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The pump head of a circulator pump is not as easily explained in one sentence as you might think – or maybe it is? Here is a definition attempt from Wikipedia:
“In fluid dynamics, Total Dynamic Head (TDH) is the total equivalent height that a fluid is to be pumped, taking into account friction losses in the pipe.” Total dynamic head – Wikipedia
I think this definition is a bit confusing and therefore I would like to explain the term “head” for circulator pumps a bit easier.
Table of Contents
Confusion about the units of head
First of all, you should know that head (H) is given in metres (m). In practice you will also see “metres of water column” (mH₂O), but this is used as a pressure shorthand, not as the unit of head. See figure 1.
This can be very confusing, because metre ‘m’ as a unit symbol suggests a length unit. For this reason, the following assumption is obvious:
A house is 10 m high, so the pump head is 10 m.
This assumption is wrong. In closed hydronic circuits, the static elevation cancels out. The required pump head (H) is the energy the pump adds to overcome friction losses and local (minor) losses at the design flow rate – not the building height.
What is head if it is not a length unit?
A circulator pump is designed to transport liquids from A to B. One of many applications is, for instance, a hydronic heating system that uses circulator pumps to transport heating water. For this transport, energy is necessary.
The rotating impeller imparts velocity head to the fluid, which then converts to pressure head in the system. This creates the differential needed to overcome friction losses and local (minor) losses in the pipework and components at a given flow rate.
Formally, the pressure difference Δp (in pascals, Pa) and head H (in metres, m) are related by the following formula, where ρ is the fluid density (kg/m³) and g is the gravitational acceleration (≈ 9.81 m/s²).
![\[\boxed{\Delta p = \rho \, g \, H}\]](https://cdn.buildingservicestutor.com/wp-content/ql-cache/quicklatex.com-b0555caf46d7ba9c2ad1d496315f8ec9_l3.png "Rendered by QuickLaTeX.com")
For convenience, pressure is sometimes given as metres of water column. Since the density of water changes at different temperatures, the “DIN 1301 Part 3: Conversion of non-SI units” specifies under point 141 the following:
| Name of unit | Unit symbol Context | Origin | Conversion |
| Millimeter Wassersäule (german), conventional millimetre of water | mmH2O mm CE mm WS | Unit of pressure | 1 mmH2O = 9,80665 m−1 kg s−2 |
Which means: 1 mm H2O = 9,80665 Pa
However, since the information is usually given as “metres of water column”, the following applies: 1 mH₂O = 0.0980665 bar = 98.0665 mbar = 9,806.65 Pa.
For simplicity:
1 mH₂O ≈ 0.1 bar ≈ 100 mbar ≈ 10 kPa ≈ 10,000 Pa
10 mH₂O ≈ 1 bar ≈ 1,000 mbar ≈ 100 kPa ≈ 100,000 Pa
Why is head not given in pascals (Pa)?
Head is specified in metres (m). In practice, metres of water column (mH₂O) is still used as a legacy pressure unit. In Germany it has not been a statutory unit since 1978 and it is also not an SI-compliant unit. Nevertheless, it is still used in various areas, including the sanitary sector. Many organisations encourage using pascals (Pa) for pressure, as ‘metres of water column’ can be confusing.
Summary
Here once again the most important facts about the head are summarized:
- A circulator pump transports liquids and adds energy to the fluid. The impeller creates velocity head that converts to pressure head.
- In closed hydronic circuits, static elevation cancels and the required pump head (H) must overcome friction losses and local, minor losses at the design flow rate.
- Head (m) and pressure (Pa) are related by Δp = ρ g H. Pump head therefore indicates the required differential pressure in a hydronic heating system via this relation.
- In practice, pump head is often shown as metres of water column (mH₂O), a legacy pressure indication based on water density, whereas the SI-consistent unit for head is metres (m).
- The SI unit for head is metre (m) and for pressure pascal (Pa).
I hope this article could help you to understand the head of a circulator pump better. If you have questions, suggestions or criticism, please use the comment function.
Greetings Martin.
Further links and sources:
Sensorsone – mH2O pressure unit
Wikipedia – Total Dynamic Head
Das große Pumpen 1×1 von Fristam
Wikipedia – Circulator pump
Wikipedia – Centimetre of Water
