Channeling
When water finds a fast path through the coffee bed instead of flowing evenly. Produces a shot that's simultaneously sour and bitter — the worst-tasting failure mode in espresso.
Channeling is what happens when pressurised water exploits a low-density path through the puck — a crack, a wall gap, or an unevenly distributed area — and rushes through, over-extracting that section while leaving the rest of the bed untouched. The result is a shot that tastes both under- and over-extracted at the same time.
How to spot channeling
- Visual: A noticeably faster blonde stream in one part of the portafilter spout, or sideways squirting from the bottomless portafilter.
- Time: Shot runs much faster than expected — 15 s instead of 28 s — despite the same grind setting.
- Taste: Sour + bitter + thin all at once. The puck has flowery patches with bone-dry pockets when you knock it out.
- Puck: Visible hole, crater, or hollow when you tap the spent puck out.
Common causes
- Uneven distribution. Clumps or low spots in the dose before tamping. Dose drops into the basket without breaking up clumps.
- Tilted tamp. Tamping at an angle compresses one side harder than the other; water flows preferentially through the looser side.
- Wall gap. Edges of the puck didn't seat against the basket walls — water races around the side.
- Updose / dome. Too much coffee in the basket means the puck touches the shower screen and gets disturbed at lockup.
- Stale puck screen / shower screen. Uneven water dispersion creates wet spots before pressure builds.
How to fix it
- WDT (Weiss Distribution Technique). Stir the dose with thin needles before tamping. Single biggest improvement to puck prep.
- Level distribution. Use a leveling tool or palm-flat technique to ensure even depth before tamping.
- Flat, full tamp. 30 lbs of pressure is overkill; level matters more than hard.
- Check dose. An 18 g basket runs 17.5–18.5 g; over-dosing compresses the headspace.
- Inspect the shower screen. Clean it weekly. A clogged screen disperses water unevenly.
- Use a puck screen. A thin perforated metal disc between coffee and shower screen distributes the initial water more evenly.
Channeling vs. fines migration
Don't confuse channeling with fines migration — when ultra-fine grind particles wash downward and clog the bottom of the basket, choking the shot. Channeling speeds the shot up; fines migration slows it down. Both come from puck-prep issues but have opposite symptoms.
Related terms
- Dial-In — channeling can derail a dial-in; rule it out first.
- Extraction Yield — channeling produces extraction numbers that don't match the taste.
- Bloom / Pre-infusion — slow ramp-up helps prevent channeling.
Log puck-prep notes alongside every shot.
Track WDT, distribution method, and tamp pressure to find the technique that works.
