Upgrading / Modifying / Fixing

Rainwater Capture for Car Washing

Last modified: 2018-07-31

7 min(s) read



The availability of De-Ionisation Water Filter Systems has done much to make car washing easier by removing the need to dry the car once washing is completed. The removal of all dissolved solids in the final rinse water ensures that there are no unsightly residues left on the finish and the car is perfectly clean.

However, the resin bed that is the active ingredient in DI systems is expensive and has a relatively short active life if not used wisely. The active life is proportional to two factors: The amount of dissolved solids in the water supply and the volume of water passed through the filter bed. Reduce either or both and the cost per wash is reduced.

Reducing the volume is easy, simply use the resin filtered water only for the final rinse. Reducing the dissolved solids is only possible by changing the quality of the water supply, and that’s where this system succeeds. In my area, near Milton Keynes, we see tap water samples as high as 350 parts per million (PPM) Samples of water from my water butt system are usually between 10 and 20 PPM.

Clearly using butt water will extend the life of the resin bed significantly.

This system is designed to capture rainwater and filter it until it is clean and free from dissolved solids.


1 x Water Butt Stand* (If you are going to fit a water level gauge, buy the 250 Slimline from Ward Strata)
12 x Snap-On Hozelock or Hozelock Style Connectors (Beware of cheap online offerings. Bunnings have a good range).
1 x Octopus Strap PK 2 (find at most DIY Stores)
2 x Snap On Hose Splitter Kit (find at most DIY Stores

1 x Xlux TDS Meter*

*link generates funds for the group when you buy (learn more about how these funds are spent on https://teslaowners.org.uk/about/tesla-owners-uk-supporter)





Rainwater is diverted into the rainwater butt by a plastic diverter inserted into the rainwater downpipe.

It is located at a height that ensures that the butt is filled to capacity. When the butt is filled, diversion ceases and rainwater flows down the downpipe and into the original drainage system.


The butt is a Ward Strata 250L Slimline Water Butt* and is fitted with a drain tap and snap on lid. The lid has been modified and a simple float and pole water level gauge installed. The butt is connected to the rainwater diverter by a flexible plastic tube.


The system is powered by a Hozelock 7612 Submersible 300W Water Butt Pump*. The pump is fitted with an inlet filter located in the base. There is no ON/OFF switch; the pump is live as soon as it is connected to a power source. Normally, the pump would simply rest on the bottom of the water butt, but as the system is designed for vehicle washing, the pump is suspended about 40mm off the bottom on a nylon strap. This allows any sediment that sinks to the bottom to simply stay there and to be removed during regular maintenance, rather than be sucked up by the pump and clog the filters.

The pump should never be allowed to run dry. To that end, it is fitted with a float switch that will shut down the pump should the water level be very low. Maintenance instructions for the pump are included with the pump,


Two Karcher Particle Filters are connected in parallel and mounted on a foam plastic board. The board is mounted on the front of the tub with an elastic nylon strap. Connecting two filters in parallel all but removes any additional resistance through the system and extends the time between filter cleaning. The system is designed so that the filter assembly need never be dismantled. Cleaning the filters is actioned by connecting the outlet of the filter assembly to a cold, mains water supply, and back or reverse flushing.


An 11.7 Litre Resin Pressure Vessel containing 10 litres of Tulson Premium Grade Resin is inserted into the system to remove all mineral solids and deliver an air dried finish free of watermarks.

The quality of the water delivered by the DI filter is monitored by measuring with a TDS (Total Dissolved Solids) tester. A TDS reading of 5 parts per million or below should deliver a perfect finish.

As the resin is exhausted the reading PPM will increase and eventually white deposits will appear on the final finish, although, if used as advised, this should be a very long time coming.


All the connections on the system are Hozelock style quick connectors. Three external hose assemblies are required. The shortest hose connects the butt to the particle filter. The medium one connects the particle filter to the DI filter when required, the longest hose, connects the system to your pressure washer. NOTE: It is not good practice to use hoses that have previously been connected to a mains supply as deposits will have coated to the inside of the hose. When pure water passes through, these will be released and contaminate the pure filtered water.


If an extended period of sub-zero temperatures is forecast it is wise to remove the pump and keep it in a warmer place, and at the same time drain the resin vessel. However, short, sharp frosts, where the temperature is in positive numbers through the day will not harm your pump or your vessel. As a guide, I did neither in the winter of 2017 -2018 but it was a mild winter.


The need for maintenance is proportional to two factors. The condition/cleanliness of your roof and guttering system, and the frequency of use. As a guide, I have cleaned my system for the first time after 6 months. You will know when the system needs cleaning when the particle filters clog quickly or the pump pressure reduces.


Empty the butt via the drain tap. Remove the feeder tube from the diverter to the butt and take the butt off the stand. Remove the lid and then cut the cable tie that prevents the cable sliding through the hole in the lid. Slide the lid down the cable and put to one side.

Disconnect the feed hose at the butt outlet connection. Remove the pump by pulling on the nylon strap that hangs it in place and again put to one side. DO NOT SUSPEND THE PUMP BY THE ELECTRIC CABLE.

Clean the inside of the butt using a hose or pressure washer. Clean the pump and the pump inlet filter by following the instructions supplied with the pump.

When reassembling the system, ensure that the nylon strap that supports the pump is located centrally between the two cable ties on the pump support handle. The pump hangs from the outlet assembly. Check that the pump float will be free to move through its entire range.

Replace the lid.

When the butt is completely empty it is very light and could suffer if strong winds are forecast. It is therefore good practice to partially fill the butt with tap water until the gauge just lifts off the bottom stop. This is not an issue in normal use as the pump will stop before the butt is complexly drained.


When rainwater is plentiful then I use rainwater for the whole of my wash cycle, i.e. Pre-Foam Wash, Foam Application, Two Bucket Wash, Rinse and Final Rinse with DI filtered water.

When rainwater is in short supply I use tap water for most parts of the wash saving rainwater for the final rinse. When the rainwater is exhausted, then of course it has to be tap water for the whole cycle, but be aware that putting tap water through your DI filter will exhaust the filter media much faster than rainwater.

Finally, good luck with your system. Many who will read this have forsaken the local Rumanian Car Wash and now enjoy, or would like to enjoy washing their Tesla. If I can be of help in any way do please call me on 07872 443 023 or. If you are within travelling distance of Milton Keynes you are welcome to come and see my system. We might even wash your car!

Pete Smoothy June 2018

My water capture area is my workshop roof. Big enough to ensure ample water in typical English weather and with easy access to keep it clean

Do you have the expertise? Share your knowledge!

Remember modifying your vehicle may invalidate part of your vehicle’s warranty.
Therefore, be careful and check with Tesla if unsure. Also any modifications will most likely need to be OK’d with your car insurance company.

To the best of our knowledge, these guides are correct and factual. However we take no responsibility if something does go wrong.

If you spot a mistake please ensure you alert us.