Preheating patio heaters: Is it a waste of energy? Since patio heating is achieved by radiant heat it is thought that you should only run them when you are there to enjoy the heat. However, there are two reasons that  fifteen minutes of preheating can increase your comfort. The first reason is that it can take several minutes for some patio heaters to reach their peak operating temperature. The second reason is that patio heaters preheat the furniture, which then warms you. To save energy turn off patio heaters when you leave.