What’s the Best Rainwater Harvesting System for Me?
Once you have determined what you are going to use your rainwater harvesting system for, whether it’s going to be garden only or a full domestic system, there are several options for pump packages.
Garden systems tend to be a more basic pump package which simply includes an external or submersible pump, a short length of pressure pipe and some way of extracting the water from the tank, so either a hose connection point, an integral hose connection box, an outside tap or a tap stand. As we said in chapter two, there doesn’t tend to be a mains-water back up unit for a garden system and this is what makes the garden package more of a cost effective, or lower priced option. On a full domestic system there are several options on mains water top-up units or pump packages which can be used. The most comprehensive of which are commonly systems that have come over from Germany, and they tend to keep the pumps outside of the tanks and keep the mains water top up outside of the tank. They work on the basis of keeping the whole tank available for rainwater and not putting any mains water into the tank, as this is the best solution because when it does rain you then have the full tank capacity to fill with rainwater and there is no mains water which is water that you have paid for filling space inside the tank. So, the way this work is they have a pump unit which is installed inside the property either in a utility room or a garage or under the stairs, or in a plant room, and there is a suction hose from the pump which has a floating ball on the end, that goes through a duct, to the tank and draws the water in to the pump which sucks the rainwater in from the rainwater tank and then pumps it directly to the toilets, washing machine or outside tap. Generally these types of pumps can only draw the water in from within around 12 to 15 metres between the tank and the pump unit so anything greater than 15 metres tends to need a booster pump inside the tank, in which case instead of the pump in the house sucking the water in, it starts to suck the water in then the pump in the tank turns on and pushes the water to the pump and then the pump unit inside the property supplies directly to the toilets and washing machine and the outlets. When the rainwater level drops on these standard ‘rain manager’ style systems there is a float switch connected from the pump unit to the underground tank and this float switch is positioned inside the tank so that when the rainwater level drops the float switch will switch-over and this will cause the pump to automatically change to supply mains water instead of rainwater from the tank. On these pump units there is a small mains water chamber or reservoir which stays topped up with mains water all the time and as soon as the rainwater level drops the pump switches over to supply from that mains water chamber instead, so you don’t necessarily know what water is being supplied unless you look whether there is water in the tank or unless you have a level sensor showing how much water is in the mains water tank. But you’ve got your continuous supply of water to your toilets, washing machine etc. so there is water readily available on demand all of the time. There are variations on this type of system; there are more comprehensive and typically more expensive systems which have additional items built in, so these may include a level gauge to show what level of water is in the underground rainwater tank. If they have a level gauge then they tend to work on a sensor instead of a float switch for when the mains water takes over when rainwater levels are low, and then there are additional parts to these which are more technical which include inline fine filters, which are generally proposed for the washing machine connection, and there are also automatic filter cleaning units which can be added to these kinds of systems which will make the whole system more maintenance free so the filter within the underground tank has a small hose nozzle attached to the top and you can pre-set how often you want it to spray a flush of water over the filter to keep it clean from leaves and debris. These types of systems require space in the house. They are good because the pump stays out of the tank so is always accessible. So for maintenance and inspection services they can be often boxed in within a cupboard to reduce the noise, and some of them are in a polystyrene casing which reduces the noise, which some say is similar to the noise of a washing machine, but of course it is only when they are pumping water, so if a toilet flushes the pump will turn on and there will be a bit of noise when the pump is filling the cistern, this is why it can be put in a utility room or garage away from the main living area in the house.
The other advantage to these systems, as briefly mentioned earlier, is that the whole capacity of the rainwater tank stays available for just rainwater and not mains water. This also gives you the option to put a submersible pump inside the rainwater tank and connect that to your outside tap supply or your external supply, so that as we mentioned if there is a hosepipe ban you can prove that you are just using rainwater from your underground tank and continue to use that during the hosepipe ban because there is not a mains water connection to the underground tank.
There are several of these types of pump packages on the market in the UK and costs vary. They are good systems which are easily maintained and easy to look after but they do require space. They would not fit in a kitchen cupboard for example, they suit a larger house, and they are all direct supply, so they require power. In the event of a power cut the pump wouldn’t work and you would be without supply of rainwater and mains water to the toilets and that should be considered on certain projects and it’s to customers preference and opinion whether that is a big problem or not.
