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Dometic GY20 mushroom vent


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The Dometic instructions are less than informative (though there is reference to closing the vent under certain conditions in a marine environment).


A combination of comments on the web, and a perusal of the cutaway drawing on the installation instructions imply that the vent can be opened/closed via a push/pull action on the central knob (push up for open, pull down for closed). (and if you break it in trying, I won't be held responsible ;-) )

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....I think I would also be careful. :-(


Various sets of sales literature refer to the vent as being "A passive ventilator with easy push/pull closing action".


The sample installation/operating dowloadable instructions here:




refer to being able to close it (particularly in the marine environment, but don't show how).


The accompanying cutaway drawing, however, indicates that a push/pull action on the centre knob would probably move the internal "mushroom" of the vent, opening or closing the aperture.


Good luck!

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  • 2 years later...
I have just had a similar issue with my recently purchased 53 plate Bessacarr E745. - My vent is permanently stuck open. Investigation reveals an additional circlip attached to the plastic shaft. The unit has to be removed (6 screws to the main body and 3 for the casing) then the circlip can be removed simply by prying off with a screwdriver. Be careful though, with age comes fragility. I suspect the circlip was added to keep the vent open at all times for air circulation. Keep on Truckin'
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This document (published September 2013) for Dometic GY20/GY21 roof vents






"Opening and closing the roof vent

- Turn the cover cap anti-clockwise to close the vent.”


As it is also advised that "The roof vent must always be open when used in a caravan”, you may well be right to suspect that a circlip was added to the plastic shaft at some stage to prevent caravan/motorhome owners from closing the vent by moving the central knob.


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