Boiler - definition
A boiler is a closed vessel in which water or other fluid is heated. The fluid does not necessarily boil. (In North America the term "furnace" is normally used if the purpose is not actually to boil the fluid.) The heated or vaporized fluid exits the boiler for use in various processes or heating applications,12 including water heating, central heating, boiler-based power generation, cooking, and sanitation.
Źródło: https://en.wikipedia.org/wiki/Boiler
Plumber - from Wikipedia
The word "plumber" dates from the Roman Empire. The Latin for lead is plumbum. Roman roofs used lead in conduits and drain pipes and some were also covered with lead, lead was also used for piping and for making baths. In medieval times anyone who worked with lead was referred to as a plumber as can be seen from an extract of workmen fixing a roof in Westminster Palace and were referred to as plumbers "To Gilbert de Westminster, plumber, working about the roof of the pantry of the little hall, covering it with lead, and about various defects in the roof of the little hall". Thus a person with expertise in working with lead was first known as a Plumbarius which was later shortened to plumber.
A plumber is a tradesperson who specializes in installing and maintaining systems used for potable (drinking) water, sewage and drainage in plumbing systems. The term dates from ancient times, and is related to the Latin word for lead, "plumbum".
Źródło: https://en.wikipedia.org/wiki/Plumber
Why a professional plumber is well-paid
Professional plumber is still a well-paid profession, because the failures occurring in the water and sewage networks are still very frequent. More often there are failures washing machines or dishwashers, which can prevent only a plumber. At the same time among plumbers, there is a lot of competition, so they are trying at all costs to attract and retain potential customers through various promotions. They can also carry out work tiling and painting if required finish the bathroom, which they do. However, many plumbers, dissatisfied with their wages go abroad in search of better-paid work and perform activities there, they do not want to do local plumbers.