Rainwater Harvesting tank buyers guide

Rainwater Harvesting tank terminology

Different manufacturers and suppliers often use different names for the same water tank components, which can make comparing products confusing. The glossary below explains the main parts of a water tank along with the alternative terms commonly used across the industry.

Thread sizes explained (BSP)

Parallel vs Tapered Fittings

Parallel thread fittings (BSPP) seal using a washer, whilst tapered fittings (BSPT) create a seal as the threads tighten together.

The majority of water tank manufacturers will use BSPP parallel threads and this is pretty universal across most water tanks. It is always worth checking whether the fitting is parallel (BSPP) or tapered (BSPT) before installation.

💡Using a tapered thread can cause irreparable damage to the threads so check before installing that the threads are compatible.

Internal vs External Diameter

ID (Internal Diameter) is the measurement across the idside of a fitting or thread, whilst OD (Outside Diameter) is the measurement across the outside of a pipe of threaded fittings, 

💡BSP sizes refer to the thread standard rather than the actual outside diameter of the fitting.

Overflow

An overflow allows excess water to leave the tank once it reaches maximum capacity. This helps to prevent overfilling and reduces the risk of water damage from an excess of water.

An overflow is common on tanks that are filled automatically, for example on rainwater harvesting systems, mains-fed break tanks and livestock water storage tanks. 

Most overflows are installed near the top of the tank and should direct any overflow to a suitable drainage point. Depending on the application, they may also be fitted with an insect screen or rodent guard to prevent pests from entering the tank.

💡An overflow only carries water when the tank is full and is not the same as a tank vent.

Inspection Lid

The inspection lid (also known as an access lid, inspection cap or access hatch) provides access into a water tank, for a number fo different reasons. This includes inspection, cleaning and maintenance through its life.

Depending on the tank, the lid can also be used to install fittings or access equipment that may be inside. 

Examples of uses for the inspection cap;

• Cleaning and routine maintenance
• To visually inspect the internal condition
• Install or service a submersible pump
• Fit float stiches or level sensors
• Remove debris or sediment

Most water tanks we supply have a screw or hinged tank lid. Typically, the larger the tank, the larger the access hatch in order to make tank maintenance easier. 

Types of lid;

The type of inspection lid can vary, with options that include;

• Vented lids – allows air to escape or enter the tank as the water level rises and falls.
• Plain lid – secure seal, ideal for potable water or tanks being transported.
• Internal lid – a lid that sits in an internal thread and protrudes into the tank.
• external lid – a lid that screws onto a neck and protrudes from the top of the tank.
• Lockable lids – a lid that can be locked

💡The inspection lid should remain closed when not in use to prevent dirt or debris entering the tank.

Nominal Capacity

Nominal capacity is a term to articulate the manufacturer’s stated storage volume of a water tank under normal conditions. For example, a 100 Litre water tank is designed to hold approx. 100 Litres of water. In practise, the amount of usable water varies on a number of variables including;

• location of the outlet
• the installation and location of other fittings such as a float valve
• whether space is left at the top for expansion or overflow.

There is also a manufacturing tolerance to consider. In most cases, this can be +/- 3%. For example, a tank that has a nominal capacity of 100 litres, could actual vary from 97 to 103 Litres due to the manufacturing variables.

💡A tanks nominal capacity can be used for naming the tank and comparison but the actual usable volume can vary slightly depending on how the tank is installed

Diverter Kit

A rainwater diverter kit helps collect rainwater from your roof and direct it into a storage tank. Connected to your gutter downpipe, it can help make rainwater collection more efficient by filtering out debris and managing how water enters and leaves the tank.

Different components are available to help keep collected rainwater cleaner and improve the performance of your rainwater harvesting system.

Filter Collectors – Filter collectors divert rainwater from the downpipe into your tank while filtering out debris such as leaves and twigs. Many designs also allow excess water to continue down the drainpipe once the tank is full.

Calmed Inlets – A calmed inlet controls how water enters the bottom of the tank. It helps prevent stored sediment from being disturbed and reduces turbulence, allowing heavier particles to settle at the bottom.

Siphon Overflows – A siphon overflow allows excess water to drain from the tank once it reaches a set level. It can also help remove floating debris from the water’s surface, depending on the design, helping to maintain water quality.

View our range of rainwater diverter kits here