We're always talking about furnace repair but there is a whole other world out there of folks who depend on hot water or steam from a boiler to keep their homes warm with radiant heat versus forced air from a furnace.
A boiler is a very different animal than a furnace. Although boilers provide some of the most comfortable heat using "radiant heat" they have several uncommon parts like a tridicator (see photo), an expansion tank, an aquastat and the list goes on. Actually, a water filled expansion tank is a pretty common problem I tell you how to fix in the tutorial How to Re-charge a Hot Water Boiler Expansion Tank.
Although some boilers use ignition systems similar to a forced air gas furnace, boiler systems are more complex and have their own issues, like noisy pipes, inconsistent heat, leaks and other fun stuff to deal with in the winter.
But I've got it all explained for you in the tutorial Troubleshooting a Hot Water Boiler complete with diagrams, and troubleshooting tips.