Circulating Force in Gravity Heating Systems

Differential pressure due to difference in density between hot and cold water is the circulating force in a self circulating heating system

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

gravity heating system

Gravity Head

The head available forcing circulation through a radiator in a gravity system is proportional to the elevation - he - of the radiator or heating element above the boiler, and the temperature difference between the flow and return pipes.

The head available for circulation can be expressed as:

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)

ρr = density of water in return (cold) pipe (lb/ft3, kg/m3)

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

Converting Head to Pressure

Head can be converted to pressure units by the expression:

p = hl ρ g         (2)


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

ρ = density (kg/m3). Using hot or cold density have very little impact on this calculation.

g = gravitation 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 pressure difference diagram

Circulating Pressure in Pa (N/m2) per m circulating elevation - he
Return Temperature
Flow Temperature (oC)
40 50 60 70 80 90
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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