This was posted today on the SM List by a former nuclear reactor engineer now certified LM tech. It's information I've never heard explained quite this way before and I believe valuable.
Many folks have the wrong idea about what the backflush actually accomplishes.
Yes it does clean the shower screen, dispersion block and everything else that is inside of and above the portafilter sealing location against the group gasket.
And Yes this is important and if backflushing or a disassembly of screen etc. and cleaning is not accomplished fairly regularly the contamination of shots from the built up gunk and oils will provide a considerably less than pleasant addition of the shots taste.
The one thing that the backflush accomplishes, that has not been mentioned, is cleaning the passageways from the location at which the brew water is injected above the shower screen backward thru the three way valve and to the drain tray. This flow path can be from several inches to almost a foot depending upon what particular machine. If not backflushed this passageway will slowly fill with solidified oils and crud until flow is completely stopped. At this point things can become expensive.
Machines built prior to the advent of the three way valve had a disconcerting habit of spraying hot water/coffee all over the operator if the pressure was not allowed to relieve itself prior to portafilter removal after pulling a shot. Before the advent of the three way valve the norm for cleaning the group head was physical disassembly, brushes soap etc. and it worked perfectly well. IMHO - ALL machines with three way valves should be backflushed regularly.
An example: I recently did some work on a nice commercial single group Brazilia that has seen regular use in a local Bed & Breakfast for a number of years. No amount of 2000 psi air and / or acid flushes etc. would clean the drilled passageways in the group head. The passageways were drilled and welded closed at the casting boundary, so physically re-drilling the holes was not in the cards. The options included drilling the welds etc. etc. ----- the cheapest solution ended up being a three week wait for a new group casting. Reasonably regular detergent backflushing would have saved these folks about $450 all told (extra labor and price of parts), as well as not having to do without a machine for three weeks.
When pulling a shot from a group with a three way valve, you should hear a slight swoosh immediately after the pump stops. This is the three way valve venting the area above the coffee puck in the portafilter thru the valve and to the drain. If you don't hear it, it is time to get serious about doing some detergent backflushing until things are cleaned up.
Mike (just plain)
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). Of course this recommendation ignores the fact the consumer will brew tainted coffee and have potential problems with blockage in the second year -- when it will no longer be under warranty.
