I just descaled my 6910 by using citric acid powder dissolved in warm water and followed the instructions in the manual. Citric acid powder is easily bought from health food shops and is not very expensive. I think I added 3 tablespoons to 1.5 litres of warm water and stirred. I pressed the Manual button twice to run through 500ml, left for 6-7 minutes, repeated the process and ran 250 ml to finish off. I then flushed with 2 litres of water.
I have used this solution to descale my trusty old Sunbeam EM2300, a Breville 800ES and now the 6910.
Can I descale the steamer with citric acid? The instruction book says I cannot descale the steamer at all but surely it must be done somehow.