**In order to calculate pressure loss in a heating or cooling system it is essential to know the volume flow rate Q. The volume flow rate is based on heat demand, temperature difference, specific energy capacity, and density.**

When the heat flow Φ is known, the flow pipe temperature tF, and the return-pipe temperature tR should be determined, in order to be able to calculate the volume flow rate Q.

The temperatures not only determine the volume flow rate, but also the heating surfaces (radiators, calorifiers etc.).

The following formula should be used:

__Φ x 0.86__ = Q

(t_{F} -t_{R})

Q = Volume flow rate in [m^{3}/h]

Φ = Heat demand in [kW]

t_{F} = Dimensioning flow pipe temperature in [°C]

t_{R} = Dimensioning return-pipe temperature in [°C]

0.86 is the conversion factor (kcal/h to kW)