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 the force created by the 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 = head available for circulation (m, ft)



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)



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


gravity heating pressure difference diagram

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

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