ulsonix USX-ST5 User Manual page 38

k) To remove sediments and limescale from the boiler while heating
water, unscrew the safety valve (6) and the plug in the lower part of the
unit (only if the pressure gauge indicates 0 and the unit is cooled down).
To unscrew the safety valve, press ir down and turn it. Pour clean water
into the opening where the safety valve is fitted until no trace of
sediment is visible in the water flowing out the bottom.