Water at my house - part 1: rainwater

This post will evolve, but the idea is that is summarises what we've done to manage the water at our place. It will contain links to other relevant posts I've written.

Rainwater

Our house was built in the 1950s, and is typical of houses from that era. It is mostly brick, with a tile roof on the main part of the house and a lean-to at the back with corrogated iron. There are four downpipes on the house, one in each corner. Those on the Northern side of the house drained to the back yard. The downpipe at the South-East corner drained to the street (that has possibly the largest flow of the four), and the pipe at the South-West drained to the neighbour's  front yard!
At the back, our garage had a gutter down the East and West side.

Some considerations for our rain water system:

• We wanted to catch all the water that fell on our roofs, and not waste any
• We didn't want big tanks in the front yard
• We wanted to use the rainwater in the house
• Where possible, we wanted to avoid pumping the water (gravity feed) (in keeping with low energy, as described in these posts)

Rainwater plan

Below is a schematic for our rainwater system

Figure 1: Schematic (not to scale) of the water systems as installed

We decided to install 5 rainwater tanks. We installed

• 2 x 15 kL tanks in the NE corner of the block as bulk storage.
• 1 x 5 kL tank near the pergola
• 2 x 1 kL tanks at the front of the house

I have performed a detailed analysis (using historical meteorological data) of whether this will be sufficient rainwater storage in this article.

This leaves one downpipe without a tank attached in the NE corner of the house. For now, I've raised the pop (within the gutter), so that the water will preferentially drain to the West (the other downpipe that's being caught by the 5 kL tank) and will only go down the NE downpipe if the rain is exceptionally heavy and the gutter is in danger of overflowing (refer to Figure 2(a) for detail). I plan eventually to run a new gutter around the house-attached pergola on the northern side.

Figure 2: (a) cross-section of a gutter showing the raised pop that will only drain when the gutter is in danger of overflowing. (b) detail of the manner of connection of the main solar hot water system

The five rainwater tanks equilibrate via blue-line poly pipe. The 5 kL and 15 kL tanks are connected by 25 mm blue-line. This is low-pressure, and I've also included a gravity fed tap in the middle of the vegetable patch to allow gravity fed watering. The two 1 kL tanks are connected to the 15 kL tanks via 40 mm blue-line. Because of the front tanks' small capacity and distance to the rear tanks, I wanted to avoid a situation where heavy rain caused them to overflow despite there being storage space at the rear tanks. The larger diameter of the 40 mm blueline allows water to more-quickly move from the front to the back of the house.

The 25 mm blue-line from the 5 kL tank is connected to a Grundfos variable speed pump which supplies the house. To use this, we also installed a valve that allows us to turn off the water supply near the street. To save energy, we run our pump at a lower pressure than the main supply (~25 psi, versus 55 psi for the supply). I think that this makes our plumbing work better (our shower in particular works better at lower pressure).

System advantages

1. Because the tanks are in equilibrium, and are scattered, we have low-pressure rainwater available around the yard. This is a very simple system, and lets us access water under its own pressure without using a pump. Simple, non-powered systems are cheap and resilient, as described in these posts.
2. Little/no rainwater is wasted. It would have been possible to create a wet sump system, whereby the stormwater from the front of the house was routed to the tanks at the back underground. This would have allowed us to avoid having tanks in the front yard. The problem with this system is that any water left in the pipe between rains tends to go bad. To avoid this bad water entering the rainwater tanks, this needs to be drained. Apart from being a waste, it is very difficult on our block which is almost flat. Although there is water inside the blue-line pipe, this water is often moving: as we use water in the house, it causes all the tanks to re-equilibrate, which cycles water through the pipe and prevents it going bad. It would not have been possible to adjust our gutters to direct water out the back without extensive modifications.
3. Because we have tanks (and not just piped water) all around the yard, the pressure we achieve by gravity feed is maximised [1]. This means that we can mostly avoid pumping water in the garden (saving electricity).

System disadvantages

1. Digging the trenches and installing the pipe was a lot of work and the components were fairly expensive. A wet sump system would be cheaper overall (there would be fewer tanks, for example) though installing the piping would be more labour-intensive.
2. Some might perceive the 2 x 1 kL tanks in the front yard as being an eyesore. I will write more about aesthetics in another post

 Observations

Through a wide-gauge pipe, it is quite amazing how much water will flow even at very low pressure. Last summer, I would water the garden through a 25 mm hose with as little as 20 cm of water head (~ 0.2 psi), and found it very effective. It doesn't spray, but a lot of water still comes out if one holds the end of the hose near to the ground. Also, the water rate can be regulated by how high one holds the hose.

If I was building a house from scratch, setting up a similar system would be simpler because I would have all the gutters drain to one or two points.

A sizable expense has been the connectors for the blue-line. If I did this again, I would seek a solution that minimised the number of connectors and provide a significant saving.


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[1] When water is moving through a pipe, there is a pressure loss that is related to the speed of the water and the diameter of the pipe. By having a column of water (ie. a tank) right next to  where we want to use it, there is less pipe and thus less pressure loss.


This article was written by Angus Wallace and first appeared at guesstimatedapproximations.blogspot.com.au