1. Post more info about your system type, etc - A reasonably sized house on a vented cylinder with poorly insulated pipes could easily contain 7 litres of cold water in the pipes which has to be drawn off before you get hot water.
7ltrs isn't that much - it'll just be what is sitting in the pipes, obviously it will cool the longer it sits there without the taps being opened. This cold water will have to come through before the hot from the tank/boiler gets to the taps.
Turn off the hot water supply, then open a tap. I bet you get more or less 7ltrs of water just from the pipework.
Hospitals and schools hot water supplies are in a continuous pumped loop, exactly for that reason, the difference being the size of the building, maybe having to run 70 ltrs before getting any hot water - would cause problems.
Reduce pipe run lengths if possible and insulate pipework.
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