9 August 2026 · Dane Whitlock

How to clean a knock box, and how often

There is no descaling, no soaking, no special product. The only thing that matters is frequency, and the only thing that makes frequency easy is whether the bar comes out.

The daily routine

Do it in the morning, when you are already at the machine, rather than at the end of the day when you are not.

1. Lift the bar out. On a box where it detaches, this is one movement. It matters because the underside of the bar and the seat it sits in are where grounds collect out of sight.
2. Tip the grounds out. Into the compost, onto a garden bed, or into the bin — all three work. Not into the sink.
3. Rinse both under hot water. Hot, not boiling. Coffee oil is what you are removing, and it comes away from a sealed surface easily.
4. Shake it dry and put the bar back. If you are leaving the box out on the counter, air-drying is fine. If it is going in a cupboard, dry the inside first.

Why daily, and not when it is full

Not because it runs out of room — a 700 ml (24 fl oz) box holds a morning of shots comfortably. Because of what spent grounds are.

A puck comes out of the machine warm and wet, which is the condition every fermentation on earth is waiting for. Two days is uneventful. Five is a smell you notice from across the kitchen. This is a property of damp coffee, not of any particular box: it happens in stainless steel, in plastic, in ceramic and in wood alike. One buyer relayed the same idea: pucks are said to go moldy over time, so a large container is unnecessary at home. It is the best argument for the small daily habit rather than a bigger box.

Every few weeks: the oil

Roasted coffee carries oils that survive extraction, and a thin film of them builds up on the inside over time. Water alone stops being enough after a few weeks.

Warm water, a drop of dish soap, a cloth or a soft brush, and it comes off. On stainless steel you can be unceremonious about it — the surface is sealed, so the oil is sitting on top rather than in anything. Rinse well afterwards, because soap residue in a knock box is the one thing that can actually reach a cup, via a portafilter you knock and then load without rinsing.

Ours is dishwasher-safe if you would rather not think about it. The bar comes out and goes on the top rack.

The bar seat, and why it is the first thing to smell

If a knock box develops a smell before anything else does, it starts at the seat — the slot or bracket the bar sits in.

Grounds get there in a way that is easy to miss. The strike throws a small amount of fine coffee sideways rather than down, and the finest particles settle in the gap between bar and body. That gap holds them by capillary action along with a little water, and it is the one place a rinse under the tap does not flush because nothing flows through it.

It takes two seconds to deal with once the bar is out: run a thumb or a cloth along the seat while you rinse. It takes months to deal with if you never remove the bar, because by then the smell is coming from a place you cannot see.

What not to use

Avoid	Why
Steel wool or abrasive powders	Scratches the surface — and scratches are exactly what hold oil where a cloth cannot reach
Bleach	Unnecessary on a container that holds coffee for a few hours, and hard to rinse out completely
Espresso machine descaler	Formulated for limescale inside a boiler. There is no scale in a knock box
Leaving it soaking overnight	Softens nothing that hot water and soap will not handle, and the bar seat stays wet

Does it need drying between uses?

Only if it is going away. A box left out on the counter air-dries on its own between the morning rinse and the next shot, and the inside of a sealed body has nothing to hold water against.

Putting a damp box straight into a closed cupboard is the one habit worth avoiding, and not because of the box — a closed cupboard has no airflow, so the moisture stays in it. Thirty seconds with a tea towel solves that entirely, and it is worth doing on the underside of the base too, where a film of water sits against the counter and takes longest to evaporate. The same goes for the bar: dry the seat rather than the visible face, since the seat is the part that stays wet.

If it has already started to smell

Wash it with dish soap, bar removed, paying attention to the seat the bar sits in and the inside corners. Rinse, dry completely, and go back to emptying it daily. On a sealed surface that resolves it, because there is nowhere for the oil to have gone except the surface you just washed.

On a porous body — raw wood, unsealed bamboo, or plastic that has scratched — the oil has soaked into the material itself, and washing reaches the surface but not the inside. That is the practical difference the material makes over a year, and it is covered in wood versus stainless steel.

The part that decides everything

Whether the bar lifts out. On a box where it does, cleaning is ten seconds and you will actually do it daily. On a box where it is fixed, grounds work their way underneath, sit there damp and unreachable, and the routine above simply cannot be completed.

It is the first thing we check before selling one, and the reason is exactly this: it is not a feature, it is what makes the maintenance possible. The full method is on how we test.