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Add-On Underground Water Tank for Expanding Rainwater Harvesting Systems
The Graf Carat S 1700 Gallon Underground Extension Tank is designed to seamlessly expand the capacity of your existing Carat S rainwater harvesting system. This cost-effective add-on tank provides 1700 additional gallons of usable storage and connects easily to your main tank, allowing you to scale your water system without duplicating filtration or pump equipment. Built with the same German-engineered quality and patented no-screw assembly, this extension tank is the smart choice for anyone needing more underground water storage without the extra infrastructure.
Perfect for homeowners, businesses, or farms that need extra capacity for irrigation, toilet flushing, or emergency backup.
Features and Benefits of the Graf Carat S 1700 Extension Tank
✓ 1700 Extra Gallons of Storage – Expand your system without needing a second riser, pump, or filter. This extension tank boosts volume at a lower cost.
✓ No-Collapse Design – Guaranteed not to collapse when empty if installed per manufacturer instructions, even in high groundwater conditions.
✓ Patented Tool-Free Assembly – Installs quickly using a screw-free quick-connect system. Centering bolt ensures perfect alignment with no leaks.
✓ Built to Withstand the Elements – Designed to be buried between 30" and 70.9" deep and remains stable in groundwater up to the middle of the tank.
✓ Modular and Stackable – Easily integrates with other Carat tanks and stacks efficiently for low shipping costs and minimal storage space.
✓ Backed by a 15-Year Warranty – Made from durable, recyclable Duralene and certified through TÜV German safety standards.
Specifications for the Graf Carat S 1700 Gallon Extension Tank
Capacity
1700 Gallons (Fully Usable)
Construction
High-Strength Duralene Plastic
Riser or Lid
Not Included (Connects to Primary Tank)
Groundwater Stability
Up to the middle of the tank
Installation Depth
30" to 70.9"
Expansion Use
Designed for connection with main Carat tank
Vehicle Load Rated
Not applicable (no access point)
Frost-Proof
Yes
Warranty
15 Years (Manufacturer)
The Tank Guaranteed Not to Collapse When Empty*
1700 Fully-usable gallons of additional storage
* When installed in compliance with manufacturer's installation instructions
The patented quick connection (illustrated in green), allows the Carat tank to be assembled without screws in only few minutes. Disassembly is possible at any time.
The first-class EPDM material used in the profile seals (illustrated in yellow), has been laboratory tested to last more than 25 years.
The centering bolt (illustrated in orange) assures the accurate and easy assembly of the two half shells preventing any leaks
Application
The global innovation – the new tank generation of the Carat rainwater underground tanks have been specially developed for rainwater harvesting
Frost-proof installation underground
Groundwater stable up to the middle of the tank due to extremely rigid construction
Logistical Advantages of the Carat Tank
The Carat tank consists of two half shells which can be stacked on top of each other, allowing multiple tanks to be stacked on a single pallet for shipping. For example, it is possible to stack five 1,700 gallon Carat tanks on a single pallet. This allows a standard 40 ft. shipping container to house a total tank volume of up to 34,000 gallons! The tanks' unique stacking feature directly reduces transport costs and environmental impact from vehicle emissions.
Is this easy to use? I haven’t had a system like this before.
Yes, it’s designed to be simple and beginner-friendly. If you’re just using a tank or a basic bundle, it works passively, rainwater flows from your roof into the tank through a downspout diverter. There’s no need for electricity or complicated setup. If you need to add a tank to an already established rainwater system, it’s just as easy, most tanks can be connected in series or used as additional storage with a few basic fittings. We can help you choose the right components to make the expansion seamless.
How do I know what tank size I need for my home or garden?
A simple way to estimate the right tank size is by calculating how much rainwater your roof can collect. Multiply your roof’s surface area (in square feet) by your average annual rainfall (in inches), then multiply by 0.62, this gives you the total gallons you could collect in a year. For example, a 1,000 sq ft roof with 30 inches of rainfall can collect around 18,600 gallons annually. From there, we can help you choose the right tank size based on your water use and how much storage you want on hand, whether you're using it for irrigation, home use like toilets or laundry, or even potable water with proper filtration.
Can I bury this tank underground?
Yes and no, not all tanks are designed to be buried. Only tanks that are specifically rated for underground installation should be placed below ground. Always check the product specifications or reach out to us before burying any tank to ensure it’s safe and properly supported.
Is this tank safe for long-term water storage?
Absolutely. Our tanks are made from high-quality, UV-stabilized, food-grade polyethylene or reinforced materials designed for long-term rainwater storage. They resist algae growth and won’t leach chemicals into your water.
Can I collect rainwater from just one downspout?
Yes. Many basic systems start with a single downspout diverter, especially for smaller tanks. This setup is easy to install and still collects a significant amount of water, especially in areas with frequent rainfall or large roof sections.
What’s the difference between above-ground and underground tanks?
Above-ground tanks are easier to install, inspect, and move. Underground tanks save space and are often used when visual impact or limited yard space is a concern. We offer both, and we’re happy to help you decide what fits your space and needs best.
Will the tank overflow if it fills up?
No worries, each tank includes an overflow port that directs excess water safely away from the tank when it’s full. You can connect this to a drain, second tank, or garden area to make use of the overflow.
Can I connect this tank to a pump later on?
Yes. All of our tanks are compatible with add-on pumps, filters, and automation systems. You can start simple and upgrade later with no problem, just let us know your goals and we’ll help future-proof your setup.
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