I am fitting a Gerberit Duofix frame and cistern. Does it need to be vented?

for Toilets > Geberit Duofix > Products , Geberit UK

Seems to hook up to your existing sewer line which is already vented, according to the installation video.

Absolutely, it has to be vented. It is just another toilet, just different design. Are you installing it in place of regular, floor-mounted toilet ?

If you are installing it in place of floor mounted toilet than you will have to do some pipe relocating to fit the new frame. Both, floor and wall mounted toilets / vents install different way.

If you are outside US than Code may be different. Check with local Building Inspector to find out what is required for your area

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Milo