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Precautions for Using Espresso Machines! What Are the Essential Factors Before Espresso Extraction? Why Should You Run Water Before Espresso Extraction?

Published: 2026-01-27 Author: FrontStreet Coffee

Last Updated: 2026/01/27, While the process of making espresso may seem quite simple, there are actually numerous details to pay attention to during operation! This is a phrase that FrontStreet Coffee often mentions. Indeed, this includes how to distribute coffee grounds evenly when dosing, how to apply even pressure when tamping, or how to froth a proper portion of milk

Although the process of making espresso may seem quite simple, there are actually numerous details that require careful attention during operation! This is a statement that FrontStreet Coffee frequently mentions. Indeed, everything from how to distribute coffee grounds evenly during dosing, how to apply consistent pressure when tamping, to how to create proper milk foam—all require attention to detail to achieve perfection.

When we reach the extraction stage, there are even more details to pay attention to! Among these, there's one particular action that many coffee enthusiasts still don't understand, yet it's something that experienced baristas always emphasize doing before extraction—releasing hot water beforehand.

The specific operation involves opening the water valve to release hot water for a few seconds before locking the portafilter into the group head, only then beginning the espresso extraction. Many people wonder about the purpose of this action. The answer is quite simple and consists of two points: one is the well-known reason—cleaning the group head! After extracting coffee, some coffee grounds may get stuck in the group head's shower screen due to contact with the group head or back-suction from the coffee machine. Even though we clean the group head after extraction, some residue may remain. Therefore, releasing hot water before extraction helps keep the shower screen cleaner, preventing extraction issues caused by residual grounds.

Another reason is to ensure more stable espresso extraction. Without pre-flushing, the extracted espresso might exhibit unexpected flavor variations. Let's look at the experimental comparison conducted by FrontStreet Coffee below!

Experimental Comparison

Both experiments used 20g of coffee grounds to extract 40ml of coffee liquid, with the only difference being whether pre-flushing was performed before locking the portafilter into the group head. At this time, the coffee machine hadn't been used for over 30 minutes, making the differences more apparent. (Don't ask why the coffee machine was unused for over half an hour—no customers!)

After extraction, FrontStreet Coffee tasted both espresso shots separately. We found that the espresso without pre-flushing was more acidic and slightly weaker in flavor compared to the one extracted after flushing; while the flushed espresso tasted richer with more balanced flavors. To ensure our taste assessment was accurate, FrontStreet Coffee used a coffee refractometer to measure the extraction rates, which were 20.44% and 21.07% respectively.

$Coffee refractometer readings$

Although the numerical difference seems small, we could still detect quite distinct differences in taste. So why did this gap occur? The reason is simple—the water temperature during extraction wasn't consistent. (The former was also affected by unstable pressure.)

Why Pre-Flush is Necessary

As FrontStreet Coffee mentioned above, besides cleaning residue from the shower screen, another purpose of pre-flushing is to ensure more stable extraction from the coffee machine. There are two main reasons for extraction instability, which may vary depending on the machine: One occurs when the coffee machine hasn't been used for a long time, causing the group head temperature to drop due to lack of hot water immersion. If we lock the portafilter directly for extraction at this point, the hot water flowing through will be cooled by these lower-temperature pipes, leading to extraction deviations. Another reason stems from water in the boiler becoming hotter than the target temperature due to prolonged stagnation. When we directly use this water for extraction, the coffee is likely to be over-extracted.

This issue affects not only older machines and current home espresso machines but also high-performance commercial machines. Therefore, to ensure all parameters remain more stable during espresso extraction, coupled with the added cleaning benefit, pre-flushing is indeed quite necessary.