Furnace and Boiler issues that can cause serious problems
The picture below is a boiler in total disrepair showing flame discolouration by the burners and condensate leaking at the venting among other things
The photographs above are examples of rust and flame damage to the heat exchangers on furnaces and boilers. These appliances create a very unsafe environment for the occupants of the homes where they are located. CO poisoning is the first thing that comes to mind.
These condition were revealed upon the customer calling for maintenance and tuneup. Even then, some techs will miss these conditions because they do the scaled down version of the tuneup; just vacuuming and wiping the appliance for 15 minutes then leaving. Of course they do it because of some homeowners looking for the cheapest service available and also they can get away with it.
Regular and thorough tuneup will reveal these unsafe and hazarduous conditions. You do not have to endanger the lives of yourself and your family.