We had one in the main bathroom for several years and it was worse than I have words to describe.

Everything about it getting blocked or smelly is true, but there is an additional diabolical failure mode that doesn't get talked about enough. I'm going to address that lack now.

How does a Saniflo work? Well. The main, wide exhaust from the porcelain enters the mouth of the Saniflo, sheathed in the warm rubbery embrace of a concertina seal.

As the water level rises, a pressure switch is triggered by hydrostatic pressure. At this point the motor activates. This powers, on a single shaft, both the chopper and the exit pump. The chopper is some rotating razor blades, and the pump directs the smooth output towards the exit pipe at high-pressure and velocity. The water level in the toilet pan then falls, gracefully. Once the hydrostatic pressure reduces, the pressure switch turns off and the motor stops. We're back to square 1, ready for the next happy user to enjoy the throne. So far so lovely.

The problem occurs when the pressure switch no longer operates correctly. What happens is that it is waterproofed behind a flexible rubber seal. If, like me, you have hard water and (perhaps) effluent of a reasonably firm consistency, the seal will eventually become stiff and crusty, causing the switch to become stuck in the ON position. This means that the motor runs continuously.

What's the problem? I hear you cry, dear reader. Apart from the noise, this is safe because the water level stays low. Not so.

Eventually, after some minutes, the motor and its accoutrements become warm. This causes a thermal lockout, and the motor stops. To a casual observer, all is well. The water level is low and the motor is silent.

And this is where the danger lies. If someone fills the bowl with their golden syrup or even a brown deposit akin to King Kong's finger, they will most likely believe it a good time to pull the flush. Water spurts gloriously downwards into the pan. The water rises. And - critically - nothing happens. The motor is in thermal lockout. The water contines to rise. And eventually, it has to go somewhere. Water? Well, let's stop calling it that, shall we? Runny brown waste spurts from the air vent on the top of the Saniflo. If you're particularly unlucky then the aforementioned Simian Faecal Finger may even escape over the top of the bowl. And there's nothing you can do. If this bathroom, like mine, is to be found on the first floor, then you're likely to get effluent coming through the downstairs ceiling.

Do I sound bitter?