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Green Building Bible, Fourth Edition
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General: Water saving: grey water top-up

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    Hello again.

    Since it's on everyone's minds right now, we've been discussing how we can use less water at home. One idea is using cold bath water to top up our WC cistern.

    Ours is a concealed cistern (Geberit Duofix) so the proper access is via a removable faceplate. The idea with this would be to introduce a separate hole and hose arrangement on one side, with some sort of inlet valve to allow topping up the water level using a watering can or jug filled from the bath.

    Any suggestions for what we could use at the filling end? All I can think of is some sort of wall-mounted tundish like the below.

    Also, is this mad? Anything we should be wary of, other than probably needing to de-scale/clean the cistern more frequently?
      tundish.jpg
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      CommentAuthordjh
    • CommentTimeAug 28th 2026
     
    Or stop having baths?
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      CommentAuthorfostertom
    • CommentTimeAug 28th 2026 edited
     
    It would apply to shower-in-the-bath water too, like ours - maybe most people who have shower trays don't realise how much water even abstemious-flow electric showers create - well worth saving - presently here for syphoning onto the garden.

    To get down and dirty, it's interesting to see what happens to two people's morning shower water after it's sat in the bath all day, for evening watering - the amount of brown dirt and mucky soapy/shampoo-y scum is impressive. I wouldn't put it direct into my loo cistern, and wonder what happens to all that when filtered in a proper recycling installation.
  4.  
    Cistern should hold 6 litres of water, if the mains fill isolator valve can be turned off.

    Bath holds around 80-100 litres

    Major mismatch of scale!

    Take a shower instead, save 50 litres that way.

    If you have to run the shower to get it to run hot, do that into a bucket. You can tip the bucket straight into the toilet pan to flush it, instead of using the cistern flush. No plumbing mods needed! Or use that water for plants etc.



    ETA my in-laws had been advised to run their kitchen sink hot tap until it runs hot, catching the water in their kettle to avoid wasting it down the drain. Their kettle is now always completely full so they boil 1.5+ litres of water for every cup of tea. They feel very satisfied that they have thus done their bit for water scarcity. :-(
    • CommentAuthormarktime
    • CommentTimeAug 29th 2026
     
    I thought the era of baths was over, especially on this forum. I use a naval method of showering, learned nearly 70 years ago an it takes me about 2 minutes to shower. Shower feed is just under 2 metres from the 30 litre hot water cylinder so you virtually step into a hot shower. Water pressure is around 3,5 bar into an overhead shower head.
    • CommentAuthorphiledge
    • CommentTimeAug 29th 2026
     
    Just be conscious that if you're using a valve flush any dirt/scale/lard on the seal may well let it it leak. Even the tiniest dribble of a leak running 24/7 will likely waste far more fresh water than you'll likely save using bath water. I'd stick to clean water in the cistern but maybe route the bath waste pipe outside for use in the garden. Definitely stick to showers where you.
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      CommentAuthordjh
    • CommentTimeAug 29th 2026
     
    We use Dudley syphon-flush cisterns to avoid the inevitable leaks from valve flush ones.
    • CommentAuthorRex
    • CommentTimeAug 31st 2026
     
    "Shower feed is just under 2 metres from the 30 litre hot water cylinder"

    Should have considered this when we built. Do have a secondary re-circulation to attempt to avoid the dead-leg, but still have to wait quite some time for hot water. The easy solution at the design stage would have been to position all hot water requirements, more of less above the HW cylinder. In our case, the ensuite is virtually diametrically opposite the HW cylinder.
  9.  
    Thanks all.

    To clarify, most of us do shower (low flow), but our youngest is still small enough that 'bathtime' is a thing for now.

    For this reason we aren't filling an enormous full tub, but it's still a substantial enough quantity to feel that it's a waste to let it all drain away.

    Having thought it through over the long weekend however, our current solution with a bucket in the shower in the en-suite is far more practical (and doesn't involve re-plumbing!).

    Once the bucket is full we just pour it straight into the WC pan, bypassing the cistern entirely. It can be a bit 'splashy' though, so an acquired skill and it might only be an emergency solution in hosepipe ban season...
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      CommentAuthorfostertom
    • CommentTimeSep 1st 2026 edited
     
    Being equipped with all the 'conveniences' of modern plumbing, what absurd manual procedures we now adopt!
  11.  
    Sounds good! A standard toilet flush is 4-6 litres so that's about half a bucketful (obviously depending how big the bucket is, and what state the toilet is in!) So don't need to wrestle a full bucket.

    In one of our a previous houses the bath, sink and toilet drains all went separately out through the wall and joined an external stack. It was fairly easy to intercept only the bath/shower drainpipe into watering cans. But our present house they all join into an internal sewage downpipe that is boxed in behind units. Better for airtightness but more awkward for recycling!

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