Flushing the Group
Espresso machine group heads are prone to an accumulation of coffee grinds covering the dispersion screen and the rubber gasket. You can significantly reduce this buildup of spent grinds by rinsing some hot water through the shower screen each time you remove the portafilter to prepare a new shot.
Volumetric machines can be programmed to flush the correct amount of water through the group. This speeds up the workflow because a barista can perform a group head flush with a single press of a button; the valve will automatically shut off the flow of water. This helps to reduce wastage, too. The heating of water is very energy intensive, so if you flush excessive amounts of water each day you will feel it in your next energy bill. If your machine doesn’t have volumetric control, keep the flush time to less than 2 seconds.
A gif showing a <2-second flush performed immediately after removing the portafilter
It is tempting to flush a great deal of water to make the group cleaner. But espresso machines are not designed to dispense large volumes of water at a time. They are designed to produce espresso-sized beverages. About 50 ml of water is enough to rinse residue from the shower screens. A more thorough scrub of the group heads at intervals through a shift will alleviate any worries about old coffee building up on the shower screens.
Timing the Flush
Espresso machines usually drip water for a few seconds after a barista turns off the valve that controls the flow of water out of the group head. For this reason, it is advisable to flush the group head immediately after removing the portafilter. If you leave this operation until after tamping, it is likely you will drip unwanted water into your freshly tamped coffee. Alternatively, you are forced to stand there waiting for the dripping to finish.