Many blockages of the heads (toilets) are caused by scale build up on the outlet pipe. This scale can reduce the diameter of the pipe by more than half, obviously this will cause blockages.
Rather than wait until it happens and you are left with a major clearing up job, a little preventative maintenance will remove this problem.
The heads piping should be removed completely from the boat annually and shaken or knocked hard on a hard surface.
This will break up the scale which can then be washed out before refitting the pipes.
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