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RW vs home water instals-wet systems 006

Rain Water Harvesting

Delivery System Options

By  Oasis Water Harvesting


In looking at how one gets water from a surface area such as a roof into holding tanks there are a number of options.  This gives some of the more commonly used ways in which it can be done.



































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The most common way is just to run a down spout or rain chain into the top of a tank from a gutter or scupper.  This is not only the easiest way to get set up, it has the least amount of complications that could arise in the future.  One thing to keep in mind is to give the downspout a downward angle so that the water moves quickly through the downspout and does not build up causing excessive weight which could result in downspout failure.  If not enough downward angle is used there is also the chance of leaf and other roof debris building up in the downspout to the point where it can get clogged.





In using this kind of system the tank does not have to be next to the roof area, it can be placed in convenient location out of the way, this opens up additional options.  The most important factor is that the top of the tank needs to be lower than the bottom of your gutter, scupper, or leaf eater device, if the later is used.  There are low profile tanks on the market that work well in these situations.  There are complicated formulas that can figure out in a 50 year rain event what size pipe to use and what the difference in height from the top of the tank to the bottom of your gutter needs to be obtained for the system not to overflow (the water can’t drain fast enough with the given head pressure created by the height difference and thus the water backs up in the pipe to the inside of your gutter until the gutter overflows and you have Niagara Falls going over the top of your gutter).  Some things to consider when doing a wet system, 4” pipe handles a lot more water than 3” pipe.  Always try to keep at least a one to two foot height difference between the bottom of you gutter and the top of the tank, closer to two feet is better than closer to one foot.  The size of your roof area will play an important roll on what size pipe to use and the height difference.  Bigger roof areas go to the larger pipe and larger height difference.  The distance from your downspout to where the tank or tanks are placed has a slight effect on how this works due to pipe friction loss.  It is important that all piping is sealed 100% with no leaks; I prefer to use schedule 40 pipe and fittings.  If you live in a climate that has hard freezes in the winter you might want to include a Tee fitting somewhere near the bottom of the piping (such as underground inside a plastic access box) which would have a reducer fitting on the tee to a threaded ¾” opening that you could screw in a ¾” brass hose bib valve to drain the piping so that is does not freeze and crack.  You only do this in hard freeze conditions as the next time it rains it is going to have to fill up the piping before water can go into the tank, but that is better than having to replace broken pipes.  One job I did was collecting off of a 7,000 square foot roof here in Sierra Vista, AZ.   We used two 6” pipes with a 3 foot height difference.




One type of system I developed on my own house and have repeated it many times over the years.  How do you collect off of an entire roof with many different pitch angles and get that water into one large holding tank that is higher than your gutters and in an out of the way convenient location.  I have 3 smaller “surge” tanks (2-50 gallon and 1-100 gallon) connected under ground using 1.5” pvc pipe into a single 200 gallon “collection” tank that has a 1hp sump pump inside of it.  The smaller tanks are on platforms to get their tops close to the top or even higher than the top of the 200 gallon tank.  When it starts to rain all 4 tanks start to fill up.  Once there is enough water in the 200 gallon tank the float switch on the sump pump will turn on the pump moving the water up and into an 8’ tall 3,000 gallon tank.  The pump I use is rated at almost 3,000 gallons of water per hour using 1.5” pipe.  At that rate if the rain water falls at a constant rate (which it usually won’t) the pump can keep up with a 2.5” rain event in one hour on my 2,500 square foot roof.  As the 200 gallon tank empties itself the water level in that tank lowers so that by using gravity water from the 3 smaller tanks is pushed into the 200 gallon tank (key rule to remember is that water ALWAYS seeks its own level)





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There are advantages to linking tanks together.  Tanks can be linked together weather they are right next to each other or on opposite sides of the house.  The cardinal rule to remember when linking tanks together is you must keep the overflows of the tanks at the same height.  Water always seeks its own level.  If two tanks are linked together and one overflow is lower than the other tanks overflow, the tank with the lower overflow will dictate the water level in both tanks.  The tank with the higher overflow will never fill up completely.


One advantage when linking tanks together it that is helps to “balance” the system.  If a larger roof area is servicing tank #1 and a smaller roof area is servicing tank #2, tank #1 is going to fill up faster than tank #2.  If the two tanks are linked together the linking will help to shift water underground using gravity from tank 1 into tank 2.  If you need water to move quickly from one tank to another use 1.5” or 2” pipe.  This is typicall when you are using smaller tanks. i.e 200 galons.  This will help to keep the water level difference in the two tanks to a minimum.  In some cases I use 1” pipe.  This is where I am connecting one large tank to another large tank (such as 2,500 or 3,000 gallon tanks).  In a situation such as this you have the size of the tank and time on your side.  Typically tanks of this size are not going to fill up in one rain event.  Also when you are draining tanks of this size even with a pump it is not going to drain in a short period of time.  The smaller pipe has the time to keep the tanks at equilibrium.


Another advantage to linking tanks together is when you go to drain one tank you are really draining both at the same time so you don’t have to move the hose back and forth from one tank to the other.


Linking tanks are also used when one wants to expand storage.  If you already have a tank with a downspout going into it you can easily set an identical tank next to the existing tank and double your storage.

Nw England, WH,front yard016 WH-Joyce Clark pics 014 RW vs home water instals-wet systems 002 Back up pics 9-13 088 New England & WH 381 WH-Rob and Val's system 006 WH-Linked tanks#1 Linked tanks#3