Flow Control: Your questions answered.

One of the most frequent questions that we field involves flow control. What is flow control? What does flow control even do? Do I need flow control? We’ll look deeper into these questions and demystify flow control as an additional variable to the espresso-making process. Lastly, we will briefly examine the differences between flow control and pressure profiling. 

For a time, the only machines in our lineup that included flow control native to the machine were the formidable workhorse by La Marzocco, the GS3, and the more affordable Lelit Bianca. There was a gulf between the two machines that needed to be filled. Thus, the brain trust that comprises our LUCCA research and development team got to work on developing a flow control system that was as smooth, reactive, and intuitive to use as the GS3 MP and the Bianca’s. Fast forward to the Spring of 2022, and we did exactly that. Our Lucca E61 flow control system can be retrofitted on any E61 group head machine or purchased with the unit already installed on the device. 

What is Flow Control? 

Mechanical E61 flow control is a straightforward process. Essentially, the paddle on top of the group head moves a spindle that opens and closes an aperture that restricts or increases water flow to the espresso puck during an extraction. Let’s back up one moment and look at the fundamental parameters of how extraction works on an E61 group head. 

• Espresso machines deliver water from the pump to the group head at a defined flow rate. Clive’s technical team advises that the flow rate be 250 milliliters in 25 seconds. This flow rate can vary depending if your machine has a rotary pump or a vibratory pump; the latter requires resistance to push water through, which is regulated by the expansion valve (OPV).

• Next, add resistance to the water flow by adding ground coffee to your portafilter. Resistance will cause pressure to build within the group head, which is related to the resistance from the coffee bed. For example, if your grind size is too coarse, the pressure in your group head will not build up to 9-bar, and you very well may see your shots extract too quickly, even gushing out of the portafilter, causing a huge mess and watery coffee. 

• As an espresso puck extracts for longer periods, the puck begins to degrade, and extraction increases—creating over-extracted and astringent flavors. Since E61 espresso machines cannot moderate their flow rate independently, the barista can influence this by changing the flow rate of water to the group head by manipulating the flow control paddle. On mechanical flow control machines like E61 machines, the relationship between water flow and pressure is not precisely direct. For example, manipulation of the flow control paddle only determines how quickly pressure builds in the group head chamber but does not change the actual pressure occurring. 

Flow vs. Pressure: An Indirect Relationship

In my last point above, I mentioned that flow and pressure are related, but the relationship between the two is indirect. The movements you make with the flow control paddle are not precise; they are more broad stroke changes. 

Turning the metered (variable) aperture increases or decreases the water flow rate by creating a larger or smaller pathway for water to flow through to the resistance point—the espresso puck. Think of it this way, when water flows from the pump through the group head with no resistance from a coffee puck—that is a direct relationship. The farther you move the paddle to the right, the faster the water flows without restriction. You’d get to 9 bar pressure on a rotary pump machine immediately. On a vibratory pump machine, you have a ramp-up of pressure for a few seconds before you get to full pressure. 

Resistance & Pressure

Thus, adding resistance via coffee creates a pressure differential. As the pressure builds in the group head with added resistance from the espresso puck, the pressure differential lessens between the pump and the group head, which means you do not have direct control over flow at all times. For instance, even if you keep the flow control paddle halfway open for the entire extraction, the brew chamber will eventually get 9-10 bar pressure. The ability to accomplish a specific flow rate at a particular pressure profile requires a machine with a brain (computer) that can alter its pump pressure at any given point during espresso extraction. 

Stop that Channeling

Ultimately, mechanical E61 flow control can be a boon to the home barista, especially where even extraction is concerned. For instance, if you’re using an espresso machine with a rotary pump, once you flip that brew lever up, the espresso puck is immediately bombarded with the full force of 9 bars of pressure. Applying full pressure right away can cause inconsistencies in your extractions and fissures in the espresso puck, also known as channeling. Being more gentle with the espresso puck at the beginning of your extraction, such as with a low 10-second pre-infusion, can be exceedingly helpful with the rest of your extraction. 

