Very often I hear "I need a plumber, my heating system isn't working properly" or "Why did my plumber not know how to install this boiler correctly". Well the answer is simple, if your heart hurt would you say "I need a general physician" or "I need a cardiologist"? Yes, both have the license to work on your body and treat you but only one has the specific expertise to work on your heart. Consider your home a body and everyone who works in it needs to specialize in the specific part of your home. It may be complicated but doing it right once will save you the trouble of fixing it over and over again.
It is true that you need to be a licensed plumber to work on gas lines which means you need to be a licensed plumber to work on a boiler. However, you need to be a heating contractor to know your heating systems. There is a term, jack of all trades master of none. This is true for everyone, plumbers included. If you need your Air Conditioner worked on then you should contact an air conditioning company, if you need you plumbing done, ie toilets, sinks, waste lines, then contact a plumber. If you need your heating system done, contact a heating professional.
a great place to get heating information and help is www.heatinghelp.com