Gravity Heating Systems

The density difference between hot and cold water is the circulating force in self circulating heating systems

A self circulation heating system operates by the force created by the density difference between hot and cold fluid.

Gravity heating system

Gravity Head

The head forcing circulation through a gravity system is proportional to the elevation - he - of the heating elements (or radiators) above the boiler, and the temperature difference between the flow and return pipe lines.

The head available for circulation can be calculated

hl = hrr - ρf) / [(ρr + ρf) / 2]                                            (1)


hl = head available for circulation (m, ft)

hr = height of radiator or heating element above boiler (m, ft)

ρf = density of water in flow (hot) pipe (lb/ft3, kg/m3)

ρrdensity of water in return (cold) pipe (lb/ft3, kg/m3)

The thermal expansion of water is approximately 4.2% from 4 oC to 100 oC.

Converting Head to Pressure

Head units can be converted to pressure units as

p = hl ρ g                                             (2)


p = pressure (Pa, N/m2) - Other units?

ρ = density (kg/m3) (using hot or cold density has little impact in this calculation)

g = acceleration of gravity (9.81 m/s2)

Circulating Pressure - Flow and Return Temperature

The forcing pressures in self circulation system with operating temperatures between 50 to 95oC are indicated in the diagram and table below.

Gravity heating system - pressure difference diagram

Circulating Pressure in Pa (N/m2) per m circulating elevation - he
Return Temperature
Flow Temperature (oC)
90           0
80         0 64
70       0 59 123
60     0 54 113 177
50   0 48 101 161 225
40 0 41 89 143 203 267

Related Topics

  • Heating - Heating systems - capacity and design of boilers, pipelines, heat exchangers, expansion systems and more

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