A water heater or boiler installed in a typical commercial application will vary output between 0% and 100% depending upon the capacity of the equipment vs the heating load but most of the operation will normally occur at an output somewhere near mid-range. To accommodate a 50% load, the equipment must vary its firing rate (cycle) to 50%. Each time the equipment is cycled OFF, it will loose heat as it cools down. This "soak heat" must be replaced when firing starts again before the equipment can regain its test efficiency. Additionally, there are other significant losses associated with cycling (purging, etc.) that are ignored in a 100% firing efficiency test. Depending on firing rate and spacing of the OFF/ON cycles, it is possible and not unusual for equipment to be making up lost "soak heat" and cycling losses nearly 100% of the time and rarely, if ever, operate at its tested efficiency.