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What size of unvented water heater....

Discussion in 'Plumbing and Central Heating' started by keep_it_simple, 21 Oct 2006.

  1. keep_it_simple

    keep_it_simple

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    would be required in order to supply a kitchen sink and bathroom basin (no bath, shower is electric) in a one bed flat? I've been looking at the LWC 50 litre cylinder but wondered if I might get away with a 15L Heatrae Hotflo.

    Cheers.
     
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  3. ChrisR

    ChrisR

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    I'd use the 15L one. Some (eg ariston) come 2kW or 3kW - obviously the latter would reheat faster.
     
  4. meldrew's_mate

    meldrew's_mate

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    ....
    but be sure to follow the manufacturer's instructions, especially regarding the length of feed pipe without valves/isolators. The hot water needs somewhere to expand into. If you don't the relief valve will soon start to weep permanantly.

    Above 15l capacity you need Building Regs Approval for unvented DHW cylinders.
     
  5. Nige F

    Nige F

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    Just done that in son`s flat.......and made a "trombone" behind sink unit for expansion of water ;) ........you don`t get 15l of hot .......remember ther`s a layer of cold @ botom
     
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  7. ChrisR

    ChrisR

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    I'm sure you got it right Nige but a lot of people don't.

    A length of pipe doesn't provide room for expansion within it at all. If there's non return valve, you need an expansion vessel no matter how long the pipe is after it.
    The confusion comes about because of the need (if no EV) to allow the water to expand back along the pipe and NOT then go out of a cold tap.
    The instructions are often ambiguous. If you work out the possible expansion on 15l of water (allowing say 4%, = 0.6 litres) you find that the length of pipe they talk about IS that volume. The instructions often seem to imply that the length of pipe will magically expand to accommodate the extra water.

    It's normal to "get away with it" if there's a float operated valve (wc, loft cistern) on the supply pipe. The pressure goes up, and the water in the pipe pushes past the valve.
    Often see small unventeds installed wrongly though, with no expansion provision. Can only assume that the expansion in the normal range, say 10 - 60 ÂșC, only say 2%, is taken up by distorting the heater vessel :eek: . Or dribbling down the prv pipe - if there is one!
     
  8. chrishutt

    chrishutt

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    The point of accommodating the expansion in the feed pipework before another cold supply branches off is to avoid warm expansion water being drawn off when a cold tap is opened. In many cases it wouldn't matter if it did, and in any case the expansion water is not all hot or even very warm, so the rules can seem a bit over the top.

    I installed a 15 litre unvented within a very short distance of a cold branch, so had to construct a serpentine feed with about 5 changes of direction backwards and forwards to get the required length of 15mm between the heater and the cold branch. In the event only about half the length got hot through the expansion of the 15 litres.
     
  9. keep_it_simple

    keep_it_simple

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    Do you mean it's not big enough for a kitchen sink?

    Thanks for tips on expansion everyone. Heatrae say that you need an expansion vessel and check valve if you have less than 4.2 metres of pipe from the 15L hotflo to the nearest cold branch. It's 2.4 metres for the 10L version. If there's a non return valve you need them regardless of how long the pipe run is.
     
  10. ChrisR

    ChrisR

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    4.2 metres, presumably 15mm copper pipe.

    4.2 metres of 15mm, wall thickness 0.7mm, contains 610 cm cubed. Point proven. :LOL:
    I'm glad they've improved their instructions , which used to be plain wrong, and their tech sales guys knew no better!

    If you used plastic pipe, you'd need more of it! 22mm and it's less...
     
  11. DIYnot Local

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