Which of the following is not considered a source of backpressure?

Backpressure occurs when the pressure within a system exceeds the pressure of the water supply that is being supplied to it. This situation can lead to undesirable backflow conditions, where water flows backward into the potable water supply.

Aspiration, which refers to the drawing in of air or another gas into the system, typically does not create pressure that would force water back into the supply plumbing. Instead, it often results in a vacuum effect that can lead to siphoning but without the characteristics of pressure that define backpressure.

In contrast, elevated piping, thermal expansion, and pumps all create scenarios where pressure can exceed the incoming water supply pressure. Elevated piping can generate additional gravitational pressure, thermal expansion can increase pressure due to heated water expanding within a closed system, and pumps actively create pressure to move water against its natural flow direction. Therefore, aspiration stands out as it does not contribute to the creation of backpressure in the same way the other scenarios do.