Because of the configuration of my kitchen, I can't plumb the machine in. I use instead a pump to automatically pump up water from a 2 gal container located under the counter cabinet to feed the machine located above. This part works well.
Eventually it will comes to descale the dual boiler. My first thought is to make a descaling solution in the 2 gal container and pump it to both boilers. Draining the brew boiler seems obvious either through the hot water wand or the brew head but for the steam boiler, how can it be done ? Can anyone has some thought about it.

Draining through the hot water wand leaves behind a lot of the citric acid solution, as would draining the brew boiler by just running water through the brew head. A better alternative, developed by Dan, is to turn off the machine and let it depressurize, remove a fixture from the top of each boiler, then use a siphon tube to empty each boiler almost completely. You can siphon in some clean water and siphon it back out to get out the final scraps of citric acid. This is much faster than flushing and refilling rinse water several times using the pump.

For siphoning, I found it really helpful to gently screw in a plastic funnel into the threaded hole after removing (in my case the anti-vacuum valve) a fitting on top of the boiler. The threads bite into the plastic and makes a water tight seal so when pouring in liquids (acid solution, rinse water etc), you can overfill the boiler easily and without spilling all over. Holds itself in place.

For emptying, a plastic tube still slides down through the funnel to the bottom of the boiler.