Are all of the ducts, plenums, couplers cross compatible then? I'm finalising my shopping list and looking at all of the options. HeatPex 75mm duct (https://naturalgreenheat.co.uk/product/75mm-semi-rigid-duct-50m/) is a fair bit cheaper than others but no point looking further if its not compatible. I assume if you have the sealing rings for the plenum / manifold then the duct should fit ok - or am i barking up the wrong tree.