Circulating hot water heat is the most popular type of residential home heating. In this type of systems a circulator pump delivers hot water through heat distribution piping. The heat distribution pipes are installed through out the home. Some of these heating systems are loop type systems in which hot water circulated through the system in sequence. Another type of system is composed of two sets of piping. One set of pipe delivers hot water to the radiators and the other set of pipes brings back cool water back to the boiler. These types of systems usually have an expansion tank located at the top of the boiler. The purpose of the expansion tank is to take up pressure from the boiler. This pressurizes the water in the system so that the water does not turn to steam.