Rainwater Harvesting Ltd.
0800 074 7234
Whats in Store

- Get a Quote Now
- Tank Size Calculator
- Technical Support

- Rainwater Systems
   -Above ground systems
   -Below ground systems
   -Shallow dig systems
   -Direct or Gravity Feed?
   -Just Garden Systems

- Rainwater Management
- Tanks and Butts
- Flat Tanks
- Pumps
- Filters and Filtration
- Pipes and Hose
- Pipe & Hose Fittings
- Technical Support
- Drainage & Flood
- Handy Accessories
- Full Product List
- Information Zone

Shop Safe
World Pay Logo
Most major cards and paypal accepted online
Contact Us

RainWaterHarvesting Ltd.
Unit A Harrier Park
Southgate Way
Orton Southgate
01733 405100
Fax 01733 230996


Home » Rainwater Harvesting Systems » Below Ground Rainwater Harvesting System Kits » House and Garden Systems with Mains Backup DUK2

Product ID: GF-372002-DUK2
Rainwater Harvesting System Carat 3750 DUK2
Rainwater Harvesting System Carat 3750 DUK2

Graf Carat tank 3750 litre underground rainwater harvesting system kit. The trusted Carat from Graf, supplied complete as a kit for use as a domestic household or garden rain water harvesting system. This complete system Features auto-mains back-up.

The graphic image above shows the main components of the Carat DUK2 System and a typical layout for a simple domestic and garden application. The item marked with a * is the Rain Backup in a Box®. This 12v automated valve will trickle mains water into the tank when it is very low due to sustained drought to allow the connected domestic appliances (e.g. toilet) to work. Read more about this control system here

For further images that give the scale of the product and show some of the fittings please click here.

External HDPE Rainwater Pipe included. It is important to identify a source of a water supply. Untreated rainwater is not for human consumption and so water companies are recommending (and in some cases requiring) that Black and Green HDPE pipe is used for piping rainwater from the tank to the point of use - eg. garden tap. The point of use should be clearly labelled to show that rain water is the source. We include 25mm black and green hdpe pipe and labels with this kit with the option to buy further rolls of pipe.

Why buy this system?

  • Home and Garden Use
  • Feeds garden tap and toilet cystern(s) 
  • Automated mains water back-up in case of long drought
  • Prefiltered rainwater delivered to the underground tank
  • Quiet (pump submerged)
  • Pressure sensitive pump only works when tap opened
  • Lifting kit not necessary - can be man-handled onto the site
  • Exceptionally strong but light two piece Duralen moulding
  • Time tested and backed by a 15 year manufacturer warranty
  • Tank construction allows for manoeuvre into less accessible locations -even though doorways
  • Components precision made and matched for fast assembly

The Rain Backup in a Box® is WRAS Approved (#1306031) and fitted inside the building where the occupiers can see and hear it.

What comes with this kit?

Components list for Graf Carat tank 3750 litre DUK2
Product Code Product Description
GF-372002 Graf Carat tank, undergound 3750 litre
GF-371001 Carat dome (turret)
GF-371010 Carat lid, telescopic, made of PE (green)
GF-330192 Calmed inlet (into base of tank)
27-20-25 1" Strainer
GF-330108 Overflow siphon (high water level outlet, fits filter)
GF-371033 Carat 3750 accessory pack including tank sealing kit
GF-375021 Carat tank dome sealing kit
GF-340037 Graf Optimax Pro Filter
RWH-HYD01 Series 3 Submersible pump and inline filter from Hydroforce Pumps Ltd
RWH-HDPE25/25 Rainwater pipe HDPE 25mm diameter 25 metre coil
RWH-BUB01 Rain Back Up in a Box
Delivery Haulage, UK mainland included (Highlands & islands on request)
GF-372002-DUK2 Complete Underground kit for domestic use

Graf Carat tank 3750 litre, Complete System 2 underground KIT includes a 3750 litre underground tank, Optimax Pro filter, Enviro pump and the accessories needed to enable you to capture rainwater, filter it and store it at steady temperature ready for use in the home - e.g the flush a toilet - or in the garden to irrigate, power wash, top up water features etc. Some standard external pipework and connectors will be needed but are not included in the kit as the location of the system will determine the amount of pipework required.

Graf Optimax Pro filter included as standard
In response to customer demand, we have included the Graf’s Optimax Pro filter which fits neatly inside the turret of theOptimax premium filter Carat tank. With its integrated overflow siphon and inlet calming device, we no longer need to cut holes in the turret other than those provided by Graf in the moulding.

The Optimax Pro Filter is self cleaning -  water arriving across the filter mesh pushes excess debris and leaves to the overflow. The optional extra hose connector permits you to connect a hose and wash it through more thoroughly from time to time. 

If you are referring to the Graf catalogue or brochure, this kit from RainWater Harvesting is the equivalent to the Graf Eco Plus Package; our kit includes items 1) the Tank 2) the dome or “turret” and 3) Filter package 3. In place of the Graf pump package we provide the high quality, popular and more reliable Series 3 submersible pump, and a inline filter to take the cleanest water first. 

