You can run the pipework in a variety of sizes. Main ducts are 150mm in diameter close to the unit, branching off to 125mm and then sometimes down to 100mm if you wish. In some tricky places you can use the rectangular equivalent of these round sizes to get round corners or reduce the size of pipwork that you are boxing in.
We finally gave in and decided to include two fitted cupboards which will hide pipe runs between the first and second floors and here the rectangular section is a neat space saver.
