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How do we get enough pressure from a rainwater catchment system?  RSS feed

 
Beverly Johnson
Posts: 4
Location: Long Island
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I am planning to be off the grid in less than a year. I am planning a rainwater catchment system in the back of my house that will be commected to the house plumbing. The tank will be large. I hope to build it out of insulated concrete forms possibly 6 feet wide x 3 feet deep x 20 feet high. I am hoping there will be enough gravity fed pressure to suit the needs of my house. bathroom shower etc.

My question is how do I know there will be enough pressure when I turn on the fauset? is there any math formula to determine the pressure when the tank is full or 1/2 full? Any ideas?

 
Brian Knight
Posts: 554
Location: Asheville NC
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Theres quite a bit of math in that which may be tough to know the variables for at this point. Gravity will probably not provide enough pressure for all but the most basic needs. Planning on a pump is probably in order.
 
Joe Braxton
Posts: 320
Location: NC (northern piedmont)
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Average household water systems operate at 30 - 80 psi. with 45 psi. being a good target. You could probably get by with less, but that would be a decision you would have to make.
1 psi. (static) of water is equal to 2.31 ft. of head, so 45 psi. will equal over 100 ft. of elevation above your faucet (not the ground).
No doubt you will need a pump to achieve pressure you are used to seeing.

Handy converter -
http://www.convertunits.com/from/psi/to/foot+of+head
 
John Polk
steward
Posts: 8019
Location: Currently in Lake Stevens, WA. Home in Spokane
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When that size of tank is full, it will contain over 20,000 pounds of water, so elevating the tank becomes impractical. If you are relying on gravity feed, you will only be able to utilize water above the level of your faucet. If your shower head is 7 feet above the bottom of the tank, you will not have any flow when the water level drops to 7 feet.

Yacht and RV stores carry 12V water pumps (RV stores are probably a lot cheaper than yacht stores).

One pump I have read a lot of good reviews (by off grid users) on is this one:
http://www.northerntool.com/shop/tools/product_37142_37142
 
Beverly Johnson
Posts: 4
Location: Long Island
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Thank you for the respnses. The pump is described as on demand. Would I need to turn it on and off everytime the water is used or could it supply water to the system up to the right psi?
 
John Polk
steward
Posts: 8019
Location: Currently in Lake Stevens, WA. Home in Spokane
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"On demand" means that it is pressure sensitive. When pressure drops, (you open a faucet) the pump kicks on. It should remain on as long as the water is flowing. Once you shut off the faucet, it will again build up pressure (to whatever it is set to), and then shut itself off again.

If you hear it frequently cycling, when not using water, that means you probably have a leak somewhere.
An occasional cycle (when not using water) is not abnormal, as pressure changes can be expected with temperature changes.

You'll be a 'happy camper' once you get off of city lights and city water. Good luck.

 
Angelika Maier
pollinator
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Location: cool climate, Blue Mountains, Australia
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The most common thing is to have a pump. We are connected to town water and have our own so we use a pump as our plumbing system is a pressured one.
 
Jeremiah wales
Posts: 137
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Pressure for what use?
I have a 500 gallon Storage tank on a "Water Tower". It is only 6 ft in the air from the ground and even with the bottom of my roof eves. But water flows fine out of my faucets, toilet and into the Tub. Now a Shower is another thing. I had to hook up a 12 volt pump to get my shower to spray out. But then I used a low volume shower head there.
I saw an appliction once where someone made a shower. They took a five gallon bucket with aprox 30 holes in it at the 7 ft mark and filled it with hot water. Took a shower under it.
 
Dave Turpin
Posts: 112
Location: Groton, CT
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The pressure we are used to seeing at the tap is there not to make our life easier, but simply to ensure that everyone who is serviced by that water has sufficient pressure to operate their appliances.

In most cases, the water that comes out of your faucet or appliances has been dropped down to atmospheric pressure by a valve. This is why most faucets are either needle or globe valves, and gate and ball valves are only used as main cutoff valves in the system. (The purpose of a needle or globe valve is to drop pressure)

Off-grid houses that use pressure tanks and pumps are doing so simply because they are using plumbing designed for houses with pressurized water mains. You don't NEED to do this, however. As long as the bottom of the cistern is above the highest point of use, water will flow, you just need to use plumbing designed for low pressure use!