In the kit version, collated for you by RainWaterHarvesting.co.uk, you get the tank, the Optimax filter pack and the accessories listed below.

Upgrade this kit - If household water use is a priority you may want to consider a Rain Director driven system instead. There's more information on this rainwater management control system here. 

Optional Upgrades.Carat tank optional lids

1) Bit more leeway on the tank depth: Opt for a Maxi sleeve giving an extra 100mm of adjustment (see above)  Product ID: GF-371011 

2) Support car traffic: (but not trucks and HGVs) when fitted with the optional dome and cast iron lid. Contractors often ask us the dimensions of the turret. The Vehicle loading lid can be found here Product ID: GF-371020

  • Height variable between 750-1050 mm.
  • Maximum tilt 5%.
  • Cast iron cover.
  • Child safety lock.
  • Dark Grey.

3) Deeper holes: You can compensate for a deep hole with a tank turret sleeve extension Product ID: GF-371003 which extends the height of the pedestrian dome shaft by 305mm

More Research?

Manufacturing expertise
The Graf Carat tank is the largest tank of its kind in the world to be manufactured by injection compression moulding. This technique provides the tank with unbeatable stability and ensures that each component is produced with the highest accuracy. Graf have invested €20m in their factory near Freiburg for the Carat range, introduced in early 2007.

Shipping, assembly and installation
Like the Graf Hercules, the Carat comes in two halves so you can carry it through a doorway as narrow as 80cms (and so we can stack many of them on one pallet for bulk transport). A centering bolt in the flange makes assembly accurate and the knock-on quick connections and patented seal ensure there are no leaks between the two sections... and yet you can disassemble it if you like. The material used is a PE variant called Duralen.

  • Designed for underground mounting 
  • (German manufactured, TÜV safety-tested)
  • Patented quick connection (no screws required for mounting)
  • Permanently watertight: durably sealed thanks to solid profile sealing
  • Easy transport thanks to low weight and practicable size
  • Durably sealed thanks to solid profile sealing
  • See table for dimensions and weight
Compare the Carat S Tank sizes
Tank 2700 Litres 3750 Litres 4800 Litres 6500 Litres
Product Code 370001 370002 370003 370004
Weight 120 kg 150 kg 185 kg 220 kg
Length 2080mm 2280mm 2280mm 2390mm
Width 1565mm 1755mm 1985mm 2190mm
Height 1400mm 1590mm 1820mm 2100mm
Total Height 2010mm 2200mm 2430mm 2710mm
Half Height ie before assembly (for access) 700mm 790mm 910mm 1050mm
Go to other tank sizes Go to a 2700L system You are here Go to a 4800L system Go to a 6500L system

Connections for underground installation: we supply the tank as in the Graf Eco Plus package, with recesses where holes for piping need to be cut.

Carat Tank Deeper Pedestrian Lid:
Carat Tank Turret extension:
Carat Tank Lid Vehicle upgrade:

Email Friend

Our Price: 2134.67 (2561.60 inc VAT)

« Previous | Next »

You may find these come in handy

Product ID: RWH-SV01
Site Visit Installation Call Out

Site Visit Option 1 - Technical or other call out including pre-installation visit. Rainwater Harvesting Ltd personnel will visit to advise customers about installation or help with technical issues.

Our Price: 157.50 (189.00 inc VAT)
Product ID: RWH-SV02
Site Visit Installation Visual Check

Site Visit Option 2 - Visual check of piping and installation just prior to back-filling. Technical mid-installation visit just to make sure all is OK before all the earth goes back in.

Our Price: 78.75 (94.50 inc VAT)
Product ID: RWH-SV03
Site Visit Commissioning Installation

Site Visit Option 3 - Commissioning Visit when installation is complete to get your system running to optimum efficiency.

Our Price: 103.95 (124.74 inc VAT)
Product ID: RWH-SV04
Site Visit Complete Installation Support

Site Visit Option 4 - Triple check: pre-installation, mid-installation and commissioning. Three site visits by Rainwater Harvesting Ltd engineers to set your mind at rest.

Our Price: 288.75 (346.50 inc VAT)

Shopping Cart
Installation Brochures
Carat Installation Manual

Carat Installation Manual

Rain Backup in a Box Installation Manual

Rain Backup in a Box Installation Manual

Rain Backup in a Box System Quick Installation Guide

Rain Backup in a Box System Quick Installation Guide

HydroForce Pump Installation Manual

HydroForce Pump Installation Manual

Rain Backup in a Box Brochure

Rain Backup in a Box Brochure

Rain Backup in a Box Video

Rain Backup in a Box Video

Carat Installation Video

Carat Installation Video

  Privacy and Cookies Policy | Terms of Use | Terms of Supply | Contact | Refunds and Delivery Policy | Pro Login  
  Copyright 2008 RainWater Harvesting Ltd VAT No GB 940 8955 90 Reg. No. 3967275 Design and SEO Internet Assets Ltd