The difference between an 80% furnace and a 90%+ furnace is that the 90%+ has a secondary heat exchanger to transfer more heat into the home thus making it more efficient. The 90%+ furnace is also known as a condensing furnace. This means it produces a small amount of water as a result of having much lower flue gas temperatures. This condensation is carried away through a drain line on the furnace. If the furnace is installed properly and regular maintenance is performed then the answer is 'False'. The answer would be true if you could still purchase a 50-year-old furnace today.