Suitability of rainwater harvesting systems for existing dwellings

Domestic rainwater harvesting systems are generally aimed at new-build.
There are several reasons for this and these are:-

• The drainage can be designed specifically with the rainwater system in mind.
• The way the internal plumbing is arranged differs slightly to separate out the potable and non-potable pipe work
• It is often easier to move the tank around the site before the building is completed.
• Excavation machinery is already available.

The costs associated with installing a system are reduced by the above factors, making them more affordable.

When considering an existing property is can be very much more expensive, starting with the fact that VAT is added to the system price. This is currently 15% (recently 17.5%). But moreover, re-routing pipe work can be very expensive and can involved fairly major cosmetic works. The external drainage is often difficult to access, either due to hidden pipe runs, unworkable invert depths and/or unknown modifications.

It can also be difficult to get the tank to the desirable location around the property. For example a semi-detached house, has only access around one side. Often this is limited. So a crane cost must be added to the project.

Space in the garden can also be an issue, as digging close to foundations can be risky and require a professionally appointed structural engineer to advise on the works.

So, with the extra costs and the fact that water is actually still relatively cheap in the UK, there isn't the financial incentive to harvested the rainwater.

At Freerain, we advise customers with existing properties to consider a system for external/gardening use. This is because the internal plumbing is changed and perhaps not all of the drainage is needed to be centralised. This reduces the costs significantly, but of course not everyone is satisfied with external water only.

For commercial projects, these are reviewed on an individual basis, this is because the very nature of some of of the sites. External drainage can sometimes be very easily accessed, above ground tanks are more acceptable and work technically better.

In summary, rainwater harvesting systems are best suited to new-build projects for domestic dwellings, but perhaps commercial sites can with certain modifications incorporate a system.