A reversing valve will only be found on heat pump systems and not straight cool. The reversing valve will "reverse" the direction the refrigerant is flowing in the system. Instead of cooling a building, it will provide heat. The reverse is accomplished by the use of electromagnetism. Signs of failure may be intermittent. Reversing valves may become stuck due to sludge and debris. When this occurs the system will typically not cycle from heating to cooling or vise versa. The problem can often temporarily be remedied by tapping the valve with a hammer which knocks the debris free. This may be however only a temporary solution with more severe problems to come. When a reversing valve fails, it can make for a expensive air conditioning repair. Replacement requires a skilled technician and is very labor intensive. In addition to the costs of the part and labor, there will also be a considerable amount of refrigerant used. The good news is that reversing valves do not fail often and typically do not need to be replaced when they do. That is because the failure is usually caused by a faulty electrical coil. Melco HVAC Services of Orlando will be able to accurately diagnose your system ensuring you aren't paying for repairs you don't need.