In this video, we’ll explain the differences between a direct and an indirect rainwater harvesting system. When it comes to using rainwater for domestic purposes, there are two types of system that you will come across a direct feed system and an indirect feed system. Both supply a property with precious rainwater and both include a mains water backup. However, they feed the appliances in two completely different ways. A direct feed system pumps water from the rainwater harvesting tank directly to the toilets, washing machine and or outside tap. This type of system has a mains water backup system, which has to include an air gap between the mains water connection and the rainwater tank to meet regulations which prevent any cross-contamination of rainwater into the mains water pipe work. This is typically done with a length of pipe from somewhere inside the house going back to the tank, which contains a float switch from a main water backup unit.
This comprises of a tundish to give you the air gap and a solenoid valve connected to a float switch, which is inside the tank. So as the rainwater level drops in the tank, the float switch drops as well. The solenoid valve then opens up and this allows main water to flow through the tundish and by gravity supply flow back through the pipework to the tank, thereby topping up the main underground tank out in the garden with mains water. It keeps filling with mains water until the float switch rises and as the water level rises, then when the float switch is in the upright position, the solenoid valve will close and the mains water top up will stop. If you’re looking to install a direct feed system, it’s important to consider the length of distance between where the mains water backup unit inside the house will be and the underground tank. From the mains water top-up unit, the water is only being supplied by gravity pressure, so at the speed it travels from the air gap arrangement to the underground tank can be quite slow. This means that the system can be topping up with mains water for what appears to be a very long time. This depends on the type of tank. Of course, the position of the float switch is tied off within the tank and the amount of height level distance between the mains water backup units and the tank. Obviously, the higher the mains water backup unit compared to the tank, the more head of pressure is built up as the mains water travels through the pipe into the underground tank. Once the mains water has got to the tank, all water supplied to the toilets, washing machine and any other outlet is supplied by the submersible pump in the underground tank.
Now, it is possible to add additional items such as a level gauge, so you can see how much water is in the tank, but these normally are not related to the mains water top-up functionality on these systems. In some systems they can be, but quite often they’re just to show the occupier of the house how much water is in the tank, and it is an added visual item to the system.
An indirect feed system works in a similar way to a direct feed system. However, instead of pumping rain mortar directly to the appliance, the submersible pump inside the tank pumps rainwater to a header tank in the roof space of the property. A header tank can be installed in the roof space and it has two connections, one for mains water and one for rainwater. There are various types of header tanks, but the idea in principle is to maximise the amount of rainwater used before switching over to mains water if necessary.
If rainwater is not available in the underground tank, so when the rain water is available, the submersible pump will pump from the tank up via the black and green rain water pressure pipe into the roof space and fill the header tank with rainwater. From there on demand when toilets are flushed and the valve on the system opens, the rainwater will be supplied via gravity feed from the header tank in the loft down to fill up the toilet. When the rainwater level drops inside the tank and isn’t available for the pump to send it to the header tank, the pump dry running protection will kick in and shut the pump off. That will mean the rainwater ball valve in the header tank will continue to drop and no water will be supplied. At this point, the mains water topup will kick in and would be set at a lower position inside the head of tank, and when the valve drops, that will open the mains water supply and mains water is then used to top up the head of tank.
The benefit of this system is that the main’s water backup supply is not relying on power. So if there is a power cut in the property, the pump wouldn’t work inside the tank and the water level in the header tank would drop. However, on a standard header tank system, if it has a mechanical valve and not a motorised valve, then the mains water will still be allowed to top up the header tank and that can be then supplied to the toilets under gravity pressure. There are alternatives that require power, so it’s not one rule that fits all. So it’s important to understand how each individual header tank system works. More often than not an indirect system, there is a way of ensuring that there’s a continued water supply with or without power, so it works whether there is a power cut or pump failure, so there won’t be a time when the toilets aren’t being supplied with water. Rainwater harvesting is a simple concept, yet many people still don’t understand how it works and how they can benefit from it. And with the UK expected to suffer from water shortages within the next 20 to 25 years, it’s now more important than ever to start putting rainwater to good use.
