A circulator pump is used to circulate gases and liquids in a closed circuit, and are commonly found circulating water in hydronic heating or cooling systems. Because they only circulate liquid within a closed circuit, they only need to overcome the friction of a piping system rather than lifting a fluid from a point of lower potential energy to a point of higher potential energy.
Circulator pumps used in hydronic systems are usually electrically powered centrifugal pumps and are often small, sealed, and rated at a fraction of a horsepower.
Circulator pumps that are supported entirely by the pipe flanges that join them to the rest of the hydronic plumbing are small to medium sized.
Pumps that are only used for closed hydronic systems can be made with cast iron components as the water in the loop will either become de-oxygenated or be treated with chemicals to prevent corrosion. However pumps that have a steady stream of oxygenated, potable water flowing through them must be made of more expensive materials e.g. bronze.