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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.

I would suggest that you not flush your boiler twice with bottled water, but flush it through 2 reservoirs full of water to be safe. I run two 3 liter reservoirs of water through my pour over machines boiler (1.5 liter) after a descale. One gets all the green water out, the second is for good measure. I also run a half to full reservoir through the heat exchanger.

When using a vibe pump, make sure you work in sessions. Continually running a vibratory pump will burn it out. Their specifications are not 100% duty cycle. For every second of use allow one to two seconds of rest and do not exceed 50-60 seconds of continuous running.

When dealing with acids, better safe than burning a hole in your tongue and dying a slow death from taste testing to see if you got all the acid out, IMHO.

Nice synopsis of the process.

I'll be sure to do this once I get my new machine. Great writeup, thanks!

cafeIKE wrote:

• 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
  - 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

Jim's directions suggest soaking the boiler with descaling solution for several hours. If I understand correctly following your simplified directions with boiler filled with descaler, then running descaler through HX and group with 10-20 minute waits repeating six times, that seems to infer only one to two hours boiler descaler soak. I like to target minimum 4 hour boiler descaler soak regardless time since last descaling.

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.

Excellent write up. I needed an HX descale "for dummies" thank you!

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

Just curious, base on?

Compass Coffee wrote:Just curious, base on?

Pique

Mine was a serious question not meant as a provocation. Is there a basis to believe a long multi-hour soak of a prosumer espresso machine's internals with a mild citric acid solution will be damaging? There may well be, I'm no chemist!

In fact, just checked Urnex site for Dezcal directions which state "Professional Use: Add 1 oz (28 g) of powder for each 16 oz (500 ml) of hot water. Stir to dissolve. Hold solution in vessel for up to 30 minutes. Rinse all parts thoroughly." This seems to support your shorter duration suggestion rather than Jim's several hours.

It really depends on the solution you use. I use a commercial descale solution that is very strong. I descale a commercial boiler that was many years old in 2 hours and it was clean as a whistle. That same solution will dissolve the edges off of brass bolts and hex heads if you leave it in solution too long, it is very strong stuff.

So the soak time needed will depend on the solution used to descale the machine, the severity of the scale and the temperature of the water. The hotter the water, the more aggressive it works.