Deciding whether to fix a furnace or not can be complex. It involves several factors, mainly the furnace's age, how much the repairs will cost, and the system's overall condition. If your furnace is relatively young, say less than 10 years old, and the repairs it needs are minor, like replacing a limit switch that cost around $100, it just doesn't make sense financially to replace it. On the other hand, if your furnace is older, say around 15-20 years or more, and the repairs it needs are major, like replacing the heat exchanger, which can cost from $500-$1500 (parts and labor) and is often a sign the furnace is on its last legs, you should seriously consider replacement. It will save you money in the long haul, and the new furnace will almost surely be more energy-efficient than your old one.