The membrane sags against the vessel wall and eventually tears.
Many have a V or the word "vertical" on their data Badge rather than an explicit instruction.
Indeed, the boiler manual tends to have the kpA for the flue and fittings to allow the flue installer (often not the boiler installer) to work out the necessaries
In answer to your other thread. The UFh should come with actuators. If it doesn't let me know and I'll add them on. They're all much of a muchness. The UFH controller is in stock now too.