Not too many replies here.
I found a solution on my own and since it may be someone else who have the same problem, I want share it with you. Heat control is for me the most important factor in a home automation system (I´m living i Norway which is a cold place) and in my opinion SmartThings should really have had a better out-of-the-box solution for this.
I solved the problem by adding a virtual device in SmartThings IDE. This can be done my clicking “New device” under “My devices”. It is important to choose “Simulated temperatur sensor” under “Type”. This device should pop up in your mobile app and you need to add it to webcore before is available online in a piston. In my piston, instead of setting a fixed temperature (which is 21,5 C in the example above), I choose temperature from this virtual device. This gives me the opportunity to change the temperature for this virtual device in the app and the piston (which is my heat control) will automatically follow the temperature in the virtual device. For me, this makes heat management considerably easier as you can change the temperature without fussing with pistons.
Hope this can be helpful to someone else in same situation.