Let's consider design of model of boiler (design of software model).

When I want imagine system in whole, I start from drawing component diagram like this:


On this figure I consider boiler as 'black box', and focused on behavior of this device from external point of view. I try to answer on question 'What I expect from this device?'.

On the next step I should define components of the whole system and expected interaction between ones. This design stage leads to components point of view on boiler:

Now, descending in abstraction layer, consider every component separately.

Valve life cycle, is here:

Valve is opened when power on and closed otherwise.

Thermo elements may has step or continuous regulation of power:

Sensors start work whith power on and take temperature, pressure, water level, plus two kinds of alarms---low and high water.