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  1. #1

    Bottom drain size

    Okay, so most of the Internet and forums say 4" pipework, regardless. It won't clog etc etc etc. I'm expanding in spring and could do without having to exhume everything within a month. I've been really looking into bottom drain construction and think I've found something that makes logical sense but I'm after a second opinion. My pond will be around 10500-11000 litre mark, 3000 gallons for you imperialists out there. I will run drain>vortex>sieve>pump>wetland filter>pond. Also skimmer>pump >biofalls>pond.

    My run will be 3m maximum, probably much less. My vortex/sieve pump will be submersible 9000lph and my skimmer will be 8000 lph. Now, according to the pondguy and a few websites, the correct size pipe for this would be 1.5". The resulting pressure meaning that nothing would have time to settle and clog. This, coupled with the fact the skimmer will be doing its job, seems to make sense. 4" so he says, should be for much larger flow rates and a pump doing 9000lph on a 4" pipe will lead to blockages a lot quicker than 1.5". I will be adding a valve for purging anyway, but before I take the plunge and get the cement out in spring, my question is, is he right? It seems to make more mathematical sense than the one size fits all, 4" school of thought. Is it as simple as matching pipe size to pump rate to get a rough turnover of 1 hour with my dimensions?



  2. #2
    Senior Member Rank = Grand Champion davethefish1's Avatar
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    there's a reason ponds use 4" pipe work for gravity feeds.
    it works.

    there would be an argument for a 3" feed on a smaller pond of 1000 gallons or less.
    but could still block up with blanket weed...
    easier to fit a purge valve than try and balance the flow rate to pipe diameter

    i don't know which website 'the pond guy' you are refering to,
    but the only way you would get 9000lph down 1.5" pipe is with a pump...

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    Moderator Rank = Supreme Champion Feline's Avatar
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    My quarantine tank has a 3 inch bottom drain. It works ok for low flow rates under 5000 lph. For a proper koi pond you would definitely need more. The correct way of running a BD is to regularly purge it. Gravity pipework needs roughly twice the diameter we would usually pump water through at that flow rate. Building something significantly different to tried and tested methods leaves you very vulnerable to finding out that what everyone else is doing was right …. the hard way.

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  6. #4
    Sorry, the pondman. See attached link and tell me what you think. I get that, that's why I'm asking, dare I stray from the well trodden path? It's just that, well it makes sense somehow. Here's the link, let me know your reaction. He's a professional pond builder as far as I know.

    https://youtu.be/5ngC64e872o

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  8. #5
    Senior Member Rank = Grand Champion davethefish1's Avatar
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    Quote Originally Posted by Billinghamkoi View Post
    Sorry, the pondman. See attached link and tell me what you think. I get that, that's why I'm asking, dare I stray from the well trodden path? It's just that, well it makes sense somehow. Here's the link, let me know your reaction. He's a professional pond builder as far as I know.

    https://youtu.be/5ngC64e872o

    to put it bluntly he's talking bollocks.
    says you need to pull 65,000 lph for a 4" bottom drain

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  10. #6
    Senior Member Rank = Adult Champion Twhitenosugar's Avatar
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    I have a 4" bd on my pond (which is about 13,000l) and a 3" BD on a quarantine/grow-on tank (that is 1,300l).

    Both work as they should do i.e. keep the bottom clear of muck, and both have purge valves - which have two uses. One to dump water quickly when doing a water change (without getting your hands wet), and secondly (and more importantly) to clear any muck settled out in the pipework.

    As mentioned above, if a 4" bottom drain didn't work effectively on a typical koi pond set up (which yours sounds like it will be) there'd be plenty of posts from people moaning that they don't work effectively.

    The only issue some people report is that muck settles in the corner of the pond.

    But that is usually more to do with the fact that a bottom drain can realistically only 'draw in' over a certain distance, rather than to do with the diameter of the bd/bd pipe (if anything, a smaller bd will probably have a smaller draw diameter).

    But to overcome that limitation (muck settling in the corners away from the bd) you can just slope the bottom of the pond down to the bd, so all the muck naturally settles down towards the bottom drain every time it's disturbed.

    I've found having an aerated bottom drain helps boost the draw of the bd. I have mine come on for half an hour in winter purely just to sweep the pond floor much further away than the bd can draw on its own.



    Sent from my Pixel 4a (5G) using Tapatalk
    13,000L fibreglassed raised pond with window

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  12. #7
    Quote Originally Posted by davethefish1 View Post
    to put it bluntly he's talking bollocks.
    says you need to pull 65,000 lph for a 4" bottom drain

    That funny huh? So is there an alternative more accurate calculator? My main objective is to plumb correctly without over spending or over engineering,

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    Senior Member Rank = Supreme Champion Ajm's Avatar
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    Sure it's 6ft pull for the bottom drain I have 1 dead spot which bugs the life out of me lol far corner from the return

    Sent from my SM-G973F using Tapatalk
    Freddyboy the legend

    "we are water keepers first"

    Johnathan

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    Senior Member Rank = Supreme Champion Ajm's Avatar
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    Quote Originally Posted by Billinghamkoi View Post
    That funny huh? So is there an alternative more accurate calculator? My main objective is to plumb correctly without over spending or over engineering,
    Go 4 inch . You go less you will have to rip up and spend 10x as much . . Money saving is good if you do it with the right bits . Don't use a 250 quid spin drifter and just use a £25 ebay kk one all I have and it works a treat

    Sent from my SM-G973F using Tapatalk
    Freddyboy the legend

    "we are water keepers first"

    Johnathan

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  18. #10
    Senior Member Rank = Grand Champion davethefish1's Avatar
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    Quote Originally Posted by Billinghamkoi View Post
    That funny huh? So is there an alternative more accurate calculator? My main objective is to plumb correctly without over spending or over engineering,
    i apologise 65,000 lph through a 4" pipe... sorry i do find that funny
    i'd love to see the examples of these systems running and working flawlessly...

    but if it ain't broke....
    how much will it cost to build from scratch to find it doesn't work and have to start over?

    over engineering is what brits do best....
    thats why (most) of our bridges don't fall into the water with people crossing them

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  20. #11
    Loving the good humour, genuinely. See, that's what I was thinking, if it works so well why isn't everyone doing it? And I definitely do not want to ever see it again once it's in. So for 4" pipe, will my 9000lph submersible cut the mustard and pull it through alright?

  21. #12
    Senior Member Rank = Supreme Champion RS2OOO's Avatar
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    4" bottom drain with 4" pipe to your filters.

    If pipe is connected to a large filter such as a nexus that should be adequate for purging, but if going directly into a drum or if you may add a drum in the future then fit a purge valve and you'll never have any of the issues described in that video.

    Personally I don't have a purge valve and have never had any problems, but a lot of muck comes out when I turn the pumps to maximum flow rate so in hindsight a purge valve should have been fitted just to eliminate the risk.

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  23. #13
    Senior Member Rank = Rokusai rolo's Avatar
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    I've watched quite a few youtube video's over the past weeks due to my back garden being dug up and having to restart. so wanting to get it right first time. now everyone is entitled to their own opinion but this guy sits in his van saying you don't need to do this and that.
    where as the peps on this forum have being there done that and learnt from there own mistakes.
    when i started a guy told me build it as big as you can first time and take your time about it. otherwise when you have the bug you will want a bigger one. how many peps on this forum are on there second even third build.
    all the best with the expansion.

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  25. #14
    Senior Member Rank = Supreme Champion RS2OOO's Avatar
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    On my 3rd Pond here.

    Was on this forum when I built the first 4 foot deep 7500 litre Koi pond. A few people said make it bigger. Not in my nature to argue but it did get my back up a little bit because I was looking at a HUGE hole in my garden and had a wife complaining it was overbearing and would look an eyesore.

    1 year later and the pond looked tiny, with the wife complaining there wasn't enough room for her 18cm Shiro that was now knocking on 45cm.

