Greenhouse heating: Flow requirements

[far2cool]

I have been tasked with designing a greenhouse heating system. I have at present calculated the required BTUs to maintain the desired internal temperature, but now am attempting to calculate the required fluid flow velocities to maintain this. I am using a pipe network with a working fluid of water. the inlet temperature is 60 degrees C. Could anyone suggest how i would go about establishing a fluid velocity in the pipes. i have tried simple calculations (Q = m.c.delta T) to establish an approximate mass flow rate, but this does not take into account such factors as pipe friction and heat transfer coefficients.