The answer to 'should I repair or replace my 15 year old furnace' is that it depends on several factors, including the condition of your current furnace, your energy efficiency goals, and your budget. In general, if your furnace is over 15 years old and has been requiring more frequent repairs, it may be more cost-effective to replace it with a newer, more energy-efficient model.
Newer furnaces are typically more reliable, energy-efficient, and have better safety features than older models. They can also save you money on your energy bills in the long run. However, the upfront cost of a new furnace can be significant, so it's important to weigh the pros and cons carefully.