How often should I backflush my espresso machine?
Backflush with plain water after every session and with detergent about once a week for daily use. Only machines with a three-way solenoid valve can be backflushed; on machines without one, clean the group and screen by hand instead.
Backflushing uses a blind basket, a basket with no holes, so that water pumped into the group has nowhere to go and is forced back through the shower screen and out through the three-way valve, carrying coffee oils and grounds with it. It only works if your machine has that three-way valve, which most pump machines with a solenoid do and most entry machines with a pressurized basket and no solenoid do not. Check your manual before you start, because backflushing a machine without the valve just pressurizes it pointlessly.
The water backflush is the daily habit. Insert the blind basket, run the pump for about five seconds, stop, wait a moment for the pressure to release through the valve, and repeat around five times. That takes under a minute and removes the loose grounds and oils that would otherwise sit on the screen and turn rancid.
The detergent backflush is the weekly one. Add a small measure of espresso machine detergent to the blind basket, roughly half a gram to a gram unless your product says otherwise, and run the same cycle. Then, and this is the part people rush, remove the blind basket, rinse it thoroughly, reinsert it and run several plain water cycles to flush every trace of detergent out. Follow that with a discard shot pulled through a normal basket. Detergent residue tastes vile and you will notice it.
Use the right chemical. Espresso backflush detergent is an alkaline cleaner designed to dissolve coffee oils. Descaler is an acid designed to dissolve mineral scale. They do different jobs, they are not interchangeable, and using descaler in a backflush can damage group seals and internals. Keep them clearly labelled and stored apart.
While you have the blind basket in hand, do the rest of the group. Every week or two, unscrew the shower screen, soak it and the dispersion block in detergent solution, and scrub the group gasket area with a group brush. A screen that looks clean from below is often caked on the upper face, and that is where a persistent stale bitterness in otherwise good shots usually comes from.