The tank size for a rainwater harvester is not entirely your choice. If the tank is too large then you run the risk of bacteria breeding over time to dangerous levels. For this reason, a tank size calculator, based on average annual rainfall and water usage, has been developed by the British Standards Institute in an effort to stop companies trying to make more money by encouraging the sale of bigger tanks.
Beware of using the Tank Size Calculator available on a leading website as some of the advice it contains is incorrect and does not comply with the BS 8515 2009. You are risking Legionnaires Disease if you oversize the tank.
The BS 8515 2009 states that the capacity of the rain water harvesting storage tank must be the LEAST of either:
- 5% of the Annual Rainwater Yield (ARY)
or
- 5% of the Annual Rainwater Demand (ARD)
To calculate the Annual Rainwater Yield (ARY) use the following calculation using the rainfall map below
Rainfall (mm) x Building Footprint Area (m2) x 0.9 0r 0.8 (Tile Roof or Slate/Stone Roof) = ARY
ARY x 0.05 = 5% ARY
To calculate the Annual Rainwater Demand (ARD) use the following calculation:
No. of Residents x 16,000 = (1) Area of Garden (m2) to be watered (if any) x 60 = (2)
(1) +(2) = (3)
(3) x 0.05 = ARD
You are not allowed to instal a rain harvesting tank that is larger than the smaller of these two figures as the water will become stagnant and a breeding ground for bacteria, including Legionnaires Disease.
