A Promax detention tank and a Promax retention tank are not the same. Sure, they look the same, and indeed, they are both stormwater tanks. But the way they are set up makes them very different. This guide explains that difference and shows you why the words detention and retention are not interchangeable.
The short answer
In short, a detention tank eases pressure on the public stormwater system infrastructure, and a retention tank eases pressure on the public water mains supply system. One exists to protect the network from the water leaving your property. The other exists to give you a supply you would otherwise draw from the mains. That single difference in purpose is what drives everything else about how the two are configured, and it is the reason a tank that is physically identical on the truck ends up doing an entirely different job once it is plumbed in.
How a detention tank works
A detention tank is designed to live its life empty. It detains stormwater for a specified period of time and slowly releases stormwater to the main network. During a rain event, the tank has more water coming into the tank than is exiting the orifice, and the tank fills up. It constantly slowly drains to the stormwater main system, but over a much longer period of time than the rain event. That is the whole idea: the same water still reaches the network, just spread across more time. Spreading it stops sudden bursts of stormwater runoff, which in turn eases pressure on stormwater networks and reduces the risk of surface flooding in the area.
Why councils ask for detention tanks
Stormwater detention tanks take the load off the drainage facilities in your area by releasing manageable amounts of water during downpours. That is why most New Zealand councils demand owners of new homes install stormwater tanks above or below their property. This helps to preserve the integrity of their existing, and often ageing, stormwater drainage facilities. A new house adds roof and hard surface to a catchment that was sized before that house existed, and detention is how the addition gets absorbed without rebuilding the network underneath the street. Whether or not you need one is covered further in do I need a consent for a water tank.
What determines the size of a detention tank
Whether or not you need a stormwater detention tank is something that only your local council can answer. If they say you must have one, then the size of the tank comes down to a range of factors, for example property size, building structures within your property, or your roof's surface area. Your council or builder will be able to provide this technical information. Deliberately, this article does not give you a number, because the inputs are specific to your property. For the sizing work itself, see size up your stormwater tank and the stormwater detention calculator.
How a retention tank works
A retention tank serves quite a different purpose. You could say it is made to serve you. A retention tank is just like a detention tank in that it collects stormwater. However, instead of releasing it over time, it retains that water until you need it. The orifice that defines a detention tank is precisely what a retention tank does without. The water stays where it lands until you draw on it, and that one change turns the same tank body from a piece of network protection into a working supply for your property.
What people use retained water for
People use Promax retention tanks to irrigate the garden, wash the car, fill the pool, hose down the driveway, flush the toilet, and more. You can do many things with all that glorious water you collect and retain for eventual use on your property, and that is why our plastic retention tanks are increasingly popular. Every one of those uses is water that would otherwise be drawn from the mains, which is the sense in which a retention tank eases pressure on the mains supply system. The water lands on your roof either way. Retention is simply the decision to keep it.
Tank farms and multiple tanks
Because Promax retention tanks effectively give New Zealanders ready access to water, many of our customers are setting up tank farms. A tank farm is simply a network of connected tanks that catch and provide as much rainwater as possible. They are common sights in rural settings or on properties with no town water supply, and they are based on a straightforward premise: the more tanks you have, the more water you have to use. It is the retention idea taken to its logical conclusion, which is why you see it most where mains water is not an option in the first place.
Choosing between the two
We have addressed the difference between a detention tank and a retention tank, but you may still have questions about the best product for your specific requirements. The honest starting point is that the choice is often not entirely yours. If your council requires detention, that requirement comes first, and retention is what you add when you want the water as well. Plenty of properties have a reason for both, which is the thinking behind a dual purpose tank. If you are building in Auckland, the SMAF stormwater rules are worth reading alongside this.
Contact us for further information and advice to help you make the right decision, or call Promax on 0800 77 66 29.