Now, this isn't as EASY as simply using off-the-shelf plumbing components. You will have to engineer each appliance you want to use. sinks, tubs, and toilets are easy. Showers will require a different kind of head (I would say a "rain" head with the restrictors removed would work fine). hot water storage and use, washing machines, etc., will require further thought.
 
Jeremiah wales
Posts: 137
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Wow, I can see I have a different take on Living off the Grid than most people these days. But then I have shirts that are older than many people on this forum.
When I hear of living off the Grid it does not seem like a huge thing to do, It all takes adjustment. When I think of it. I consider it saving money, Not spending money to say you are saving money.
I have lived a basic lifestyle for that past 45 years. Keeping costs down, No Dishwasher, No Special appliances. Basic stuff. My water is heated with wood, My house is heated with wood. Not that I bought, that I cut and got for free. Storage water flows just fine out of every faucet and into my toilet very easily. No I dont think I can an automatic dishwasher with my water flow. But that is because my dishwasher is me.
How much do you really save catching water and then using it for regular lifestyle? To me it would cost more to catch it and save it than it I would save.
 
Kacy Wallace
Posts: 5
Location: Toronto, Ontario and Westport, Ontario
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Jeremiah wales wrote:Pressure for what use?
I have a 500 gallon Storage tank on a "Water Tower". It is only 6 ft in the air from the ground and even with the bottom of my roof eves. But water flows fine out of my faucets, toilet and into the Tub. Now a Shower is another thing. I had to hook up a 12 volt pump to get my shower to spray out. But then I used a low volume shower head there.
I saw an appliction once where someone made a shower. They took a five gallon bucket with aprox 30 holes in it at the 7 ft mark and filled it with hot water. Took a shower under it.


Hi Jeremiah. I have a 275 gallon rain tank encased in a galvanized steel cage. I'm researching ideas on how to elevate it. I was considering using concrete masonry units (CMUs), but would be curious to know how you elevated your tank safely, which is much larger. By my calculations, my tank will weigh over 2,500 pounds when it's full.

 
Alder Burns
pollinator
Posts: 1379
Location: northern California
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I recently put four such water "totes" on top of a shed. I reinforced the interior with some 4X4 braces near the corners of each tank....all of one edge is along one wall. It is a stoutly build shed with 2X4 framing and plywood sheathing, floor, walls, and roof. It's been comfortably holding up 10K lbs. of water all summer. Of course I had to pump the water INTO these tanks, but it will flow fine by gravity out of them, and is at sufficient height to run a shower (provided, as stated above, that the showerhead is low pressure. I made one out of the "rose" of a watering can one time!). Getting a barrel or tank even six feet up in the air will run drip tape. If the site has any elevation differences at all, one can contrive to have tanks or cisterns near the high point and then gravity feed the whole site. At one of my places in Georgia we had a well placed near the high point of the property and then ran water to cabins and gardens downhill from there. The well was on direct-solar with a float valve in the tank, so it would top up automatically.
An additional advantage to low-pressure, gravity-fed water systems is that most of the plumbing can be done with garden hoses! These are easy to work with, easy to repair leaks, and are not as likely to be damaged by freezing.....
 
Richard Nurac
Posts: 52
Location: north Georgia
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Having sufficient water pressure and avoiding airlocks needs planning and experimentation. Most of my catchment areas are at the bottom of the hill and I pump to two 285 gal tanks at the top of the hill from which I irrigate my plantings. Rainwater provides all my irrigation needs and wellwater my personal needs. The system requires frequent supervision since algae does grow and clogs my low pressure bubblers and drippers. But it is satisfying, efficient and, as water shortages loom, ensures that my well (or my neighbors' wells) will not run dry because of my irrigating. My website has details and pics.
 
Corey Schmidt
Posts: 155
Location: Kachemak Bay, Alaska (usda zone 6, ahs heat zone 1, lat 59 N, coastal, koppen Dfc)
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If i understood the original post right, the proposed water tank would be 20 feet high, meaning there would be 20 feet of vertical water in it when full.  if that's the case, please be sure you properly engineer it!!!  I had a tank 5 feet high explode (twice) due to the weight of water in it.  it was made of wood and built in too much of a hurry, some fasteners were forgotten....but 20 feet, thats a whole other story.  even if you have plenty of continuous rebar, and you will definitely need it horizontally as well as vertically, you will need to make sure somehow your mortar joints don't blow out.  Also, there is a reason most water tanks are cylinders, its much easier to have structural integrity with that shape.
 
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