The small metal button on the side of a moka pot boiler is a pressure relief valve, and it is a genuine safety device.
What it does
If pressure in the boiler rises dangerously — because the filter is clogged, the coffee is tamped, or the grind is far too fine — the valve opens and vents steam.
Without it, the boiler could rupture. Moka pots operate at real pressure and this is not theoretical.
The rules
Never fill water above the valve. If water covers it, it cannot vent steam — it would vent boiling water instead, and the safety margin is gone.
This is the reason for the “fill to just below the valve” instruction that everyone reads and some ignore.
Never tamp the coffee. Tamping raises pressure far beyond the design.
Keep it clean. Scale can seize the valve shut. Descale periodically, particularly in hard-water areas.
Checking it
Occasionally push the pin with something blunt; it should move freely and spring back. If it is seized, replace it — they are cheap and widely available.
If it vents during brewing
Something is wrong. Take the pot off the heat, let it cool, and check the grind, the filter plate and the gasket.
A properly working moka pot should never trigger it.
The gasket
Replace annually with regular use. A hardened gasket leaks steam and prevents proper brewing.
Last reviewed 8 August 2026