Chris Nachtrieb has communicated that my descaling instructions on the FAQ are woefully incomplete. Judging by all the emails I get, I think he's right.
The key is the autofill sensor on the boiler:
-- when descaling the HX, you absolutely do not want the boiler refill to run
-- when descaling the boiler, you want the refill to run beyond the autofill setting and to flood the boiler to near the top (I actually like to be able to run the descaling solution out of my steam wand although this is not strictly necessary (both Jack Denver and I have found it reduces problems on the pressure stat diaphragm and the valves if one descales them occasionally).
I'm going to post instructions here and people can copy, report and use them as they please. It'll also give me a crib sheet when I do the FAQ rewrite.
The autofill is a "normally closed" switch. This means when the sensor is grounded, normally by making contact with the water in the boiler, and does pass current, it does NOT refill. If it is not grounded, it DOES refill.
-- To prevent it from overfilling, one can remove the wire from the top of the wire and ground it on the boiler case (or use an insulated wire with alligator clips on the ends to ground it). Do this when descaling the HX circuit
-- To force it to fill, once can simply pull the wire off, and let it hang in the air. Do this when overfilling the boiler. Use the machine's on/off for control. When overfilling the boiler, make sure the machine has reached operating temperature on the regular boiler level, since over filling it when the vacuum breaker is open will be very messy!
-- If you plan to descale regularly, attach a 3 position "descaling switch" to the autofill circuit. Cut the wire running from from the brain box to the autofill wand. Attach the wire from the logic box to the middle terminal. Attach the wire to the autofill wand to one of the outer terminals. Attach a wire to ground (there's ground terminals on all machines, or stick it on the piping, if you don't trust your house wiring) to the other outer switch terminal. When you switch this to ground, the autofill never works, when you switch it to the wand, it works normally, in the middle position it just keeps filling. This switch can (and should) be left inside the machine, since one doesn't want to operate it inadvertently. Any small Radio Shack low voltage switch (1 amp is plenty) will work, just make sure it's designed to operate up to 250F.
-- If you don't understand or don't want to understand these instructions, my advice is to learn how the machine works before doing this, or not to attempt descaling, and use softened water instead.
The descaling solution of choice is 1.5 to 2 table spoons of citric acid powder per liter/quart of water. It is at proper concentration if you see a light blue/green tinge in the flushed water when descaling. The reason this is the choice is that some citric acid will remain in the boiler when you descale that, unless you can drain it from the very bottom. Citric acid is odorless and, in low concentrations, tasteless, so it ill not mess up your steaming to have a little left. The alternatives are not as good: Vinegar smells, and industrial solutions (as well as CLR) are based on hydrochloric acid and are not food safe -- they would require a lot of extra rinsing flushes to remove.
Descale the HX like you descale a home machine:
-- ground the autofill wand (see above)
-- fill the tank with the descale solution.
-- flush the group with four ounces worth, wait 10 minutes, and repeat.
-- repeat this 4 to 6 times.
-- refill the tank with water
-- restore the autofill
-- flush the group till the taste is gone (start with 1 quart before tasting). If anyone has one with a big honking HX like the Cimbali's, please jump in with revised quantities.
Descaling the boiler is more of a pita:
-- The machine should be at operating temperature.
-- turn the machine off, and run as much water out of the hot water tap as you can. Tilting the machine towards the side the pipe attached to the boiler helps.
-- Fill your tank with descaling solution.
-- Turn the machine on, and let the autofill fill the boiler to the normal level with descaling solution.
-- Wait for the machine to reach operating temperature.
-- Disconnect the autofill (see above) and let the boiler overfill. Open the steam valve slightly as this is happening. When the solution starts squirting out of the steam arm (into the bowl placed there), the fill is done. Turn the machine off, and restore the autofill to its normal postion.
-- Let the machine descale hot for several hours. If doing the HX, you can do that while this is happening.
-- To rinse the boiler afterwards, repeat this entire process, including the overfilling and running water through the steam arm once.
-- Repeat the emptying of the boiler through the water tap, and ordinary refill (no extra water) until the drained water is tasteless (for me this happens on the second round -- your results may be different).
I hope this is complete enough to answer all questions.