modify equation system during the simulation

Hello,

I started using COMSOL 4.3a only recently and I'm experiencing some troubles with my model.
My simulation is somehow analogous to the boiling water process, so I'd like to ask how you would simulate this very simple process.

Let's assume to have a mass of water m [kg] in a pot, heated by a flux Q [W], starting from a temperature T=20 [°C]. Rho is water density [kg/m^3], cp is water heat capacity [J/kg/K] and DHev is water enthalpy of vaporization [J/kg].

The system I would solve would be:

dm/dt=0
V*rho*cp dT/dt = Q

if T<=100°C.

I would then change my system to the following, when T>100°C:

dm/dt = - Q/DHev
dT/dt = 0

How can I solve such a problem, where I basically have to modify the equation system as a consequence of the temperature condition?

Thank you,
Alessia