Radiator connection pipe diameter ⋆ REGULUS System

Hydraulic balancing of a heating system means controlling the flow of the heating medium (hot water) through the system peripherals (pipes, radiators), aiming at optimal distribution of heat generated in a boiler to individual rooms in amounts desired by the user. The water transport route (pipes) must be adequate to the needs, must have adequate flow rate.

An element regulating the flow of the heating medium (water) in the heating system is the suitable graduation of pipe diameters (e.g. DN 22-18-15), which ensures its adequate distribution to individual circuits and branches. Another commonly used method of hydraulic balancing of a system are presets, shut-off valves, and more recently rotameters (flow meters).
The diameters of pipes, the same as on the supply lines to radiators, are used on the return lines to the boiler.

Specific heat of water in CGS system is 1 cal/(g C), and in a SI system it is 4.18 J (kg K). Watt – unit of energy flux in the SI system denoted by W. In mechanics, Watt is briefly defined as a joule per second. Therefore, radiators with a power of 1000 W at a standard flow rate of the heating medium (usually water) cooled to a specified temperature (usually 10°C), take over from the water and emit heat into the environment in quantities of a thousand joules per second. For this to be possible, the radiator must be not only of a certain size but a specific amount of water, which later will be cooled, must pass through it.

From each radiator, we can “squeeze out” more power by speeding up the flow of water (e.g. more efficient circulation pump or pump working at a higher gear), but this may be accompanied by clearly heard sound of flowing water.

To avoid this, the radiator connection branches must have an appropriate diameter.

We can accept the general principle that high power radiators, i.e. above 2700 W, must be necessarily supplied using connecting pipes with a diameter of ¾”.
Smaller heaters can be powered with connecting branches with a diameter of ½”.

The condition for good, high-efficiency, no-noise operation of the system, as well as the condition for a long period of its service life, is its careful construction, consistent with good installation practice. After selecting the material of which the system will be built, carefully check the rated power (size) of radiator to choose the right diameter of connecting pipes. Appropriately selected diameter will guarantee an adequate flow. The internal diameter of the connecting pipes is dependent on the material from which the system is made (copper or plastic) and the size (rated power) of the radiator. A system made of copper has a much thinner walls than pex-al-pex or polypropylene pipes. Hence, the inner diameter of the copper pipes is larger than the inner diameter of polypropylene or pex-al-pex pipes, because with more powerful radiators (grey area in the chart), you can still use the 1/2 inch copper pipes, but plastic pipes should be 3/4 inch.

It is the best to use connections with a diameter compatible with the calculation in the system design plans.

Chart showing the relationship between the minimum diameter of the connection and the rated power (size) of a trench radiator with a fan (W)

LEGEND

TYPE	lenght [mm]	750	1250	1750	2250	2750
TRIOVENT		590	1170	1770	2360	2960
QUATTROVENT		980	1940	2900	3910	4880

Minimum diameter of the copper connections	The minimum diameter of the pex-al-pex and propylene connections
1/2 ”	1/2 ”
1/2 ”	3/4 ”
3/4 ”	3/4 ”

Chart showing the relationship between the minimum diameter of the connection and the size of a trench radiator with natural convection.

Related content