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I have a drilled well which is about 80m deep with a submersible pump. Its about 10m from my basement, where I have a pressure vessel (at 4 bar) and connections to other parts of the property. All pipework is 26mm ID HDPE.

One of the locations is a run of about 35m from the basement, and I get over 60 liters a minute there.

I had an iron removal filter installed on Friday. It has a venturi for injection of atmospheric air into the oxidation vessel. The vendor said I should get a pressure drop of no more than 1/2 bar and I should barely notice the difference in flow rate

I now get 20 litres a minute to the same location. If I bypass the iron filter, it is back to 60. Clearly I notice the difference in flow rate :) I am unsure how to measure the actual pressure differential, but it seems like it must be a LOT more than a 1/2 bar drop to result in such a reduced flow.

I ran the tap for 1 minute before conducting the tests in each case, as I assumed that may be enough time for the pressure to stabilize.

What are my options here? Why would it be getting slowed down so much? The vendor is suggesting removing the venturi and installing an air compressor. Is it likely that would fix the problem? Do I need a larger filter system to deal with the high flow rate? Is this even considered a high flow rate?

Any advice appreciated!

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    Your system should have some design specs, including design flow rate and pressure drop. This should be included in the instruction manual, or on a label attached to the system. Is it rated for 60 liters/min?
    – Mark
    Mar 21 at 15:19
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    I would talk to the vendor. It seems something was not adjusted right or there was some type of miscommunication.
    – crip659
    Mar 21 at 15:31
  • @Mark The system is only rated for 25 litres/min. We were told by the sales person that this would be adequate for our needs, as it would just mean untreated water would come through for the few minutes a day when we need the increased flowrate. Clearly this isnt the case :/
    – Matt
    Mar 21 at 19:27
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    DIY.stackexchange.com is for residential / home improvement questions. A typical home flow is less than 20 litres/minute (5 GPM). A shower is 6 litres a minute (maybe 10 l/m for a high-flow American showerhead). The unit appears to be designed for a residence with a ~25 lpm (6-7 GPM) max flow which is appropriate for a dwelling. Generally someone who needs more is a commercial application like a micro-brewery, municipal pool, farm, or multi-unit housing - off topic here.
    – Harper - Reinstate Monica
    Mar 22 at 23:17
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    Also if you have a requirement like irrigating a massive lawn, generally that isn't put on the water purifier. The lawn doesn't care.
    – Harper - Reinstate Monica
    Mar 22 at 23:27

1 Answer 1

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A venturi works (by definition) by means of a flow restriction. In your case, a substantial restriction is needed to reduce the pressure within the venturi to below atmospheric pressure so that it can draw in atmospheric air.

Air compressors are awfully noisy, and I don't recommend going that route unless you're sure the noise wouldn't bother you. If your water tech has a silent compressor, you might consider it.

Other substances besides the oxygen in air could oxidize your dissolved iron and precipitate it, such as potassium permanganate or hydrogen peroxide, but at a cost in money and maintenance. Air is free and unlimited.

If this were my house, I would add a second pressure vessel after the iron filter to give you whatever unimpeded flow you wish up to the drawdown capacity of the tank.

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  • Yeah, im thinking that the pressure vessel is probably the way we will have to go. We only need a few hundred litres of water twice a day really. The basement is not a part of our accomodation, and is already noisy with a sump pump, etc, so that compressor may be an option. Im just wary about going that route and finding it doesnt work and im further out of pocket.
    – Matt
    Mar 21 at 19:30
  • @Matt If / when you decide to go that route and need to decide on a tank size, remember that both tanks will be contributing to flow in proportion to their flow rate capacity, of which the first tank now has somewhat reduced flow capacity due to the venturi. The math probably includes Cv or valve coefficient. The takeaway is that if your original tank was big enough, you'll be able to get away with a smaller tank after the filter. Exactly how much smaller I couldn't say.
    – MTA
    Mar 21 at 21:57
  • Last paragraph: "up to the tidal capacity" or "up to the total capacity"? That could be what you meant (please explain if so) or a simple typo - I'm honestly not sure...
    – FreeMan
    Mar 22 at 15:37
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    @FreeMan I meant tidal. A more common phrase would be drawdown capacity. The OP resides in the UK and I don't know how tank sizes are specified there. I.e., if a tank with a 300 liter volume can only deliver 200 liters because the rest is bladder, is it sold as a 200 or 300 liter tank in the UK? Dunno. So I said tidal to mean the amount the tank can deliver, not necessarily its volume. But drawdown is a better word. Thanks for having me clarify, I've edited the answer.
    – MTA
    Mar 22 at 21:44
  • @MTA Many thanks for your help. It turns out that we were sent the wrong equipment. They had intended for us to use the undersized tanks with a compressor and larger intake and dip-tube to meet our infrequent demand for high flow, but the engineer did a regular install. We now have the compressor and fatter pipework (and they installed the forgotten non-return valve!) and the drop in flow rate isn't noticeable unless I measure it. I may still add a tank after the filter to reduce the unfiltered water at high flow, but its already working like a dream. Much appreciated!
    – Matt
    Apr 5 at 10:47

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