Cooling of the majority of hit-and-miss engines is by hopper cooling, with water in an open reservoir. There were a small portion of small and fractional horsepower engines that were air-cooled with the aid of an incorporated fan. The water-cooled engine has a built in reservoir (larger engines usually don't have a reservoir and require connection to a large external tank for cooling water via pipe connections on the cylinder). The water reservoir includes the area around the cylinder as well as the cylinder head (most cases) and a tank mounted or cast above the cylinder. When the engine runs it heats the water. Cooling is accomplished by the water steaming off and removing heat from the engine. When an engine runs under load for a period of time, it is common for the water in the reservoir to boil. Replacement of lost water is needed from time to time. A danger of the water-cooled design is freezing in cold weather. Many engines were ruined when a forgetful operator neglecting to drain the water when the engine was not in use, and the water froze and broke the cast iron engine pieces. However, New Holland patented a v-shaped reservoir, so that expanding ice pushed up and into a larger space rather than break the reservoir. Water jacket repairs are common on many of the engines that still exist.