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Thetford cassette toilet jamming


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monique.hubrechts@gm - 2019-01-21 4:26 PM


The wheels on the casette is like you go shopping whit luggage. About 20 kg. Thetford do not like full cassette. Up to 2/3.. So before you drive they should be empty all the time. There is coming an end to this in 2020.



Ha ha! What is coming an end to this in 2020........?



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Naebother - 2019-01-21 1:14 PM


Seem to remember a post about cassette blades jammimg at altitude, this was caused by pressure in the cassette being higher than outside. If it happens a few days after use it may be a build up of pressure in the cassette. Just guessing, but worth a thought


This was certainly true for Thetford C2, C3 and C4 bench-style toilets (long since out of production) and swivellling-bowl C200 models. These had an AVV (Automatic Vent Valve) with a foam float (photo of this AVV attached) in the cassette’s top and designed to open when the cassette was inserted in the cassette-locker (allowing equalisation of the pressure within the cassette and the outside air) and to close if the level of the waste in the cassette reached the float.


However, these AVVs had a nasty habit of sticking closed and, if high pressure built up inside the cassette as a result of waste decomposition and/or altitude change, opening the toilet’s blade could be inhibited. This was not necessarily a bad thing, as the blade on those toilets slid across the bottom of the cassette’s rubber seal and opening the blade was an abrupt action. A combination of a lot of waste and high pressure in the cassette, and the blade suddenly opening, could result in an eruption of waste being blasted upwards out of the toilet’s bowl.


The AVV of Thetford C250/C260 and C400 toilets is a completely different design, being inside the cassette and activated by a mechanism in the cassette’s underside. The blade also moves differently, sliding into postion below the cassette’s rubber seal, then finally moving vertically upwards to contact the seal itself. This reduces seal-wear and, as blade-opening is 2-stage, if there were a pressure build-up inside the cassette, pressure equalisation should occur as the blade moves downwards when the ‘hole’ in the base of the toilet-bowl would still not be properly open. There used to be dire warnings about the potential effect of cassette pressure build-up when C2/C3/C4 and C200 toilets were commonplace (Once experienced never forgotten it was said!) but I’ve not read anything about this issue applying to more recent Thetford toilets, nor of them being affected by altitude change.


I notice in the Thetford C260 User Manual




that, when preparing the waste-cassette for use, the following advice is given


"To depressurise the waste-holding tank, press the vent button before placing the tank back in its position.”


I‘m not sure what this action is supposed to achieve, but it’s not something I’ve ever bothered doing.


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