Anyone who drags the hose around the garden every evening in summer is wasting time and water. An automatic sprinkler system takes over at the set time – even on holiday and early in the morning, when evaporation is low. The heart of every automatic system is the solenoid valve: it opens and closes the water supply on command from the controller. Anyone who wants to automate an irrigation system properly must above all get two things right – dividing the system into zones and sizing the valves to match the flow rate.
How a solenoid valve works
A solenoid valve is an electrically controlled valve. A solenoid coil generates a magnetic field that lifts or releases a diaphragm – the valve opens, water flows to the sprinkler system. Without a signal, the valve closes and the water path is sealed. The controller switches these coils on and off at the programmed time.
When it comes to powering the coil, there are two common variants that differ fundamentally:
- 24V alternating voltage (24V AC): the classic solution for permanently installed systems. The controller is connected to the 230 V mains and supplies 24 volts of alternating current to the valves. The valve is only open as long as current is applied – if the current fails, it closes automatically. This variant is robust, allows many valves and long cable runs and is the standard for larger, permanently installed systems.
- 9V direct voltage (9V DC, bistable): the battery-powered solution for locations without a power connection. Here a bistable valve works: a short current pulse opens it, a second pulse closes it again – in between, no holding current flows. This is exactly what makes battery operation possible over a whole season. Ideal for remote beds, allotments or systems without a mains connection.
Valves with an interchangeable coil are practical: with the Poelsan solenoid valves, the 9V and the 24V coil sit on a common thread, so the same valve can be fitted depending on the controller. A manual open/close and venting function also makes commissioning easier. You will find the matching models in the solenoid valves category.
Dividing irrigation into zones
Hardly any water source delivers enough to sprinkle the whole garden at once. That is why the system is divided into zones (irrigation circuits) that run one after another. Each zone is switched by its own solenoid valve. The controller works through the zones one by one – this way, each circuit has the full pressure and full flow rate available.
The most important rule when planning irrigation zones is:
The sum of the flow rate of all sprinklers in a zone must not exceed the available source flow rate.
So add up the flow rate values of all the nozzles, sprinklers or drippers in a circuit (in m³/h or l/min). If the sum exceeds what the source and line can deliver, the zone is split and an additional valve is added. As a rough rule of thumb for lawns: about one additional valve for every 100 m² of lawn – depending on the sprinkler type and water availability.
The easiest way to find out how much your source delivers is with a bucket test; our guide Measuring water flow rate explains the procedure.
Example
Your source delivers 3 m³/h. A pop-up sprinkler consumes 0.6 m³/h. This means that at most five sprinklers may run per zone (5 × 0.6 = 3.0 m³/h). If eight sprinklers are to cover the lawn, you split them across two zones (e.g. 4 + 4 sprinklers), each with one solenoid valve. The controller runs zone 1 and zone 2 one after another – the source is never overloaded, and each sprinkler reaches its full throw radius.
Which valve size?
A common mistake is a valve that is too small: it throttles the flow rate and costs pressure. The decisive factor is the flow range for which the valve is designed – it must match the amount of water in the zone. The following table is based on the Poelsan solenoid valves (PoelsanARC series, working pressure 1–10 bar):
| Valve size | Connection | Flow range | Typical use |
|---|---|---|---|
| 1″ | 1″ female thread | approx. 0.05–9 m³/h | home garden, individual lawn and bed zones |
| 1½” | 1½” female thread | approx. 5–27 m³/h | large gardens, landscaping, several sprinklers per zone |
| 2″ | 2″ female thread | approx. 5–34 m³/h | sports fields, agricultural areas |
Choose the valve so that the zone flow rate lies within the specified range – not at the very edge. The 1½” and 2″ valves are also available in a flow-adjustable version, with which the amount of water per zone can be finely adjusted. The values are guide values; the manufacturer’s specifications and the actual demand of your system are decisive. An overview of all sizes can be found under solenoid valves.
Controller & valve box
The controller (irrigation computer) is the brain of the system. It switches the zones according to a programme – by time of day, day of the week and run time, with modern devices also by soil moisture or rain sensor. Make sure the controller has at least as many outputs (stations) as you plan zones, and that it matches the coil type: mains-powered computers control 24V valves, battery-powered controllers control the bistable 9V valves.
The valves themselves belong protected in the ground. A valve box houses one or more valves, keeps them accessible for maintenance and protects them from soil, frost and damage. You can find matching boxes under valve boxes. A filter is also recommended before the first valve, so that no dirt gets into the valve and nozzles.
FAQ
9V or 24V solenoid valve – which is better?
That depends on the location. If there is a power connection and a permanently installed system, a 24V system is the robust standard choice. Without a mains connection – for example in an allotment – a battery-powered 9V controller with bistable valves is the right solution. Valves with an interchangeable coil can be converted later.
How many valves do I need for my garden?
As many as needed so that no zone exceeds the available source flow rate. Divide the total demand of all sprinklers by the measured source flow rate. As a guideline for lawns, about one additional valve per roughly 100 m² applies – the exact number results from the flow calculation and the sprinkler type.
What happens if the valve is too small?
An undersized valve throttles the flow rate, creates pressure loss and the sprinklers do not reach their throw radius. So choose the size according to the flow range of the zone and not by the line thread alone.
Where is the solenoid valve installed?
In the ground in a valve box, easily accessible and frost-proof. This keeps the valve clean and easy to reach for maintenance. Several valves of one system can be grouped together in a common valve box.
With the right division into zones and appropriately sized valves, automatic irrigation runs reliably for years. The complete planning – from flow measurement to zone division – is summarised in our guide hub Planning your irrigation.
Expert advice from KLN: unsure about valve size, coil type or zone planning? As a distribution partner for Poelsan products, KLN & Heumann GmbH advises you professionally on your automatic irrigation system – from the design to the selection of the right components.