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# Gravity Heating System

## The density difference between hot and cold water is the circulating force in a self circulating gravity heating system.

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

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)

where

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)

where

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.

Circulating Pressure in Pa (N/m2) per m circulating elevation - he
Return Temperature
(oC)

Flow Temperature (oC)
405060708090
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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## Unit Converters

Temperature

oC
oF

Length

m
km
in
ft
yards
miles
naut miles

Area

m2
km2
in2
ft2
miles2
acres

Volume

m3
liters
in3
ft3
us gal

Weight

kgf
N
lbf

Velocity

m/s
km/h
ft/min
ft/s
mph
knots

Pressure

Pa (N/m2)
bar
mm H2O
kg/cm2
psi
inches H2O

Flow

m3/s
m3/h
US gpm
cfm

3 10

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