How to increase the amount of water you can store in your raintank #1

There are several inlet designs for rainwater tanks and many of don’t allow you to store all of the volume of water and some are doing you out of quite a few litres.

I’ll show you in a short series of posts, how to squeeze a little more storage capacity from the tanks you already have. You’ll have to observe the amount of flow that goes in and out of your tank before choosing the method that you prefer.

The first method I’ll show you is restricting the overflow.

Many tanks are designed with an overflow that is too large for the amount of water flowing in and out of the tank. The tank can only hold water up to the bottom of the overflow outlet, then the water runs out so blocking a few centimetres at the bottom of the outlet pipe can give you a considerable amount of increased storage.

In this example, the surface area of the water in the tank is tank is 570mm × 1700mm. That means that for every centimetre we raise the bottom of the overflow, we can store an extra 9.7 litres of water (rounded up) calculated as 1.7 × 0.57 x 0.01

In this case, I’ve added approx 4 cm to the base of the pipe, which will give me an additional 39 litres capacity (rounded up).

How I did it –

This method calls for a slip-on cap with one side to slip into a pipe and one side to slip another pipe into.
This method calls for a slip-on cap with one side to slip into a pipe and one side to slip another pipe into.

In this case, my outlet is for 90mm pipe. I simply get a slip-on cap that has two sides – a side to fit inside a pipe and a side to slip another pipe into. I used the cap to make a little dam to block off the lower part of the outlet pipe.

I carved out just over half of the blank wall of the fitting with a spade bit, leaving the outer wall intact. Then I filed it down so that it looks nice and professional(ish). I’d suggest not going much over half of the centre of the fitting, just in case of an unexpected, huge downpour.

I know from observation that this size is well within the input flow for Spring, Summer and Autumn from the rain gutter that this tank is attached to. I will remove it in Winter or any other time when the weather forecast indicates downpours look like they’ll add a greater flow than normal, otherwise my tank may overflow and make a mess! Fortunately, using the slip-on cap makes installing and removing this little dam very easy.

Then this fitting slips into the outlet hole and the overflow pipe fits into the other side. This effectively blocks off just under half of the outlet, allowing it to hold back another 39 litres of water. Because we didn’t cut the outer wall of the cap, there is no gap for mosquitoes to get in.

I used  a spade bit to cut out most of the center of the fitting
I used a spade bit to cut out most of the center of the fitting
and tidied it up with a bit of filing.
and tidied it up with a bit of filing.
Slip in the modified fitting to block off the lower half of the overflow.
Slip in the modified fitting to block off the lower half of the overflow.
Slip on the overflow pipe and you're done!
Slip on the overflow pipe and you’re done!

Capturing and storing as much water as possible whenever we can is the only way we can keep gardens and home food production going. The 38 extra litres in this case may not sound like a lot but all of the climate projections point to less rainfall coming in heavier downpours because the warming atmosphere can hold more water and is more reluctant to let it go in small amounts. Because global heating adds a degree of uncertainty to any projections, we may see increased localised rain in some areas but, overall, things will be drier.

Working like a dream!
Working like a dream!

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