Gravity fed water for the home - how much head do you really need?

Discussion in 'Designing, building, making and powering your life' started by spud, Dec 6, 2014.

  1. spud

    spud Junior Member

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    I've been googling this for ages, but I am getting conflicting reports as to what sort of head is really necessary. Some are arguing for upwards 10 or 20 metres (30 or 60 foot) of head, and then I read a New Zealander talking about 0.9m (3 feet) of head being sufficient and a common practice over there.

    I get that if one wants the usual levels of pressure (that those with pumps / mains water are accustomed to) you're probably going to need quite a lot of head, but suppose you just want enough head to operate a low pressure shower - will something like 0.9m actually work?

    And what if you additionally need to get that water through some kind of a heater - probably solar or wood combustion? Does that complicate things?
     
  2. Grasshopper

    Grasshopper Senior Member

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    A 4 metre head (bottom of the tank can be 6 metres when full) gives me about half pressure compared to mains out of a standard hose.
    1 inch fitting if I had 4 inch fitting and only reduced to a standard hose at the end it would be way stronger
     
  3. 9anda1f

    9anda1f Administrator Staff Member

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    Grasshopper hit on the essence of low head/low pressure water systems: the delivery piping cross-sectional area must be larger than the total area of your shower head (or faucet, or whatever).
    If your shower head outflow area total is greater than the delivery pipe area, your pressure will drop to essentially zero when the shower is turned on and you will get various levels of "dribble" (the water will drain out, but not "shoot" out like a real shower). But if your available flow from the delivery pipe is greater than the flow allowed through your shower head, you'll have pressure available to "shoot" the water out into a satisfying shower.

    My portable, plastic-bag solar shower hangs from a tree branch and might have a total head of only half a meter, but the hose from the bag to the shower head is large enough compared to the accumulated holes in the little shower head that the shower is quite acceptable (i.e., wife can wash her hair).
    Does this make sense???

    Flow times pressure equals power. If your pressure is low, your flow must be high to achieve equivalent power, which means using large diameter supply piping. Standard mains water pressures are high (>50 psi) so the supply piping diameters can be much smaller.

    Yes.
    = )
     
  4. spud

    spud Junior Member

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    Thanks Gandalf and Grasshopper, that does make sense.

    I'm feeling pretty confident now that I can design our house with only a couple of metres of head.

    I realise that different water heating alternatives will require differing amounts of water pressure, but generally will I require substantially more head/pressure if I want to heat the water too via solar / combustion / rocket stove heating systems?
     
  5. 9anda1f

    9anda1f Administrator Staff Member

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    No, as long as you can keep your available flow rates high, which means using larger (and more expensive) pipe and fittings. Minimize pipe lengths to reduce cost and friction. Overall, you'll be trading off the high cost of a supply tank at great height with the costs of using large supply pipes (there ain't no free lunch).
    The "it depends" moment comes when considering which water heating technology to use and how to maintain your system flow rates. For instance, plumbing an array of solar collectors in parallel could keep the total flow rates high. And of course, when using any form of combustion heating you'll need to make absolutely sure there's no possibility of generating steam under pressure ...
     
  6. andrew curr

    andrew curr Moderator

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    There are friction charts for differant size pipes /joints!
    Capture and store energy as high in the landscape as u can!
     
  7. spud

    spud Junior Member

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    Terrific! Thanks Gandalf and Andrew.
     
  8. dreuky

    dreuky Junior Member

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    We had arranged our place with the top tank 10m or so above the house. Easy we live on the side of a hill. Great pressure no worries until 30 years down the track bathroom needed a redo. talked to plumber, yes no worries plenty of head. WRONG government has now decreed "Thou shalt only sell low flow taps" they work when you are talking mains pressure but when you have not got mains pressure they are CRAP. The moral of the story is do not allow a plumber who has only ever worked with mains pressure any where near a system that is not mains pressure. They do not know what they are talking about.
     
  9. 9anda1f

    9anda1f Administrator Staff Member

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    Can one modify "low-flow" taps by removing a restrictive insert???
     
  10. dreuky

    dreuky Junior Member

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    Answer NO. The low flow bit is not an additive it is cast as part of the tap so there is no way of removing it. Bugger! The only way of getting better pressure is to install a pressure pump. Which is what lots of people do. So much for saving water, you end up using the same amount of water as you would without the flow restrictor plus power. Makes prefect sense!!
     
  11. Unmutual

    Unmutual Junior Member

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    Equipment attached to the water system may have it's own minimum requirement(mostly to do with heating water). But I also thought I read that there was a minimal water pressure you want for safety reasons, like 20 psi or some-such, so you don't have ground water seeping in to your system. I only half remember it, so it could have been something else entirely.
     
  12. Tasman

    Tasman Junior Member

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    We are nearly done with our house. Its a fairly conventional house. But its not connected to council water. We placed our top tank 15m above the house (about 100m length of pipe). We are satisfied with the water pressure we get, though its not comparable to mains pressure. Here are a few notes:

    1.) We couldn't find a plumber that would advise us. I suspect they are worried that if, in the end, we complain that they said it would be OK they might have to fork up for remedial work.

    2.) We have a tank at house level. We setup the pipework so that if we were unhappy with the pressure we could pump from the bottom tank directly into our house, thus switching over to a more conventional pressure pump setup. Since we are happy with the water pressure that pump just functions as a transfer pump.

    3.) We checked dishwasher and hot water system in advance to make sure input pressure would be OK. They work. Washing machine is still in storage. So we don't know for that one.

    4.) We oversized pipes. Internal pipes bumped from 1/2 inch to 3/4 inch. Pipe from top tank to house 50mm blue line polypipe.

    5.) We used greens tapware. They are a New Zealand manufacturer that has low pressure mixers. I think we would have had more options if we had gone for taps (but wife wanted mixers).

    cheers,
    tas
     
  13. sweetpea

    sweetpea Junior Member

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    I have a gravity flow system with quite low pressure, and I haven't been able to buy any fixtures in a hardware store that work, even the low-flow shower heads, they still block it way too much. I found a plastic nozzle on a camping shower bag that works really well.

    It's pretty easy to get cold water to flow from above through a shower head or faucet that's attached at a high spot on the wall. You do have to protect it from freezing.

    But the hardest part is getting the hot water under pressure.

    Did you ever find a solution?
     

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