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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.

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.

Gravity Heating System - Circulating Pressure vs. Temperature Difference
Return Temperature
(oC)
Circulating Pressure in Pa (N/m2) per m circulating elevation - he
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.

## Related Documents

• ### Density vs. Specific Weight and Specific Gravity

An introduction to density, specific weight and specific gravity.
• ### Design of Hot Water Heating Systems

Calculate gravity and forced circulating hot water heating systems.
• ### Hot Water Heating Systems - Online Design Application

Free online design tool for designing hot water heating systems - metric units.
• ### Hot Water Heating Systems - Online Design Application, Imperial Units

Online design tool for hot water heating systems.
• ### Hot Water Heating Systems - Pressure Loss in Steel Pipes

Pressure loss nomogram for hot water steel pipes.
• ### Hot-Water Heating Systems - Classification

Hot-water heating systems can be classified by temperature and pressure.
• ### Volumetric (Cubic) Thermal Expansion

Volumetric temperature expansion calculator.

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## Citation

• The Engineering ToolBox (2003). Gravity Heating System. [online] Available at: https://www.engineeringtoolbox.com/gravity-heating-systems-d_189.html [Accessed Day Month Year].

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9.19.12