Furnace or boiler?
New Jersey’s houses span more than a century of building, and the heating shows it. Newer homes mostly use a furnace, which warms air and pushes it through ducts. Many older homes still run on a boiler that heats water and sends it as hot water or steam to radiators and baseboards. Some have both: a boiler for heat and a separate air handler for central air.
What wears out
Each kind of system has parts that wear on their own schedule. On a furnace the usual suspects are the igniter, the flame sensor, the blower motor and the control board. On a boiler it is more often the circulator pump, the expansion tank or, on a steam system, the water level controls. Most of these are ordinary repairs. A cracked heat exchanger in a furnace is different. It is a safety problem, and on an older unit it usually means replacement.
Repair or replace
The honest answer depends on the age of the system, what the repair costs and what is likely to fail next. A repair on a 17-year-old furnace buys time, not a new furnace, and a repair warranty normally covers the part that was fixed rather than the rest of the system. When a system is close to the end of its life, it makes sense to price a replacement before the next breakdown, so the decision happens on your schedule and not on the coldest night of the year.
The costs and planning guides walk through the numbers. If you are weighing a switch away from gas, start with heat pumps.