If you’ve ever looked at a pump datasheet and wondered what “10m head”, “20m head” or “40m head” actually means, you’re not on your own.
Head is one of the most important figures when choosing a pump, but it’s also one of the easiest to misunderstand.
Put simply, a pump’s head rating describes the pressure difference the pump can generate, expressed as a height of liquid. For water, 1 metre of head is roughly equivalent to 0.1 bar of pressure.
But there’s an important catch: a pump rated at 20m head won’t necessarily deliver 20m of head when it’s installed.
To understand why, you need to look at the relationship between head and flow rate.
What is pump head?
Pump head is a way of expressing the pressure difference a pump can create and it is normally measured in metres (m).
For water, 1m of head is approximately 0.098 bar, so as a useful rule of thumb:
- 5m head ≈ 0.5 bar
- 10m head ≈ 1.0 bar
- 20m head ≈ 2.0 bar
- 30m head ≈ 3.0 bar
These are approximate figures for water. The exact relationship depends on the density of the liquid being pumped.
It’s also important not to confuse pump head with system pressure. Head describes the pressure difference generated by the pump, rather than the total pressure shown on a system gauge.
What does a “20m head” pump actually mean?
A pump’s maximum head is generally its shut-off head, meaning the maximum head it can generate at or very close to zero flow.
As the flow rate increases, the available head normally decreases. For example, a pump might have a performance curve showing:
| Flow Rate | Available Head |
|---|---|
| 0 L/min | 20m |
| 33 L/min | 18m |
| 67 L/min | 15m |
| 100 L/min | 10m |
| 133 L/min | 4m |
The important thing is that you cannot select a pump based on maximum head alone. You need to know how much head the pump can provide at the flow rate your system requires.
Head vs flow rate
This is one of the most important things to understand when choosing a pump.
Flow rate tells you how much liquid the pump needs to move, while head tells you how much pressure difference the pump needs to provide to overcome the system’s resistance. Flow rate is commonly expressed in litres per minute (L/min), while head is expressed in metres (m).
For example, if an application requires 100 L/min at 12m head, you need to find a pump that can deliver at least that duty point.
A pump capable of 20m maximum head isn’t automatically suitable. You need to check its performance curve to see what head it can actually produce at 100 L/min.
What is a pump curve?
A pump performance curve shows the relationship between flow rate and head. You’ll normally see flow rate along the horizontal axis and head along the vertical axis.
For a typical centrifugal pump, the curve slopes downwards. At low flow, the pump can generate more head. As flow increases, the available head decreases.
This means that saying “I need a 15m head pump” doesn’t give you enough information. A better specification is “I need X L/min at X metres of head.”
This combination is known as the duty point.
Does head mean how high a pump can lift water?
In an open system, such as when transferring water, the vertical height between the source and destination can form an important part of the head the pump needs to overcome.
However, this doesn’t apply in the same way to a closed heating system.
Closed heating systems are different
In a typical closed heating circuit, the water is already filling the system. The circulator isn’t simply lifting water from the bottom of the building to the top.
Instead, it primarily needs to overcome the hydraulic resistance of the system, including pipework, bends, valves, fittings, heat exchangers and other components.
That’s why the height of a building isn’t simply added to the required head when selecting a circulator for a closed heating system.
So, if you’re choosing a heating circulator, don’t assume that a three-storey building automatically needs a pump capable of “lifting” water three storeys.
What happens if the pump has too little or too much head?
If a pump can’t provide enough head at the required flow rate, the system may not achieve its intended performance.
Depending on the application, this could mean:
- Insufficient flow
- Poor circulation
- Reduced heating or cooling performance
- Low pressure at outlets
- Longer transfer times
But more head isn’t automatically better either.
An oversized pump can produce excessive flow or pressure, increase energy consumption and potentially cause unwanted noise or control issues.
The aim isn’t to choose the pump with the biggest numbers. It’s to choose a pump that matches the system.
