Step Response of a DAE/ODE system

Hi Guys!

I need to model the dynamics of a thermal energy system. My system is described by a DAE system (differential algebraic). However I need to simulate a 'step up/down' event. ie one of my parameters in the model (not any of the ones I want to integrate) is to be increased at say t=100s.

So any ideas how to program the above? I know that it has been done in the past (many studies with such a method) just explain me how to connect my ode15 solver with a step input.

Thank you in advance,

Warm Regards,

Chris