The water HTF massflowrate is given by:

  m = (Q x 3600) ( ro x c x DT)

Avec :

m = water massflowrate in l/h

Q = Thermal power to  transfer in W

ro = Density    kg/m3

c = water heat capacity  kJ/kg K

DT = Température in K. (T° in - T° out in K)

Thank you very much for your help Prof. Mohammedi

But, if I'm not mistaken, there was supposed to be a division sign in between..

That is : 

m = (Q x 3600) / ( ro x c x DT)

Can you please confirm.

Thank you very much in advance.

Flow rate can be determined by an energy balance: heat loss of water = heat convected to air in heated rooms.

The velocity is limited by the possible noise generation due to turbulences ( if it is a space where human beings work or live). since water has a high convection coefficient low velocity is not a problem.