Channeling is water taking the path of least resistance through the puck: it finds a crack or loose patch and drives a channel through it instead of pushing evenly through the coffee. Parts of the puck get too much water and turn bitter, parts too little and stay sour. Both arrive at once — the signature.
A bottomless portafilter does not cause it. It removes the spouts that were hiding it.
Signs
During the shot:
- flow starts on one side, or jumps between spouts
- jets or sprays off to the side instead of a single stream
- a good cone of syrup suddenly turning pale and thin (“blonding” early)
- the shot accelerates as the puck opens up
In the puck afterwards: holes, cracks, or an uneven surface. A soupy puck is consistent with channeling but proves nothing on its own.
In the cup: bitterness and sourness together, thin body, no crema, a different result every pull.
A correct extraction is a centred, syrupy cone — “rat tail” — with no spray.
Causes
- Bad distribution. Voids, mounds and uneven density — what WDT fixes.
- Inconsistent tamp — tilted, or pressure that differs per pull.
- Grind too coarse. A less resistant puck opens channels more easily. Too fine and a bad tamp also cracks it.
- Wrong dose for the basket. Under-dosing leaves headspace and the puck expands unevenly.
- Stale coffee. No CO₂ left, so the puck behaves unpredictably.
- High pressure. At 9 bar the water forces its way through any weak point. Dropping to ~6–7 bar is the biggest improvement on a stock Gaggia Classic, and helps most with light and medium roasts.
- Dirty or worn basket and shower screen. Clogged holes redistribute the flow.
- Uneven grind. A grinder producing boulders and fines gives the water a pre-made path. WDT cannot fix particle size.
Why medium roast still channels with WDT
WDT solves distribution and nothing else. Medium roast sits in the awkward middle: not as tolerant as dark, not as forgiving as it looks, so the remaining variables — particle uniformity, tamp consistency, dose versus headspace, pressure, absence of pre-infusion — still open channels. The fixes that matter most:
- grind finer, just before the shot chokes
- reduce pressure toward 6–7 bar
- make the tamp identical every time
- match dose to basket capacity
Small upgrades that help: puck screen, dosing funnel, levelling before tamping.
Why dark roast hides it
Channeling happens with dark roast too; it is less visible and less punishing:
- the brittle beans shatter into fines that plug micro-channels
- they are more soluble, so the rest of the puck extracts acceptably anyway
- the puck forms more uniformly, masking distribution errors
- the strong roast flavour covers the defect
So a dark roast can drink fine while the technique is wrong — and then a medium roast from the same roaster tastes broken. Forgiving makes it a poor training ground.