Do your homework. I have a natural gas boiler circa 1974, when I was in Afghanistan we had a carbon monoxide problem in Our basement, and the boiler wouldn't stay lit. Wife called repairman out, he looked at our boiler and said it was to old and he couldn't work on it, it was to bad of shape to fix, couldn't get parts etc. but happily quoted us a new one at $25k. He also scared my wife to death over it and just about had her talked into buying a new one. We called another company and they came out and cleaned everything, took it apart and cleaned it. He then relit the pilot and took a bunch of readings and everything was fine. The problem was the exhaust gases could escape as they were supposed to, after cleaning everything worked fine. In the end, the second company was surprised at how good of condition our boiler was in and said we shouldn't need a boiler for many many years.
I tell you this because in today's society it's easier to remove and replace components and systems instead of good old fashion trouble shooting. It also generates more money scaring people into new equipment that's not built as well as the older stuff and generates more service calls for the new stuff later down the road. They try to sell you on efficiency ratings and safety features that replaced common sense. That "more efficient" piece of equipment at $XX,xxx could take decades to pay for itself in the immediate small money savings from being more efficient. Think about it, even if it saves you $300 a year, but cost $10,000 it would take over 30 years to break even, then you are back at square one again, old equipment and service techs telling you that a new system is needed.