noun
filling the bathtub with hot water
Refers to the act of running hot water into a bathtub, often as a household chore or before bathing.
お湯張りが終わったら教えてください。
Please let me know when the bathtub is filled.
毎晩、子供たちのためにお湯張りをする。
Every evening I fill the bathtub for the children.
湯沸かし refers to boiling water in general or a water heater, while お湯張り specifically means filling a bathtub with hot water.
Compound of お湯 (oyu, 'hot water') + 張り (hari, the noun form of 張る, 'to fill'). The prefix お- adds politeness.