What is pump head and why it’s important?

The head of a heating 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 pumps a bit easier.

Confusion about the units of head

First of all, you should know that the head “H” is usually only given in meters (m) or “meters of water column” (mH2O) – see figure 1.

This can be very confusing, because meter “m” as a unit of measurement suggest a unit of length or height. 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, because the delivery head of a pump has nothing to do with the system height or the height of a house.

What is the head if it is not a length specification?

A pump is designed to transport liquids from A to B. One of many applications is, for instance, a heating system that uses pumps to transport heating water (heating system with circulating pump).  For this transport kinetic energy is necessary.

The kinetic energy is produced by the rotation of the pump impeller inside the pump. This creates a pressure build-up by the pump, which is necessary to overcome the friction and loss along the pipeline inside the heating system at a certain flow rate.

The head is therefore an indication of pressure and nothing more than a conversion from pascal to meters 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:

Which in means the following: 1 mm H2O = 9,80665 Pa

However, since the information is usually given in “meters water column”, the following can be said: 1 m H2O = 0,0980665 bar = 98,0665 mbar = 9.806,65 Pa

For simplicity, the following can be noted:
1 m H2O ≈ 0,1 bar ≈ 100 mbar ≈ 10.000 Pa
10 m H2O ≈ 1 bar ≈ 1000 mbar ≈ 100.000 Pa

Why is the head not given in Pascal?

The head is mostly given in the unit “meter water column” and is an obsolete unit for measuring pressure. 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 associations try to change this and enforce Pascal as the unit to be used because the unit meter water column is confusing.

Summary

Here once again the most important facts about the head are summarized:

1. A pump transports liquids from A to B and thereby transfers kinetic energy to a pumped medium.
2. The kinetic energy of the pump must be high enough to overcome the weight of the pumped medium and the friction and loss along the pipeline inside the system at a certain flow rate.
3. Pressure is a unit of measurement for resistance, so the delivery head of a pump is an indication of the required pressure in the heating system.
4. The head of a pump is often given in “meters of water column”, which is an outdated unit for measuring pressure.
5. The conformal SI unit for the head is Pascal.

I hope this article could help you to understand the head of a 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