How does a gravity hot water heating system work?

by | Last updated: Jun 10, 2021 | 6 comments

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In this article I will introduce you to gravity hot water heating. Gravity hot water heating is a heating system based on the physical principle of gravity and works due to the change in density of water. Thus, gravity heating does not require electrical energy for a circulating pump (as in a heating system with circulating pump) and works solely based on the physical properties of water.

The principle of gravity heating

The principle of gravity heating is explained as follows: Heating water is heated in a heat generator and thus changes its density. The heated water expands and has a relatively low density at 90 °C ( \rho=965.30kg/m^3 ) and is therefore lighter. This gives the water a so-called buoyancy movement and, due to its lightness, it can rise in a specially provided riser (flow) and distribute the water to the radiators via an upper distribution.

IMPORTANT: Gravity hot water heating systems were almost always built with high-level flow pipes for the heat distribution.

In the course of the heating system, the water gives off its heat to the rooms and the building. The water thus cools down to approx. 70 °C, becomes heavier again and has a higher density ( \rho=977.76kg/m^3 ). This results in a so-called downward movement. The heating water drifts back to the heat generator in the return (downcomer) and can be heated again.

Gravity heating thus makes use of the temperature difference between flow and return and the associated change in density of the water. As a result, a constant cycle takes place. The lower density of water in the flow and the higher density of water in the return ensure a low differential pressure in the heating system. This is called the circulation pressure.

The following sketch is intended to clarify the principle of gravity heating once again.

Figure 1: Sketch of the scheme of a gravity heating system
Figure 1: Sketch of the scheme of a gravity heating system

The circulation pressure in a gravity heating system allows the heating water to flow in the heating system without an electric drive (e.g. circulation pump). In the figure below you will find an exemplary diagram of a gravity heating system as a closed system with upper distribution.

Figure 2: Gravity heating with upper distribution in a closed heating system.
Figure 2: Gravity heating with upper distribution in a closed heating system.

Conditions for gravity heating

The magnitude of the circulation pressure and thus the flow velocity in the gravity heating system depends on several factors. Without these conditions, it is difficult to operate a gravity heating system.

  • System height: The higher a building is, the faster the water can flow in a gravity heating system. However, it should also be noted here that the building must not be too high, otherwise the water will cool down too much on the upper floors and the circulation may slow down again. So, the system height is also limited and must be calculated.
  • Pipe diameter: The larger the pipe diameter, the lower the frictional resistance of the pipes and the faster the water can flow. However, this also results in higher installation costs.
  • Temperature level and temperature difference between flow and return play a decisive role, because the larger they are, the greater the circulation pressure and flow velocity.

Note: The higher the temperature difference, the higher the temperature level, the larger the pipe diameter and the higher the system height, the higher the drive pressure.

At the same time, this also means that gravity heating has very unfavorable values for today’s low-temperature systems and is therefore rarely used.

Advantages and disadvantages

In the table below, I have once again listed the main advantages and disadvantages of gravity heating:

Advantages gravity heatingNachteile Schwerkraftheizung
Simple systemLarge nominal pipe diameters due to the low driving force
Completely silent operationPoor controllability and very sluggish
No additional energy for circulating pumpsThe heat generator must always be located at the lowest point of the heating system
 High system temperatures are required (e.g. 90/70)
Gravity heating system requires an accurate calculation

How can I recognize a gravity heating system?

In renovation cases, existing gravity heating systems are often converted to circulating pump heating systems. Therefore, it is only possible to determine whether a gravity heating system is in operation or not if a precise system analysis is carried out. Furthermore, gravity heating systems can be operated with open systems and closed systems, since the circulation pressure is not dependent on the system pressure.

In a gravity heating system, the circulating pressure must overcome the pipe friction losses, so large pipe cross-sections are necessary in gravity heating systems and are an indication of this heating system. In addition, risers and downcomers were installed that were as straight as possible and had a short and horizontal connection to the radiators.

Conclusion

Today, gravity heating systems are rarely built. Examples of installation locations are areas where there are no or few power sources or where the power grid cannot be operated reliably enough.

I hope I could give you a little insight into the function of a gravity heating with this article. If you have any questions, suggestions or criticism about gravity heating, I look forward to your comments.

Best regards! Martin

Further links and sources:
Bosy Online – Schwerkraftheizung
Wikipedia – Schwerkraftheizung
Amev Heizbetrieb 2001
Gravity Hot Water Heating – A Q&A

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About Me

Martin-SchlobachHi, my name is Martin and Iโ€™m a passionate engineer in the field of buildings technology. Here you can read who I am and why I write this blog.

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