i want to heat water between 20c -40c
do you mean 20-to-40 degrees Celsius or 20 to -40 Celsius?Same tech will do it but various components may need to be switched if its below freezing.What you need is a feedback loop that checks the temperature and changes the output of the (presumably heater) to suit the temperature requirements. There are a million ways to do it, the simplest being a thermostat connected to a heater. It can be active, passive, use analog components or go digital with a microcontroller.
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Can't "heat" water to -40C, there is a slight problem.....
solid water can exist below -40C, where it can then be heated to -40 and above.English has a way of adding extra words (like ice)...but water is still water when its ice or steam.