encyclopedically about plumbing - Wikipedia
Wikipedia tells us a little about what plumbing really is:
Plumbing is the system of pipes, drains, fittings, valves, and fixtures installed for the distribution of potable water for drinking, heating and washing, and waterborne waste removal. "Plumbing" also refers to a skilled trade which deals with installation and maintenance.
The plumbing industry is a basic and substantial part of every developed economy. The word derives from the Latin plumbum for lead, as the first effective pipes used in Roman era were lead pipes.
"Plumbing" often denotes the supply and waste system of an individual building, distinguishing it from water supply and sewage systems that serve a group of buildings.
Why we call plumber?
When it comes to repairing things around the house, men often feel compelled to do it themselves, rather than spent money on overpriced expert. More often than not, it is worth the effort, but what happens when you mess with crucial installations, that you know very little about?
Broken chair or squeaky hinges are one thing, but if you mess up with your drainage or plumbing, all hell can break loose. From time to time, it is really wise to call in the plumber - you may pay more than you would want to, but not necessary more than it is worth. Especially if you would do this yourself and mess up.
Besides, do you really have all the necessary things for all the repairs? I doubt that, let alone the tools, which you may or may not have. If you buy unnecessary spare part, what will you do with it? Plumber can always use it, but he probably wouldn't make the mistake in the first place.
To sum up, it is good to do things around your house by yourself, but don't be afraid to call in the specialist from time to time.
Broken boiler vs. plumber
There are repair jobs, that almost anyone can do themself, but broken boiler isn't one of them. Boiling hot water and immense pressure - those are the things that define boiler, and also the things you should be wary of.
Calling in professional plumber isn't any shame in this case, it is necessesity. It is different from a ticking bomb, but close enough, especially if the safety valves are at fault. All in all, you need expert on this - let him do his job, and relax. He is aware of the risks involved and already knows how to avoid any mistakes. All you need to do is coffee and maybe some snacks, while he do his job.
If you are not convinced by this, and ready to repair you boiler alone, all i have to add is this: search the internet for movies of exploding boilers, I dare you.