Rainwater harvesting at a detached house in Kerry

Study of a typical domestic dwelling

Key points to be assessed:

  • existing plumbing and fittings
  • Number of persons using the property and any special requirements
  • Suitability of the roof and building for rainwater harvesting
  • Determining measures for reducing consumption
  • Design of a suitable rainwater harvesting system
  • Survey of existing plumbing and fixtures:

    Ground floor items which must use potable water

  • 1 bath
  • 1 shower
  • 1 hand basin
  • 2 sinks for kitchen use
  • Ground floor items which could use non potable water

  • 1 toilet with 9 litre flush
  • 1 washing machine
  • 1 outside tap
  • First floor items which must use potable water

  • 2 hand basins
  • 1 bath
  • 1 shower
  • First floor items which could use non potable water

  • 2 toilets with 3.5 / 5.5 litre flush mechanisms


    3 toilets, 1 washing machine and 1 outside tap could be supplied from rainwater harvesting.

    The washing machine is a low water use design.

    No leaks or obvious wastage is apparent, all taps and ball cocks etc.. are drip free.

    Persons using the property:

    The property contains 2 double bedrooms and one single bedroom so peak occupancy can be assumed as 5 persons.

    There are two teenage children who use the showers frequently but no other special considerations.

    Suitability for rainwater harvesting:

    The property is a dormer bungalow with a main 30 degree pitched roof covered with artificial slates. There is a separate roof over the kitchen at one gable end and another over a sun room at the other.

    The rear of the house is served by two down pipes whilst the front has three plus another taking the sunroom roof at the western end.

    There is ample space and hard standing to accommodate above ground storage tanks on the North Eastern corner where they would benefit from the most shade from the sun.

    This position also affords almost direct connectivity to the down pipes on the rear of the building.

    All but one of the toilets and the washing machine are against the outside rear wall making plumbing simple.

    Dimensions were obtained from the original plans.(see diagram)

    No figures are available for actual water consumption as there is no meter.

    Currently there is no charge for water which is supplied from a local lake via a private scheme, this will most likely change soon as the council has taken over the scheme and is in the process of adding a treatment plant and reservoir.

    Measures to reduce current consumption:

    From the assessment of the plumbing it was noted that the ground floor cistern could have its capacity reduced (this has now been reduced to 6 litres by adjusting the float level) and the ground floor showerhead can be replaced with a more efficient, water saving model.

    Educating the children in better ways to use water can not be underestimated and will be implemented.

    All three toilets, the washing machine and the outside tap could be adapted to use rainwater.

    Finally installing a meter to allow accurate observation of actual use may encourage better water management.

    Rainwater Harvesting:

    Questions to be answered:

  • Demand
  • Potential available supply from rainfall
  • Roof area and it’s ease of connection to storage tanks
  • Tank size and location
  • Type of plumbing and means of supply
  • Costs and practicality
  • Health and safety
  • Demand: based on BS 8515:2009

    Using the simplified method and referring to figure 3 table d (page 11 of BS 8515)

    Assume average daily consumption of non potable water to be approx 50 litres per person.

    5 persons @ 50 litres per day gives 250 litres / day

    For the recommended period of 18 days for storage need, a total of 4500 litres is required.

    Potential rainfall available, roof area and ease of connection:

    From Met Eireann,Valentia observatory available rainfall is 1430 mm per year

    Using the rear roof, the kitchen roof and including the section of the front roof which discharges onto the kitchen roof a total area of 84.12m2 can be utilised.

    From figure 3 table d this will give the maximum required capacity of 4500 litres.

    These roofs currently discharge via down pipes at the North East corner of the building making connection very easy.

    Tank size and location:

    The tank required would have a capacity of 4500 litres and be mounted above ground at the North East corner of the building.

    Type of plumbing and means of supply:

    There is no room in the attic area to facilitate a tank so a gravity system would be impractical.

    A direct system could be used with either a submersible pump mounted in the tank or an external pump mounted alongside the tank. Either type would require a pressure switch or controller and a back up for when rainwater was in short supply.

    The most convenient solution would be a self regulating booster pump mounted alongside the tank feeding a new supply pipe to the rear of the house where the ground floor toilet and washing machine could be accessed directly through the wall.

    A further supply pipe could be taken up the wall to supply the first floor toilets, this would fit within the cover around the soil pipes and allow for insulation.

    A simple tap mounted above the tank would allow for topping up in the event of insufficient rainfall and would provide the necessary air gap to prevent cross contamination.

    proposed rainwater harvesting solutionclick on picture for full size image

    Costs and viability:

    To install a 4500 litre tank system would require;

  • 4500 ltr tank price (nearest size is 5400 ltr but better than too small) €1125
  • Booster pump €240
  • Filter assembly €168
  • Gutter and down pipe fittings €85
  • Plumbing pipe and fittings €92
  • Insulation €67
  • Electrical supply to pump €100
  • Total Parts €1877

    External labour (electrician) €80

    Remaining labour €720

    TOTAL labour €800

    TOTAL…………………… €2677


    The current current water charges in this location are zero but this will change.

    If water rates are introduced then this system might save 50%, from an environmental point of view it would offer some ecological benefits over sourcing water from a processing plant.

    It could offer some independence from the council supply but might increase the possibility of coming into contact with harmful pathogens whilst for instance washing the car.

    On balance it is a matter of personal choice, would you rather see something we should consider an often wasted and precious resource used or allow it to run into the drain?

    Health and safety:

    Following the guidelines set out in BS 31100 : 2008 a risk assessment was undertaken and key points of concern were identified.

    The principle risks were found to be:

  • Algae or pathogenic organisms multiplying in storage tanks and pipes
  • Contamination of potable water by poor plumbing
  • Actual risks encountered during installation work
  • In order to address these issues the following solutions are proposed;

  • Suitable filtration will be incorporated to minimise ingress of pollutants.
  • All tanks and pipes are to be located and insulated in such a manner that they are not subject to solar heating.
  • In the event of long term disuse these should be drained.
  • To prevent cross contamination a suitable air gap will be provided between the tap for topping up and the tank.
  • No switchable connections will be made between the rainwater and mains water systems.
  • All plumbing to be tested and constructed to current BS codes.
  • All electrical installations to be certified by a qualified electrician.
  • All work to be carried out to a schedule and in a safe manner (eg. Ladders to be tied and protection against falling objects provided etc.)
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