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The water that comes out of the heads doesnt come from the boiler in a standard HX commercial machine. The only water that comes from the boiler is from the steam wand and the hot water tap.
The boiler will heat up quicker then the group heads so by running water through group head then the machine will be up to temp quicker. Every machine is different so the time to heat up will vary.
But most machines will warm up in 30 mins without the need to flush water through the heads. So when you walk through the door in the morning turn on the coffee machine first.