I understand the point of a quick flush before clicking the portafilter into the group on an HE machine. I'm wondering whether it's necessary on a DB with PID? How many of you flush the group for 3 seconds or so if you have a DB macine. If you do, what advantage is there?

On an HX, it's a cooling flush to get the incoming water down to brewing temperature and, on some machines (usually dragons) that have been on for a long time, to cool the group a bit. On a DB, I do a flush, but merely to ensure that the water path and group is warmed up to the brewing temperature; there's no need to flush to get the brewing water down to the right temperature, as the brew boiler is controlled and compensated to send water of the proper temperature to the group. The flush on a DB only takes a second or two, while the flush on an HX, especially dragons, can last for even 20 seconds. Newer HX machines, like the Mara, have design changes that reduce the flushing time.

I flush for a few seconds with the portafilter and puck screen in place. It brings them all up to temp and wets the puck screen evenly.

No flush will bring the portafilter up to temperature.

If it's not bottomless, it's going to cool the shot to little above room temperature.

A couple of seconds on the DB clears the cooler water from an e61 thermosyphon.

Our machine is on a Wemo programmable timer but if I'm in a bit of a hurry to pull a shot before the hour warm-up is up I'll do a quick brew boiler pre-flush, otherwise not.

Never found running water through an e61 group sped up R E A D Y

If I'm in a hurry, tea towels over the group can get 'er done in 15-20 minutes