How a pressure vessel stops the pump short-cycling, what volume suits your building, and whether you still need one alongside a smart controller.
The pressure vessel is one of the most misunderstood parts of a building's water system. Many people are unsure what it actually does and whether it is still needed once a controller is fitted.
What a pressure vessel does
Inside the vessel there is a rubber bladder and a volume of compressed air. When the pump runs, water enters the bladder and compresses the air further. When a tap opens, that compressed air pushes the water out — without the pump having to start.
The result is simple: the pump does not start for every small draw, such as opening a tap for a few seconds.
Why that matters
Every start draws a high inrush current and stresses the motor, the capacitor and the pressure switch. If the pump starts and stops dozens of times an hour, its life shortens sharply. In the trade this is called short cycling.
A pressure vessel reduces how often the pump starts; a controller reduces how hard each start is. The two complement each other rather than replace one another.
What size to choose
Volume depends on pump flow and how the building uses water. This estimate is practical for typical residential buildings:
| Building | Typical pump | Common vessel volume |
|---|---|---|
| 1 to 2 units | 0.5 to 1 hp | 24 litres |
| 3 to 6 units | 1 to 1.5 hp | 50 litres |
| 6 to 10 units | 2 hp | 80 to 100 litres |
| more than 10 units | 2.5 to 3 hp | 150 litres and above |
These figures are estimates based on common installations. Larger projects need a proper calculation from flow and the cut-in/cut-out pressure difference.
Do you still need one with a controller?
HYDROSET works both with and without a pressure vessel. Without one, it still steadies the pressure and starts the pump gently. But a vessel reduces how often the pump starts, and in high-demand buildings that makes a noticeable difference to pump life.
- Have a vessel? Add a controller for steady pressure and a protected pump.
- No vessel? A controller works on its own; you can add a vessel later.
Maintenance notes
- Check the vessel's air pressure once a year. It should normally sit slightly below the pump's cut-in pressure.
- If the pump suddenly starts short cycling, the first things to check are the air pressure and the bladder.
- Install the vessel where it can be inspected and drained.
