A hydronic heating system, typically referred to as a hot water or steam system, is one that uses water or steam as a medium of transporting heat from the heating unit. The heating unit itself is called a boiler because it heats or boils water in a vessel made out of cast iron or steel. The heated water is then distributed through the desired areas of the dwelling to create an even temperature throughout. The desired temperature is controlled by the thermostat. The boiler goes on and off automatically to keep the water temperature within it to a range which enables the maximum benefit of the system. Almost all hydronic systems in operation today use either gas or oil for fuel. Most systems have significant controls for safety and efficiency. It is our job as professional inspectors to operate the system using normal controls to expose defects or maintenance requirements. It is possible though not typical to find combination steam and hot water systems. These installations are usually seen in expanded older systems and can be prone to maintenance problems because of the two types of operation from the same vessel.