It’s important to pay attention to subtle problems with your heating system, no matter how small they may be. Even the smallest problems should not be ignored, as a furnace cannot fix itself and can only worsen.
Of course, there are those problems you couldn’t ignore if you tried, too. When your furnace puts out cold air instead of warm, you want to do something—fast. So what’s the deal? Could you repair it on your own? Will you have to replace the furnace?