Which type of pump is NOT mentioned as a type used in heating systems?

In the context of heating systems, the diaphragm pump is typically not used as a primary type of pump. Heating systems commonly utilize centrifugal pumps due to their effectiveness in circulating hot water or steam throughout a system. These pumps are designed to move fluids by converting rotational kinetic energy to hydrodynamic energy, making them ideal for handling the higher flow rates associated with heating applications.

Vacuum pumps also have their specific uses, such as removing air and other gases from a system, but they do not play a role in the actual heating process. Duplex pumps, while not mentioned frequently, can refer to a type of setup where two pumps work together for increased reliability or flow. They are generally used in applications that require continuous operation rather than direct heating.

In contrast, diaphragm pumps are most commonly found in applications requiring the transfer of fluids with a higher viscosity or in situations where precise dosing is necessary. They operate by using a flexible diaphragm to create a vacuum that draws fluid in and then pushes it out, which is not typically aligned with the needs of a heating system.

Thus, the diaphragm pump's specific operational characteristics lead to its exclusion from the list of pumps traditionally associated with heating systems.