How will Flow Control Impact Your Coffee?

What’s the point of this? You may love your standard 9-bar e61 shots or lever machine shots. That’s completely awesome! Remember that flow control is an additional variable to the espresso-making process and potentially can impact your espresso outcomes. Those results are highly subjective and depend on other variables such as grinder, coffee, espresso technique, etc. One of our favorite flow control recipes involves a low-pressure pre-infusion, a gentle extraction, and a tapering down of pressure at the end of the shot. Let’s break espresso extraction down into three parts. 

1. Pre-infusion: a two to three-bar even saturation of the coffee puck. 10-12 seconds at the beginning of your shot will thoroughly wet 18-20g of espresso evenly. Mainly, you’re allowing the coffee to be wet, minimizing the chance for channeling, and building the body of the espresso shot—experiment with upping the pre-infusion by a second or two.
2. Infusion:  after around 10-12 seconds of pre-infusion, slowly ramp up the pressure by moving the paddle to the right. On the Lucca flow control gauge, this is indicated in the “build” range. Here is where you can get creative, especially given your coffee choice. You can slowly ramp up with bar pressure to 9 or even peak it lower at 7 or 8 bar. The time you extract during the infusion phase varies depending on your coffee, roast level, and varietal. Are you tired of those traditional 25-30 second shots? Try how your espresso tastes at 60 seconds.
   
   
3. Post-infusion: the longer you extract the solubles from the espresso puck, the faster water will flow through the puck and the quicker it degrades—leading to bitter, over-extracted flavors and a very astringent mouth feel. To avoid these undesirable flavors, start tapering down the pressure by moving the flow control paddle back to the left, restricting water flow, and slowly continuing until you end your extraction. Remember, you’re not changing the pressure applied to the puck, only lessening the water flow. So, the longer your extraction is, the faster water will flow through the puck. Tightening the water flow will keep your extraction flowing steadily until you decide to stop it.

Disclaimer: It’s important to note what flow control shouldn't be used for. Flow control is not a silver bullet solution that will cure lousy shots. Nor is it something to simply slow your shots down. Flow control is not a substitute for a good grinder and freshly roasted coffee. 

A Quick Note on the La Marzocco GS3 MP

The GS3 MP allows you to use flow control; however, it is slightly different from mechanical E61 flow control. On the GS3, you're moving the paddle from right to left. When you actuate the brew paddle, a switch turns on the pump and opens a bypass valve where excess water bypasses the brew chamber and diverts into the drip tray. 

La Marzocco considers this a form of pressure profiling and that you’re manipulating pressure with the paddle. Because moving the paddle only influences pressure on the brew chamber side of the flow restrictor, the resistance of the puck itself determines the flow rate. The manometer on top of the group head will be zero until you have a restriction. 

Pressure Profiling and the Rocket R Nine One

The Rocket R Nine One is the only machine in our lineup where you have a direct influence over pressure and flow at all times. Yes, you read that correctly. You can specifically program the machine to alter the pressure at any point of your extraction accurately. The R Nine One can do this because its computer works with a gear pump. This means that no matter the puck resistance at the group head, you have independent control over the flow rate. You can create a profile where your machine Play the flavor game at home. Decide what works for you. Every coffee is different, but the potential is limitless. You can practice with a manual shot; if that is good, you can save it as a profile! 

Do I need Flow Control?

Yes! Also, no. It’s important to remember that flow control is not essential to everyday espresso brewing and is not critical to pulling amazing espresso shots. However, it can be enjoyable and exciting to someone who likes to experiment and tinker. It can also help improve extraction issues such as channeling. Flow control and pressure profiling are important to modern home espresso machines because it gives you the same tools in the home setting that more commercial machines typically come with, making those skills more accessible than ever. If you’re on the fence about flow control, you don’t have anything to lose, and you will have great espresso either way.