    So I rebuilt it to 12,000 litres.

    Now on pond 3 which is 15,000 litres / 3300 gallons.

    Average Koi size is up from 15cm to now over 50cm, anything smaller looks tiny.
    Last edited by RS2OOO; 08-12-2022 at 05:00 PM.

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  27. #15
    My build is kind of dictated by my geography really, and the pond isn't going to "be" the garden, rather just a pleasant feature. Going to end up somewhere close to 2.5 ◊3m by 4ft deep so around 11000l. Great advice on here. I'm not running to a nexus, I'm having diy settler>diy sieve> pump> wetland filter and on top I'm having skimmer and waterfall. So I guess I want a few things.
    1. To know my pipes are the best possible size to give me good flow.
    2. To know that my pump has enough oomph to pull water from bd (at present its non variable 9000lph) and
    3. To know that it'll give me a decent turnover time with all things considered(also bearing in mind there'll be an 8000lph submersible running my skimmer and falls.
    4. I'm neither overengineering or underengineering.

    You're all right, it's all opinion, but I'm trying to get to the nib of who's is the right one. It's true a smaller diameter should in theory go through faster and therefore less settling time, and I've never suffered from blanket weed as I'm under trees, hence the new skimmer, however, if everyone is saying 4" it's pretty hard to ignore....the advice has been great so far thank you

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  29. #16
    Senior Member Rank = Grand Champion davethefish1's Avatar
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    i think it's a pretty good way to build a pond starting smallish and then upgrading....not the cheapest way,
    but i've seen a £40,000... 5,000 gallon block built and fibreglassed pond built as a first pond... and then get bulldozed flat 2 years later when they lost interest

    plus if you often change the way you want to run your pond, and have different ideas on how you would build it over time.
    if you put pencil to paper once every two years, you would never design exactly the same pond twice....

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  31. #17
    Totally Dave, and that's not what I'm after, I don't want a display cabinet with water and fish in it. It does look impressive that is true, but each to their own and I don't think any one design is the right one for everyone. Basically I started off with what I've got now...hole, liner, fish, pump, filters. Its 4ft deep and alright but I've learned so much since then, the biggest thing being what a propeller does to fish crap and what happens when we get 40 degree sun in under oxygenated ponds . My design for expansion has evolved organically from speaking and watching. I'll be happy with my set up so long as:

    1. The equipment works how I intend it to
    2.the water is healthy
    3. The fish are happy
    4. I can maintain it easier than I currently do.

    Anyone got an answer regarding my pump?

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  33. #18
    Senior Member Rank = Grand Champion davethefish1's Avatar
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    Quote Originally Posted by Billinghamkoi View Post
    Totally Dave, and that's not what I'm after, I don't want a display cabinet with water and fish in it. It does look impressive that is true, but each to their own and I don't think any one design is the right one for everyone. Basically I started off with what I've got now...hole, liner, fish, pump, filters. Its 4ft deep and alright but I've learned so much since then, the biggest thing being what a propeller does to fish crap and what happens when we get 40 degree sun in under oxygenated ponds . My design for expansion has evolved organically from speaking and watching. I'll be happy with my set up so long as:

    1. The equipment works how I intend it to
    2.the water is healthy
    3. The fish are happy
    4. I can maintain it easier than I currently do.

    Anyone got an answer regarding my pump?
    pump size depends on how high, how far, and how many bends,
    but if you get a variable pump you can over size a bit and adjust it how you want.

    if you want 9000 lph 'actual' real lph, i'd get a 20,000 vario and slow it down, and use 2" pipework for the return
    as a 10,000 vario will do around 6,000 lph flat out 'actual' lph in the real world...

  34. #19
    Thanks I'll bear that in mind. To answer that a bit, the run won't be far at all and the return is going to be via a wetland so not an actual pipe return.

  35. #20
    Just thinking then, so if I'm losing a bit, that'll give me about a 2 hour turn over. Anything wrong with that? Do you need the big flow for any other reason than turnover time considerations?

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