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Engineering ToolBox > Hot Water Heating Systems Design
In a hot water gravity heating system, the circulation of water is a result of the difference in density between the hot water in the supply line and the cold water in the return line.
The hot water tends to flow upwards and cold water tends to flow downwards. The elevation of the supply and return lines are therefore important for proper functionality.
The low differential pressure in the system demands increased pipes and valves dimensions. The gravity system will also have a relative low heating capacity because of low medium temperatures in the heating elements.
The gravity system is simple but limited to smaller hot water heating systems.
With a reversed return pipe the length of the supply and return pipes for the heating elements are more or less the same. This makes it easier to select proper piping dimension for balancing the flow.
The expansion of water (4.7% from 0oC to 100oC) can be collected in an open expansion tank above the rest of the system, or inside a closed pressurized tank close to the boiler and safety valves. In modern systems the closed tanks are preferred.
In a forced system a pump will produce the circulation .
Water is circulated regardless of the temperature forces between hot and cold water. It is not necessary to be careful with the piping elevations. The pipes, valves and heating elements, radiators and air heaters, can be downsized because of higher flow and higher mean temperatures.
Forced circulation is in general the only practical alternative in bigger systems.