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Most of this is covered in another thread, but this is a nice printable summary with a couple of tips to make it simpler.

When descaling the HX boiler it's important to descale above the water line. Scale builds up on the water level probe, gradually changing the boiler water level. Unless the boiler is overfilled, scale could remain on the sensor. When the sensor is completely clean, refill pump action is deliberate. When the water level sensor is scaled, refill pump action can be spasmodic.

Simple Boiler Overfill Method :
After the boiler is filled with descaler and up to temperature, raise the water level sensor side of the machine with a couple of espresso cups under the feet. This should cause the pump to start and overfill the boiler. Set the machine level to raise the water level above the water line.

OTT Engineer Solution :
On the heavy Vibiemme, the steam exit is on the opposite side of the water level sensor, so I opted to add a Miniature DPST NC Momentary switch in the Auto Fill Sense Line. Wire the double poles in parallel for redundancy. [I added mine through one of the ventilation slots in the base of the machine. I had to file an extra flat for it to fit. I added a finger guard and positioned it far enough inboard to preclude accidental operation. After installation I snotted it over with RTV to ensure water rejection. One could use a sealed switch instead, but I had this one in an old sample kit.]

Let's call it the Manual OverFill Switch or MOS for short.

Mix up solution of citric acid in a separate container. Mix 1 liter / Quart for HX only descale or treble that for typical home HX and Boiler. Concentration should be 1.5 to 2 Tablespoons per Qt / 25-30ml per L

• To Descale HX Only
  - Heat the machine to normal operating temperature
  - Open Hot Water tap until pump starts and let boiler refill
  - Open the steam wand valve slightly and tilt the machine or press the MOS for 30 to 60 seconds to overfill the boiler. The trick is to keep enough pressure in the boiler to keep the vacuum break valve closed. The overfilled boiler won't ingest any cleaning solution during the HX descale
  - Close steam wand valve and lower the machine or release the MOS.
  - Place fill tube in descaling solution and return in a separate container.
  - Draw 4oz / 125ml through the group.
  - Wait 10-20 minutes depending on how long since last descaled
  - Repeat 6 times.
  - By the end, the water should be almost clear.
  - Place the fill tube in fresh water and flush and repeat for 1 L / Qt through the group
  - Open Hot Water tap until excess water is removed from boiler.

• To Descale HX and Boiler
  - Heat the machine to normal operating temperature.
  - Turn OFF Machine
  - Place two espresso cups under the machine feet on the OPPOSITE side to the Hot Water exit.
  - Open the Hot Water tap and drain as much water as possible.
  - Remove espresso cups
  - Place fill tube in descaling solution and return in a separate container
  - Refill boiler and let machine reach operating temperature
  =============================
  - Without an MOS, raise the sensor side of the machine.
  - Open the steam valve and let the pump run until it stops or water runs out the steam wand
  - Close steam valve and level the machine
  OR
  - Open the steam wand valve slightly and press the MOS until water comes out of the steam wand
  - Close steam wand valve and release MOS
  =============================
  - Draw 4oz / 125ml through the group.
  - Wait 10-20 minutes depending on how long since last descaled
  - Repeat group flush 6 times. By the end, the water should be almost clear
  - Place the fill tube in fresh water and flush and repeat for 1 L / Qt through the group

• Boiler Flush
  - Turn OFF Machine
  - Place two espresso cups under the machine feet on the OPPOSITE side to the Hot Water exit
  - Open the Hot Water tap and drain as much water as possible
  - Remove espresso cups
  - Place fill tube in clean water and return in a separate container. Use bottled distilled RO water for the refill to minimize boiler scale build up. There seems to be enough citric acid left to ensure operation of the auto fill. Of course, if you use boiler water for other than steam generation, use your favorite water
  - Refill boiler and let machine reach operating temperature
  =============================
  - Without an MOS, raise the sensor side of the machine
  - Open the steam valve, let the pump run until it stops or water runs out the steam wand
  - Close steam valve and level the machine
  OR
  - Open the steam wand valve slightly and press the MOS until water comes out of the steam wand
  - Close steam wand valve and release MOS
  =============================
  - Repeat twice from Boiler Flush or until the water is tasteless
  - Open Hot Water tap until excess water is removed from boiler

2021 Update:
Now that I have a collection of used pumps, I remove the vac break, pump the boiler empty, fill w slightly more descaler than water pumped out, replace vac break & heat and rather than using the machine pump. Take care not to overfill the boiler. A bamboo chopstick makes a good dipstick. A hand pump could work as well to empty and refill. Two complete boiler refill flushes after descale is complete. Replace the vac break
or at least the copper crush washer when complete.

from "The Insanely Long Water FAQ"
Once the boiler is filled with descaler, leave it in for two and half hours with the machine off, or one hour with the machine on.

The method works well for a typical home use HX descaled every 3-4 months.
By the 3rd or 4th iteration the solution water is almost clear.

Four hours is a long time to leave metal in hot acid, however mild.

Compass Coffee wrote:Just curious, base on?

Pique