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.
Table of Contents
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 ( ) 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 ( ). 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.
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.
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 heating||Nachteile Schwerkraftheizung|
|Simple system||Large nominal pipe diameters due to the low driving force|
|Completely silent operation||Poor controllability and very sluggish|
|No additional energy for circulating pumps||The 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.
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
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