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Water or Coffee Tastes or Smells Off: Causes and Fixes

Updated August 3, 2026 · Home and commercial machines

Start by tasting the water on its own. Run hot water from the machine into a clean cup with no portafilter fitted and taste it once it has cooled enough. If that water tastes stale, flat, or chlorinated, the problem is on the water side, and the usual cause is water that has sat in the tank for days growing warm and stagnant. If the water tastes clean but the coffee tastes rancid, bitter, or ashy, the problem is old coffee oils in the brew path, and that calls for detergent cleaning rather than descaling. Those two paths need different products and confusing them wastes time. A musty or mouldy smell is its own category and usually means biofilm in the tank or tubing. Sharp chemical or metallic notes right after a descale mean the rinse was insufficient, and more rinsing is the fix.

Causes, most likely first

Work down the list. Each cause has a check you can perform before you change anything, so you are diagnosing rather than swapping parts.

  1. 1. The water in the tank is stale

    You can fix this

    How to confirm it: Note how long the current tank has been sitting. Water left in a warm machine for several days tastes flat and slightly sweet-stale. Empty it, refill with fresh cold water, run two cups through the group, and taste again.

    What to do: Refill daily with fresh cold water and empty the tank if the machine will sit unused for more than a day or two. Rinse the tank when you refill rather than topping it up on top of old water.

  2. 2. Rancid coffee oils have built up in the group, screen, and baskets

    Routine maintenance

    How to confirm it: The plain hot water tastes clean but the coffee tastes bitter or rancid. Wipe under the shower screen with a white cloth: heavy dark residue confirms it. Smell a dry, freshly rinsed portafilter basket, which should smell of nothing.

    What to do: Soak the portafilter, baskets, and shower screen in a cleaning-tablet solution, brushing the group seat while they soak. If your machine has a 3-way solenoid valve, run a detergent backflush cycle with a blind basket. Machines without that valve must not be backflushed, so clean the removable parts instead.

  3. 3. The water filter is exhausted or the source water is heavily chlorinated

    Routine maintenance

    How to confirm it: Taste your tap water side by side with the machine's hot water. If both taste of chlorine, the source is the issue. Check the filter's fitted date and its rated volume, since an exhausted cartridge can taste worse than no filter at all.

    What to do: Fit a fresh filter cartridge, prime it as directed, and reset the filter counter. If your tap water is very hard or heavily treated, use filtered or bottled water intended for coffee rather than distilled or softened water, both of which extract poorly.

  4. 4. Descaler residue was not fully rinsed out

    Routine maintenance

    How to confirm it: The off taste is sharp, sour, or chemical, and it appeared immediately after a descale cycle. It fades slightly with each cup rather than staying constant.

    What to do: Run at least two full tanks of fresh water through both the brew group and the hot water and steam outlets, discarding everything. Repeat until the water tastes neutral, then pull and discard one shot before drinking.

  5. 5. Biofilm or mould has formed in the tank, lid, or intake tube

    Routine maintenance

    How to confirm it: The smell is musty or earthy rather than sour. Look at the tank corners, the underside of the lid, and the intake tube for a slippery film or pink or black spotting, and feel the inside wall for sliminess.

    What to do: Wash the tank and lid by hand with warm soapy water and a soft brush, paying attention to corners, then rinse thoroughly and dry. Replace the intake tube if the film is inside it. Leave the tank lid off when the machine is idle so the inside can dry.

  6. 6. Scale in the boiler is affecting taste and temperature

    Routine maintenance

    How to confirm it: The taste is dull and flat, flow has dropped, and the machine is overdue on descaling with hard water. Look for white crust around the steam tip and inside the tank as supporting evidence.

    What to do: Run a full descale cycle at the stated dilution, then rinse thoroughly with two clean tanks. Set the machine's water hardness to match your supply so future prompts arrive at the right interval.

  7. 7. New machine plastic or a new hose is off-gassing

    You can fix this

    How to confirm it: The machine is new or recently serviced, the taste is faintly plastic or waxy, and it is getting weaker with every tank rather than staying level.

    What to do: Run three or four full tanks through the brew, hot water, and steam outlets and discard them. Most machines lose the taste within the first few tanks. If it persists beyond that, contact the manufacturer while the warranty is live.

When to stop and call a technician

Never bypass, disable or remove a safety device — a thermal fuse, pressure relief valve or lid interlock is the part standing between a fault and a fire. Do not open a pressurised boiler or work on mains wiring.

What the fix needs

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If your machine shows a code

A code on the display narrows this down considerably, because the machine is telling you what it thinks is wrong. Look it up by brand →

Other faults

Frequently asked

Should I use distilled water to avoid scale?
No. Distilled and heavily softened water extract poorly, taste flat, and on some machines the water-level sensors will not read them. Use filtered water with moderate mineral content instead.
My coffee tastes burnt. Is that the machine?
Usually it is old oils in the brew path or a dark roast pulled too slowly. Clean the screen and baskets first, then check your shot time against about 36 grams out in 25 to 30 seconds.
Does descaling fix a rancid taste?
No. Descaler dissolves mineral scale in the water path and does nothing to coffee oils. Rancid or bitter flavours need a detergent cleaner on the brew path.
How often should I wash the water tank?
Weekly by hand with warm soapy water, and more often in a warm kitchen. Tanks are the most common place for biofilm because they hold standing water at room temperature.