Line-Pressure Pre-Infusion

Line-pressure pre-infusion is an espresso-machine technique that wets the coffee puck using incoming mains-water pressure before the pump raises the system to full brewing pressure. It is most commonly associated with certain direct-plumb rotary-pump and commercial hydraulic designs.

What actually happens

When the brew valve opens, supply-line pressure can reach the group while the pump is not yet building full pressure. This lower-pressure water begins saturating the puck. After a programmed delay, mechanical transition, or control input, the pump engages or the hydraulic path changes and extraction continues at higher pressure.

Line pressure is not puck pressure

If the building supply is 3 bar, that does not mean the entire puck instantly experiences a uniform 3 bar. Flow restriction through valves, gigleurs, tubing, the group, and the coffee bed creates pressure differences throughout the circuit.

Why baristas use it

Gentle initial wetting can help the puck swell before high flow arrives, potentially reducing abrupt channel formation. The result depends on coffee, dose, grind, basket, puck preparation, duration, and actual water debit—not simply the pressure number.

Machine design determines behavior

Reservoir machines generally cannot perform true mains-pressure pre-infusion because they are not connected to pressurized plumbing. Some machines simulate a similar effect with pump control, chambers, valves, or flow restriction. Consult the manufacturer before changing regulators or hydraulic settings.

For broader shot control, see our published pressure profiling guide and the Coffee Glossary.