Heating and cooling systems contain hundreds of parts, which means that almost anything can go wrong. Some warning signs are obvious. For example, if the air conditioner is blowing hot air or if the furnace won’t shut off, it’s time to call a professional. Other malfunctions can cause subtle decreases in comfort and efficiency. Regardless of the problem, responding quickly is generally the best way to avoid more significant repairs and expenses. Homeowners can protect themselves by recognizing the following warning signs: Unusually high energy bills Excessive mechanical noise Starting difficulties Sudden shutdowns Continuous airflow Temperature imbalances High humidity Unpleasant odors Ice buildup on the coils Leaks or water damage