Model Predictive Control with on/off heater

Hi all. I'm working on Model Predictive Control of a Heating System with an on-off heater. When it comes to the input constraints I have difficulties at that point. Since my heater(manipulated variable) is on-off type, how can I specify minimum and maximum values for heat input at constraints part. I know my maximum and minimum(which is zero)heat inputs but when I specify this maximum and minimum, my heater works in the range of min. and max. values. But as I mentioned this heater is on-off type,its heat input must be maximum or minimum,not in this range. How can I solve this problem. It will be great pleasure for me if you respond my interest. Thanks all.

Tarik Yucel