The other types of pump package involve a submersible pump and either a direct supply or an indirect supply system. A direct supply works in the same way as the pump package we have just been talking about where water is pumped from the rainwater harvesting tank directly to the toilets, washing machine or outside tap so this particular system is reliant on power. The difference is that the submersible pump direct supply systems don’t require anything inside the property in terms of a control unit. They have a mains water back up system which has to include an air gap between the mains water connection and the rainwater tank to meet regulations which prevents against cross contamination of rainwater into the mains water pipework, so the way this is typically done is with a length of pipe from somewhere in the house going back to the tank which contains a float switch from a mains water back unit, which comprises of a tun-dish 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, the solenoid valve then opens, and this allows mains water to flow through the tun-dish and by gravity supply flow back through the pipe to the tank and it tops up the main underground tank out in the garden. It keeps filling with mains water until the float switch rises as the water level rises and then when the float switch is in the upright position the solenoid valve will close and the mains water top up will stop. On these systems it’s important to consider the length of distance between where the mains water back up unit is inside the house and the underground tank. From that mains water top up unit the water is only being supplied by gravity pressure so the speed it travels from the air gap arrangement to the underground tank can be quite slow, meaning 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, the position the float switch is tied off within the tank and the amount of height level distance between the mains water back up and the tank. Obviously the higher the mains water back up the more head of pressure is built up as the mains water travels through the pipe and into the underground tank. Once the mains water has got to the tank, all water supplied to the toilets and the 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 that you can see how much water is in the tank but normally these are not related to the mains water top up functionality on these systems. In some systems they can be, but quite often they are 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.
The other alternative to this is a submersible pump indirect system, in which the submersible pump inside the mains water tank which works in the same way as the direct, so it’s got a floating water intake on the pump which draws water in from just below the surface water level inside the tank which is where it’s at its cleanest, it goes through a fine filter and it draws the water into the pump and the pump then sends it up to a header tank in the roof space of the property. A header tank is 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 rainwater is available the submersible pump will pump from the tank up via the black and green pressure pipe into the roof space and fill the header tank with rainwater. From there, on demand when toilets are flushed, the valve on the system opens and the rainwater will be supplied via gravity feed from the header tank in the loft down to fill up the toilet. It’s important on this type of system that a traditional indirect header tank system is designed into the internal plumbing of the house so that the most direct route is provided for the pipework from the header tank to the outlets so that it doesn’t slow down the rate in which the water fills the cistern because it’s not being pumped under pressure it’s just being supplied under gravity pressure from the header tank. So, when the rainwater level drops inside the tank and isn’t available for the pump to send it to the header tank the pumps dry running protection will kick in and the pump will shut 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 top up would kick in which would be set at a lower position inside the header tank, and when that valve drops down that will open up the mains water supply and mains water is used to top up the header tank, making water always available to supply to toilets and washing machines etc.
The benefit of this system is that the mains water back up supply is not reliant 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, but on a standard header tank system if it has a mechanical valve not a motorised valve, then mains water will still be allowed to top up the header tank and that can then be supplied to the toilets under gravity pressure. There are alternatives that require power so it’s not one rule fits all, so it’s important to understand how each individual header tank system works, but more often than not on an indirect system there is a way of ensuring there’s a continued water supply with or without power so it works whether there is a power cut or a pump failure so there won’t be a time when the toilets aren’t being supplied with water, which is of particular importance to housing associations who have several properties who have rainwater harvesting systems and they need to keep their tenants supplied with water, and they need to be confident that they are not going to be getting phone calls about their toilets not filling or their washing machines not working, or there’s a problem with the system and they don’t know what it is. So, these indirect systems give an added peace of mind under that scenario. Having said that, header tank systems are not everyone’s preference. It’s seen as quite traditional and ‘old-fashioned’ to put a tank of water in the loft or roof space of the property and it’s not the most modern way of doing it. In certain countries it’s not done at all. In Germany, for example they never use header tanks in our experience and our German colleagues say it is very unusual to put header tanks in the roof space of a property to supply water to toilets, it is not done over there and therefore they don’t see it as a very attractive system at all. Having said that, the majority of their systems are installed in basements as a lot of their houses have basements or cellars, as standard and these are ideal for the location of pump packages. Of course, in the UK, basements and cellars are not as common, especially not on large scale housing developments.
To summarise on types of pump packages and mains water back-up units the main thing to realise is that there is such a choice, and some systems will suit particular properties better than others, so it’s important to consider what best meets your needs and what you prefer the sound of in terms of what maintenance will be required and how often you need to check the system, and if you need to do anything with the system at all. So the main point in this chapter would be to consider all options and not just opt for the lowest price system that gets proposed because that might not be the case in the long run, depending on whether it’s an individual private build for your own home or whether you are building a housing site development of a number of homes, there will always be a level of maintenance required on any type of system but this can be minimised by choosing particular products and pump packages that are more maintenance free and that work automatically. So, it is important that you understand what the options are and how they work so you can determine what works best for you.
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