[Getdp] Heat equation

[yoshin76@libero.it]

I'm an italian PhD student and I'm trying to solve the 2D heat equation:

                       du/dt-div(grad u)=f(u,t)         on a volume V

with the Neumann boundary conditions:
                      
                       (grad u) ° n=0         on  S (surface that contain V and n unit vector normal to S) 
 
If i search a steady state for this problem, I need to fix the integral constraint:

 
                        int_V(f(u,t)dxdy)=0                 (int_V integral over V)

otherwise the temperature u increase without reach an asymptotic value.
In which way can I fix this global constraint in GetDP? 
Should I use a GlobalTerm or a GlobaEquation?
Can I you give me an example?

Thanks in advance and best regards,

Fabio Vicini.
Roma, Italy.