What ho one and all,

This is just an 'out of interest' question.

On one of our walking routes, along an expensive road, one house is installing an Enduramaxx 10,000 Litre Rainwater Tank that stands around 3.7m tall.

I was chatting with the groundworker and was told that it will be in the back garden, set onto a solid base and will be set into the ground by around one meter. That leaves around 2.7m above ground. The water inlet will be from the bottom of the tank.

Since water finds its own level, when the tank is full, I assume that water in the downpipes attached to the house will be full of water to around 2m + high. In my mind, this does not make much sense or am I missing a fundamental principle?

Certainly a top feed makes sense but then the supply pipe will have to be overhead and personally, I cannot see the owners of this house doing that. but what do i know.

It is not up and running yet but I will see what is happening on future wanderings.

The feed could go into the tank underground and then be led up in a pipe inside the tank to discharge into the air at the top of the tank. So it would still need a head of 2.7 m in the downpipes to drive water into the tank, unless there was a lift pump as Gareth suggests. The head in the downpipes would only be needed temporarily whilst it was raining. But given there needs to be some filtering somewhere I'd suspect a pump is more likely.

We have an extensive rainwater collection system on our various buildings, all DIY and added and improved over the years. The well usually dries up in summer, so we then become totally dependent on rainwater.

The main requirement is that when it rains we want the full output from the gutters to fill the tanks. The water butt small pipe attachment is totally inadequate and fails to trap all the available precious water.
Conversely once the tanks are full we want all the water diverted straight down the drains - otherwise the tanks would quickly and unnecessarily fill up with the winter's leaves and debris.

I haven't so far been able to devise a way of doing this automatically. The simplest is just to have a section of convoluted pipe just above tank level and manually move the downpipe outlet from pipe A to tank to pipe B to drains.

I can't see any purpose in feeding in water at the bottom - it's not a hot tank, so the effects of thermal layering are not relevant. Also it would needlessly stir up sediment. The exit pipe is taken from about 6" above the bottom, to avoid drawing in sediment.

Once in a long long while we drain the tanks and tip them on their sides to flush and clean them out. In theory I could get through the screw lid on top, but the claustrophobia would be terrifying. For this reason the tanks are not buried, but hidden by shrubs and climbing plants.

They appear to all operate on small bore water pipes. Diverting a rainwater down pipe from one pipe to another would need something capable of controlling the flow in a 3" pipe and switching it between two other similar pipes.