Cooling Flush for HX Machines

The Cooling Flush on Heat Exchanger (HX) Machines

Central to improving shot quality in an HX machine is the "Cooling Flush and yes you need to do one each time you prepare a coffee (mabye only a small one if it's the 2nd or 3rd coffee in a row).

Lots of mystique around this one, or so it seems for the beginner. First a quick explanation of what it is.

The HX machine flash heats brew water by passing cold water from the pump, through a copper tube (heat exchanger) which passes through a boiler filled with very hot water (127C). As the water passes through this tube, the surrounding water and steam flash heats this water to the correct brew temperature.

The problem: Well the HX has a volume of around 30-50cc and the water sitting in this becomes overheated and eventually reaches boiler temperature (127C) which is a lot hotter than the 90-96C required for decent espresso. So we "flush" it out, the "cooling flush".

How: Well it's quite simple, remove the portafilter, bend down and have a look (don't worry, once you get used to it, you won't have to bend down and look each time). Start the pump and watch the water bubbling and steaming at the shower screen, after a short while this water will stop bubbling the "water dance" and steady up (you will also hear the difference). Thats it, your now ready to pull your shot.

Only 2 complications

Q1. How long do I wait before pulling my shot

A1. This depends on your machine, with some you lock and load straight away and pull the shot. With others you may have to wait 10, 15 20 etc.. seconds before pulling the shot for things to come up to temperature a little. If your shots are sour, try waiting a little.

Q2 After doing water dance to get to the desired temp and pulling a shot, what does one do to pull another shot and achieve the same result? Should another water dance be performed or is the machine at the proper temp to go ahead and just pull the shot? My thinking would be that the water refilling the boiler is cooler and therefore it would need to be heated, otherwise cooling everything in its path, so a new water dance would need to be done but at a shorter interval because of prior heating, am I making sense?

A2. I tend to always pull a cooling flush after the first one, but the size of the flush varies depending on how long the machine sits idle. Now because there are so many things that can affect this e.g. temp of inlet water, boiler pressure etc.. rather than have a set amount I flush, or a set time, I use the following simple rule of thumb to get repeatable shots.

"Flush until the water stops flashing/bubbling off to steam, then wait how long you normally wait before pulling your shot"

Once you know your machine, you will be able to hear this steady flow, without needing to look. Also for subsequent shots this might be 1oz, it might be 2oz it might be 3oz etc……it doesn''t really matter, as with all other things being unchanged since the previous shot, it's going to put you back into the same position you were after your first cooling flush and before your first shot. The boiler itself will normally be back up to to temperature extremely quickly, so this shouldn't be a factor.

I think it's easy with coffee (or anything else) to get carried away with numbers and fixated by numbers/routines and ritual and whilst there is a certain comfort in these, it's more important to trust your senses, forget the stopwatches, thermometers and scales. Of course these things are great to learn with, but not really designed to help you improve your own technique in the longer term. I think it's very important to become used to watching listening and tasting and you will find you cease to need many of these things and your "routines" become just things you do by second nature that dynamically adapt to the situation.

Fixation with routines and numbers has been the primary cause for many an accident, showing that all the equipment/routines in the world can't sometimes make up for trusting your judgment….what was that phrase "I was just following orders"