Everything you need to know about Combi Rainwater Harvesting

Are you familiar with “combi” rainwater harvesting systems? Like most people, you have probably never heard of this terminology, but Combi systems are now one of the most popular types of commercial rainwater harvesting systems in the United Kingdom.

Although rainwater harvesting sounds like a relatively simple technology, it can be achieved in several different ways depending on the size, type, and use of the building. A “combi” system simply refers to how rainwater harvesting is achieved within a building.

Before looking at how a combi rainwater harvesting system works, it is important to understand the other ways of achieving rainwater harvesting. One way of recycling rainwater is through a non-pressurised rainwater harvesting system (often referred to as a gravity-fed system). In this arrangement, recycled rainwater is pumped from the main storage tank via a submersible pump to a smaller, secondary storage tank located at a high level. The supply of rainwater then gravity feeds to points of use below.

Best practice for non-pressurised or gravity-fed rainwater harvesting systems has changed in recent years. Now, instead of locating the header tank at a high level , the secondary storage tank is located at a low level with a booster pump to supply outlets. The change was made to protect rainwater harvesting systems against legionella which is more common in water sources located higher up due to the increased temperature.

The other way of harvesting rainwater is through a pressurised rainwater harvesting system (also referred to as a boosted system). In this arrangement, rainwater is pumped from the storage tank via a submersible pump to a pressure vessel. 

• Float switches instead of toggle sensors – Unlike other suppliers of similar arrangements, the Stormsaver Combi Control Unit uses float switches instead of cheaper toggle sensors. Toggle sensors are susceptible to failure when used in a rainwater harvesting system due to the naturally occurring biofilm which can easily clog a small toggle sensor. Using mechanical float switches protects the facility against costly maintenance visits to replace failed toggle switches.
• Fail-safe ball valve – Stormsaver Combi Control Unit is supplied with mechanical float valves to prevent the system from continuously filling in case of a solenoid valve failure which controls the supply of incoming rainwater and mains water into the secondary break tank. Similar systems do not have a fail-safe ball valve in case of a solenoid failure. This could lead to an alternative system continuously demanding a supply of water that overflows into the drain until the next service visit – don’t get caught out with a large water bill!
• GRP construction – Stormsaver combi Control Units are constructed with GRP (glass-reinforced plastics) unlike alternative suppliers which are supplied with steel housing. Steel housing is prone to rusting from condensation generated by the wet environment. This degrades the lifespan of the system and will eventually need a full replacement.
• Programmable logic controller (PLC) – Stormsaver uses PLCs to electrically control all the pumps and valves within the rainwater harvesting system. Other manufacturers use PCB electronic controllers which will need a full replacement if components were to fail. If a component fails within a PLC controller, then individual relay components can be replaced relatively easily. This reduces unexpected downtime and prolongs the lifespan of the system.
•  Easily accessible components – the break tank on the Stormsaver combi control unit can be easily accessed by a maintenance engineer should an internal component fail. Whereas, on alternative units, the internal break tank only has a small opening for maintenance and is hard to access due to the location of the steel housing. This is a problem because the small toggle switches mentioned above are in the internal break tank and are sometimes inaccessible for replacement. For end users, these issues could result in higher maintenance costs for full system replacements.
• Easy pump changes – the Stormsaver Combi Control Unit comes equipped with individual valves to shut off the water supply to each pump, whereas other systems need completely draining down when a pump fails. This could lead to the maintenance visit taking longer and an increased cost as a result.
• Type AA Air Gap – The Stormsaver Combi Control Unit comes with a Type AA air gap which is equipped with ingress protection. This prevents mosquitos and other insects from entering the control unit. Other systems come with a type of AB air gap that does not have this protection.

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We are proud to design, manufacture, and test our rainwater harvesting systems here in the UK through our own internal team. We maintain thousands of systems each year from all types of manufacturers and the knowledge we develop from our maintenance sites feeds back into the continued development of our rainwater harvesting systems. We are always on hand to answer any questions you may have. Contact us here.