What is Back Flushing? How to do it? Why? [Gaggia Class Pro Maintenance]


Hi everyone!

In this previous post, I explained how to use the Gaggia Classic Pro.

Once you know how to use a machine, you need to learn how to take care of it.

The Gaggia Classic Pro, like other properly made espresso machines, can serve you for more than 10 years if you take care of it.

Back flushing is one of the maintenance routines you should perform most often.

It’s not that difficult, let me show you how.

What is back flushing? Why do you do it?

Back flushing can only be preformed on machines like the Gaggia Classic Pro and Rancilio Silvia, which have a 3 way solenoid valve.

Let me quickly explain how these machines work, in order to get a better understanding of what we are doing.

A very simplified diagram an espresso machine’s group head and boiler is like this.

The boiler is the chamber that heats the water. When you start pulling your espresso, the pump pushes the hot water out of your boiler into the group head and enters the basket. The water is blocked by the coffee grounds in the basket and pressure builds up in the basket.

The solenoid valve controls the flow of the water in your machine. When pulling your shot, it opens a path between the boiler and the group head and lets water flow.

When you finish pulling the shot and switch off the pump, it also changes the position of the solenoid valve. In this state, the valve cuts off the group head from the boiler and instead connects it to the drain pipe.

The basket is highly pressurized when the pump is turned off, so the coffee in the basket wants to leave. The solenoid valve releases the pressure and coffee stuck in the basket by providing a path to the drain pipe. This is the liquid you see out of the drain pipe when you turn off the brew switch.

Another key function of the solenoid valve is to block the coffee from flowing back into the boiler.

As you can see from the diagram, coffee flow back through the group head and out the drain pipe. This means that you will get coffee residue in this path. You obviously don’t want old rancid coffee in your group head.

Back flushing is a way to clean out this path.

The idea is very simple. You take a blind basket, which is a basket without any holes, and put some detergent in it. You lock this into the group head and turn on the pump as you would to pull a shot.

Hot water will run into the basket, dissolving the detergent. It has no where to leave so the pressure will build up.

Once you turn off the brew switch, the pressure will drive the detergent water (indicated as pink) back through the group head and out the drain pipe, thus cleaing the path on its way out.

Let’s try it out

Things you need

There are a couple of things you need.

First, you need some detergent. You should pick up some coffee detergent which is specifically made for this purpose. A popular option is Cafiza. You can’t go wrong with this one.

The next thing you need is a blind basket. As mentioned it’s a basket without holes. It might also be called a back flush disk.

I actually got in a bit of a trap here. I made sure that I bought a 58mm blind basket so it does fit my Gaggia Classic Pro. But when I actually put it in the portafilter and locked it in, it didn’t lock in properly. The handle should be pointing strait out, but it did not turn further than this.

It turns out that the rim of the blind basket was much thicker than my normal baskets. The left one is the blind basket, the right is the normal one. This extra thickness made the portafilter not lock in.

I bought this basket from a generic no brand shop. It was send over from China. The product page did not have a specification on the thickness of the rim. To be honest, I have never actually seen any baskets with this spec, so I just assumed it was standardized. It you have easy access, it might be safe to buy a Gaggia branded blind basket.

Well anyway what did I do?

I actually bought a bottomless portafilter. And it turned out that this one barely fit and I was actually able to lock it in.

I probably has to do with this slight difference in the tab shapes.


Maybe take a look at your group head before cleaning. I usually do some purge cycles after I pull a shot, but its pretty dirty with coffee grounds.

OK, let’s finally start back flushing.

Put the blind basket in your portafilter. Put about 3g of Cafiza in it. (You don’t have to measure it exactly. This is just so you get a feel of 3g)

Lock this in to the group head.

Turn on the brew switch. You should do this once the boiler is heated up. This should allow the Cafiza to dissolve easily.

The pump will turn on and after a few seconds you should hear the sound change. This indicates that the basket is fully pressurized. Wait about 10 secs after the change in the sound, then turn the brew switch off. You should see some foamy water flow out of the drain pipe.

Repeat this about 5 times. ON -> wait for the sound to change -> wait 10secs -> OFF.

After 5 cycles take the portafilter out, and dump the contents.

Rinse the basket and lock it back in. Still with the blind basket.

Repeat the cycle about 5 to 10 times. What you are trying to do here is to rinse out any remaining detergent in the pipes.

After a few cycles, the water should not have anymore foam.

Finally, replace the blind basket with a normal one and run some water through, to give it a final rinse.

If you are in a cafe, they do this every day (or even multiple times a day). Depending on how often you brew with your machine, adjust how often you want to back flush. I usually pull 1 or 2 shots a day, so I back flush once a week.

After back flushing, the group head is looking much cleaner!

Bonus tip : using Cafiza

Cafiza it a very good detergent especially for removing any coffee residue. You can use it to clean all sorts of parts.

Just get about 500ml of warm water and dissolve 3g of Cafiza in it. Throw in your baskets or other things you want to clean and leave it for a while.