why does the water pressure drop a little bit once the water heats up?

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When I have a shower, I leave the hot water on to heat up and I can hear the drop in pressure when it’s done. I also notice this when using the kitchen sink and the water seems to become less oxygenated(?) Always wondered why, if the water heats up in the pipe shouldn’t it expand and the water pressure rise?

In:

Do you mean water flow? Usually rubber in taps expands when it heats so the flow decreases.

It shouldn’t have anything to do with the water per se, but with the supply chain, since technically, if you heat water, the pressure should only increase.

Cold Water comes directly from the pipes that supply your whole area, there the water pressure is maintained to be fairly high. Hot water, however, is usually produced inside your house, there will be a heater somewhere that heats up your water. Since that is local and takes time, the pressure used will not be that high.