The answer to 'should I repair or replace my 15 year old furnace' is that it depends on the condition of your furnace and your specific needs. In general, if your furnace is more than 15 years old and has required multiple repairs, it may be more cost-effective to replace it with a newer, more energy-efficient model. Newer furnaces can save you money on your energy bills and provide more reliable heating for your home.
However, if your furnace is still in good working condition and has only needed minor repairs, it may be worth considering a repair. Furnace repairs can be less expensive than a full replacement, and can extend the life of your existing system. Before making a decision, it's a good idea to have a professional HVAC technician inspect your furnace and provide a recommendation based on its current condition and your specific needs.