15 mm pipe with 1 mm wall thickness so copper cross-section of about 50 mm². Guess length is 300 mm through the wall. Conductance is A/l × k = 50e-6/300e-3 × 400 = 0.067 W/K or about 1.3 W heat loss at 20 °C temperature difference.

    Not exactly an emergency but something you'd rather avoid. How about lagging the pipe some of the way up towards the boiler and sleeving the pipe in something reasonably temperature resistant so the cold copper doesn't cause any condensation in the inboard side of the wall?
    • CommentAuthorDantenz
    • CommentTimeApr 18th 2013 edited
     
    A simple question you might think, but regarding compliance the answer is not so straightforward. Plastic pipe usually cannot be used within 1 metre of the boiler because of the maximum flow temperatures that can be achieved by boilers (and including those temperatures under a fault situation) versus the failure point of plastic pipe under pressure. However, the discharge pipe work is the final safety mechanism for the system and must be of a suitable material that is able to take the release of excessive pressure, which is also directly related to excessive temperature, and must be able to discharge safely externally to atmosphere. It is my understanding therefore, that plastic pipe cannot/should not be used for PRV discharge purposes.
    • CommentAuthorEd Davies
    • CommentTimeApr 18th 2013
     
    On the other hand, a plastic pipe which is close to open to the atmosphere at one end isn't going to have excessive pressure.
    • CommentAuthorDantenz
    • CommentTimeApr 18th 2013
     
    Posted By: Ed DaviesOn the other hand, a plastic pipe which is close to open to the atmosphere at one end isn't going to have excessive pressure.

    I hear what you say but I believe a Gas Safe registered engineer worth his salt will pull you on using plastic pipe for the PRV discharge - I would have to!
  1.  
    When the thermostat on my immersion heater failed the pex pipework it vented through had the consistency of cooked spagetti. That was with a release valve set at 1.5 bar. Was fine when it cooled but as above wouldn't recommend it.

    Assuming this is a pressurized system? If not then don't believe it needs to be piped anywhere but I'm out of date on regs.
    • CommentAuthorDantenz
    • CommentTimeApr 18th 2013
     
    Posted By: HeadScratcherWhen the thermostat on my immersion heater failed the pex pipework it vented through had the consistency of cooked spagetti. That was with a release valve set at 1.5 bar. Was fine when it cooled but as above wouldn't recommend it.Assuming this is a pressurized system? If not then don't believe it needs to be piped anywhere but I'm out of date on regs.

    I rest my case!
    • CommentAuthorTimSmall
    • CommentTimeApr 18th 2013 edited
     
    <blockquote><cite>Posted By: Dantenz</cite>I hear what you say but I believe a Gas Safe registered engineer worth his salt will pull you on using plastic pipe for the PRV discharge - I would have to!</blockquote>

    Well yes, that's what I thought, but I googled it anyway, and was then surprised to find a US PEX-B supplier explicitly saying it's fine for use downstream of a relief valve (and the US judicial system is not known for awarding low damages for personal injury claims!).

    That having been said, it may well be that the US typically has temp+pressure relief valves (e.g. opens at > 90 Celsius or > 3 bar), whereas I think (BICBW) that just plain pressure relief is more typical in the UK.

    My personal conclusion was similar to Crusoe's i.e. it'd probably fine (particularly if the PEX was completely within the wall - I can't really see that there would be any personal injury risk at all) - but that the average Gas Safe bloke, or BCO might start to vibrate if they saw it.

    Perhaps I'll go for stainless - I found a just-the-right length bit of 15mm x 1.5mm wall SS for a few of quid on ebay - heat loss at 20 Celsius difference is 0.036 watts through that piece of tube - which is well into gnat's fart territory, so I think that even I can put up with that...

    Is any BCO / Gas Safe likely to complain about 15mm stainless do you think?
    • CommentAuthoradi
    • CommentTimeApr 18th 2013 edited
     
    I’m currently trying to finalise my own heating system for a bungalow which I’m trying to renovate to the Passivhaus Enerphit standard. I also have a question related to this topic! Can I discharge a boiler temperature/pressure relief valve via a tundish to an internal drain? I know you can discharge a pressurised tank via this method but is this allowed from a boiler? The reason I would like to do this is to avoid any more penetration through my air tight barrier and the obvious thermal bridge.
    • CommentAuthorDantenz
    • CommentTimeApr 18th 2013
     
    Is any BCO / Gas Safe likely to complain about 15mm stainless do you think?

    no - NEVER
    • CommentAuthorEd Davies
    • CommentTimeApr 18th 2013
     
    Posted By: HeadScratcherWhen the thermostat on my immersion heater failed the pex pipework it vented through had the consistency of cooked spagetti. That was with a release valve set at 1.5 bar. Was fine when it cooled but as above wouldn't recommend it.
    Was the PEX directly on the release valve or was there a metre or more of copper to cool the steam somewhat first?
    • CommentAuthorTimSmall
    • CommentTimeApr 19th 2013 edited
     
    Posted By: adiCan I discharge a boiler temperature/pressure relief valve via a tundish to an internal drain?


    I don't think the penetration through the airtight barrier is significant - just use an EPDM grommet (either home made or otherwise) where it penetrates the air barrier layer, or some carefully cut tape, or even some low-air-permeability expanding foam. Likewise for the thermal bridge - if done in stainless steel, then at circa 0.036 watts, it represent about 0.002% of the heating budget for my house. If done in copper, then it's still under 0.1%,
  2.  
    <blockquote><cite>Posted By: Ed Davies</cite>Was the PEX directly on the release valve or was there a metre or more of copper to cool the steam somewhat first?</blockquote>

    This was pipework from the hw tank and the whole route to the valve was pex. As the immersion is a heat source theres a good argument for the venting route to be copper, I didn't consider it. To give the pex its due every joint held. Suspect temps from a boiler with a similar fault would go far higher.
  3.  
    <blockquote><cite>Posted By: TimSmall</cite><blockquote><cite>Posted By: adi</cite>Can I discharge a boiler temperature/pressure relief valve via a tundish to an internal drain?</blockquote>

    I don't think the penetration through the airtight barrier is significant </blockquote>

    Not sure about that Tim. I just realised this today. Having got down to 0.75 m3/m2/h on a retrofit tested prior to connecting blow off when the penetrating pipe was therefore taped I've just realised that the outside pipe end is open and so is the tun dish so that's a 20mm diameter hole in my envelope. Heating engineer and I were discussing the possibility of putting traps off some kind in. Anyone got any thoughts?
    • CommentAuthorEd Davies
    • CommentTimeOct 29th 2014
     
    Wouldn't traps in pipes which aren't normally used dry out? Can't see anybody remembering to refill.
  4.  
    Ressurecting an old conversation but I don't think a tundish would be much use for whats likely to be water and steam at 3bar.
    • CommentAuthormarkocosic
    • CommentTimeOct 30th 2014
     
    32 mm push fit (not solvent weld - it isn't temperature rated) here for the combined condensate drain/pressure relief valve. In accordance with manufacturer's instructions is the key bit though: they're satisfied that their internal controls will keep discharge to 80C
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