The mathematicians have a good way of doing it: declare it trivial.
In your heater example, let's say that you demonstrated some results in a liquid. Then you start your section with:
In this section, we present results obtained from heating a liquid sample with a Widget2000. These results can be trivially extended to samples in a gas or solid phase. Since the liquid case is of particular interest for this thesis, other types of sample are not described in detail.
As long as your readers know very well that your statement is correct and intuitively agree that the other cases are trivial, without having to think about why they are equivalent, this is a good way to do it. If you have some doubt - e.g. because somebody has to have a certain level of background in your field to know that it is indeed trivial to demonstrate the equivalency - it is also advisable to add a citation of a source which discusses the equivalency.