Design of Hot Water Heating Systems

Gravity Circulating Heating System

In a hot water gravity heating system the circulation of water is a result of the density difference between hot water in the supply lines and cold water in the return lines.

The hot water tends to flow upwards and cold water tends to flow downwards. The elevation of the supply and return lines are therefore important for proper functionality.

The low differential pressure in the gravity system requires increased pipes and valves dimensions. The gravity system has a relative low heating capacity due to low medium temperature in heating elements.

The gravity system is simple and limited to use in smaller hot water heating systems.

With a reversed return pipe the length of the supply and return pipes for all heating elements are more or less the same. This makes it easier to select proper piping dimension for balanced flow.

The expansion of water (4.7% from 0^oC to 100^oC) can be collected in an open expansion tank above the rest of the system or inside a closed pressurized tank close to the boiler and safety valves. In modern systems closed tanks are preferred.

Forced Circulation Heating System

In forced systems the circulation is forced by pumps.

Water is circulated by the pump regardless of temperature forces between hot and cold water. It is not necessary to be careful with piping elevations. Pipes, valves and heating elements, radiators and air heaters, can be downsized because of higher flow and higher mean temperatures.

Forced circulation systems are in general the only practical alternative for larger systems.

Temperature ^oC
K
^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 kg_f
N
lbf

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

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm