Should I be rainwater harvesting?

UK SuperHomers Judy and Charles Ainger have been rainwater harvesting since 1995. First they installed water butts. In 2008 they then added a pumped rainwater harvesting system with a 3,300 litre underground water tank. It was an interesting experiment. In this article they explain how life in their 1960s bungalow changed as a result.

Roof rainwater harvesting

When we retrofitted our bungalow in 2008 we installed a roof rainwater harvesting system. Rainwater from the roof passes along gutters and pipes to water butts and an underground water tank.

Underground water tank 

Burying a 3300L underground rainwater tank means relaying the lawn

Burying a 3300L underground rainwater tank means relaying the lawn

A 3,300 litre water tank is buried under the lawn at the back of the house. A digger was needed to make the large holes for the tank and soakaway. The hole was 2.5m deep, 3m long and 2m wide.

The plastic tank was placed in the hole hanging from the digger bucket. Then it was carefully back-filled with sand dug in around it. The lawn was re-laid over the top.

The garden needed replanting afterwards so it is best to do it during other building work as we did. After installation, just the access lid of the underground water tank remains visible, with a stone slab over it. We open it once a year, to take out the filter (from the top) to clean it.

Benefits of rainwater harvesting

Only the lid of the underground water tank remains visible after the installation

Only the lid of the underground water tank remains visible after the installation

We live in a ‘hard water’ area, whereas the rainwater is soft. Our vegetables prefer it, and in the washing machine there is no need for lime de-scaling, and we use less detergent and fabric softener.

Overall the system provides about a fifth of our water use. Our water bill has reduced and it allows us to water our vegetables during very dry periods. We have also altered our behaviour in subtle ways.  We are extra careful in our use of the washing machine because we don’t want our supply of rain water to run out, so we always  use it with a full load.  Our system allows for mains water to kick in automatically if we do run out of rainwater, but it has become a matter of pride to try to avoid this happening.

The harvested rainwater now supplies water for our:

  • washing machine for clothes washing
  • toilet cistern at the back of the house
  • two garden taps

The amount of rainwater we collect is about 1/2 the amount we could get off the whole house roof. It was limited by the fact that we were only re-plumbing half the house, so it draws from an area of roof which is 110m².

Collecting rain water with minimal losses

We have installed measuring instruments including a rain gauge in the garden. These show that we only manage to get about 1/3 of the rainfall that theoretically hits the roof into our tank – much less than the 80% that suppliers assume.

We do not understand this fully; but think is partly direct evaporation from the roof after light rain, and small leaves blocking the downpipe entry points (they catch there and stop the flow down the pipe, and the water then just flows over the edge of the guttering). Also there can be overflow from gutters in the more frequent heavy rainstorms, and builders do not always fix the gutters with the correct 1 in 200 slope to the down pipes, to minimise this.

We have re-fixed the gutters to the correct slope, and tried widening the holes in our guttering above the downpipes, and  removing the coarse wire meshing that was meant to catch the leaves. There is a filter at the entry to the underground water tank to catch the leaves, which we clean once a year.

The rainwater harvesting pump

Rainwater harvesting pumps - membrane pump mounted on wall, old centrifugal pump bottom right on floor.

Rainwater harvesting pumps – membrane pump on wall, old centrifugal pump bottom right on floor. Click to view.

Originally a centrifugal pump was installed in the tank to pump rainwater into the house pipes to where it was needed. This requires electricity, which also means CO2 emissions.

Our ‘one-house’ system is much less efficient than the large mains system,  so replacing mains with rainwater actually increased our energy use and CO2 emissions per litre, to around 3 times that of mains water.

The pump was inefficient, and further research suggested that a membrane pump would be better. So we switched pumps and monitored the difference. The new pump, when running, has energy and CO2 use which is 2/3 lower – just about as low as mains water. But it only runs for an hour or less per day; and if we leave the standby on, energy use doubles.

Cost and environmental benefits 

The roof rainwater harvesting system only draws on the half of the house that was being re-plumbed.

The roof rainwater harvesting system only draws on the half of the house that was being re-plumbed.

The cost of the tank, soakaway, pipes and pump was around £2500; but the ‘civil’ and plumbing work for installation can add £3000 – £9000, depending on how much you have to replace, how you commission it (varying from a DIY approach to a project managed contract), and how much garden restoration you have to do afterwards.

It was a sizeable investment to recoup but, in the context of a whole house retrofit we wanted to try it. The actual metered water cost saving in 2012 was only £19 (water is very cheap, compared with energy), so it would take a very long time to ‘get our money back’.

From the calculations above, even taking great care, with the most efficient pump available, it is very unlikely that a pumped single house rainwater harvesting system will reduce your CO2 emissions or your costs by much at all. If you can do it without pumping, it will.

So, if this is not a reason for doing it, does it actually ‘save’ water, and help the environment? To answer this, we need to consider how the whole water system works.

Whether it is extracted by the water company from rivers or underground water, or by us from our roof, we are taking the water from the annual quantity flowing through the ‘hydrological cycle’. In a drought, it will be essential to leave a minimum amount of water flowing in the rivers and maintaining underground water levels, to preserve the local biodiversity and environment.

Rainwater harvesting from our roofs has to come from that same cycle, so it does not actually add any extra water to that available. What is does do, in a drought, is reduce the rate at which the water company has to draw down its reservoirs (or groundwater) and this extra storage helps reduce the time during which water restrictions have to be applied. So even if your rainwater harvesting tank runs empty in the drought (as it very likely will), you have still helped..

But if you want to help the environment, it is important, first, to reduce your total water use, as well as considering rainwater harvesting.

How much water can a home save?

Short showers can be a big water saver compared to taking baths

Short showers can be a big water saver compared to taking baths

Our water use has always been low. In 2012 we each used 85 litres per day, compared with the UK national average of 150. Of this, 16 (19%) was from rainwater harvesting. So, our mains water use is now less than half the national average. Our annual water bill was reduced by 11% (£19). If we had been able to use our whole roof area, these numbers might double.

We only have showers and may have a bath on very rare occasions. Even with showers, we do not have a ‘power shower’, just an ordinary one. On average, we shower 5-6 times a week in the winter and 6-7 times a week in the summer.  We also took the advice of someone who advocated that people should ‘sing shorter songs’!

Both our toilets cisterns are of the old larger capacity, but have a ‘hippo’ in them. We sometimes adhere to the slogan “if it’s yellow let it mellow”.  However we feel we could use the two toilets more strategically, i.e. use of the toilet at the back of the house (served by the RWH) when it has been raining a lot. Our dishwasher is a slim-line one that we run only on a full load. This fits in with the size of our family, now that we don’t have children living at home any more.

There is a lot of advice available on low water use – in appliances, but also in low flow taps and shower rate limiters, from Waterwise, the Energy Saving Trust, Which and your local water company. Reducing hot water use is even better, because the energy cost and CO2 savings that makes are much larger than those from water itself.


Would we go for the 3,300 litre tank and pump if we did it all over again?

No. We would spend £100s rather than £1000s, and avoid the need for CO2 emitting pumps, and large holes in the ground. We would fit large ground level water butts, possibly with small solar PV powered pumps; or possibly, one of the new systems which store the water in the roof space and deliver it to the house by gravity.

eco renovation - talk to homeowners

© Judy and Charles Ainger, SuperHome 83, Mar 2013

Further Information:
You can find out more about this rainwater harvesting system at Judy and Charles’ SuperHome at Open Days in September. Also see rainwater harvesting and water saving devices at green open house events in September.

Also See:
At Home with Water report on domestric water use