If obstructions make it impossible to run vertical tube lines through partition walls the best and easiest alternative is to go through the insides of cupboards, particularly when a cupboard on the second floor is directly above a cupboard on the main floor. Look for other possibilities as well, such as boxed-in soil and vent pipes or under stairs.
Drill a horizontal pilot hole through the centre of a suitable inlet valve location on the wall. Probe for hidden obstructions in the wall with a bent piece of coat hanger. With the pilot hole as a centre drill a 2 1/2" hole through both sides of the wall. Using a keyhole saw cut a 2 1/2" x 4 3/8" hole in the outside wall (the side that will have the valve).
Feed the low voltage wire through from the inside of the cupboard, thread it through the top hole of a trimmed mounting plate and attach it to the lugs on the back of the valve. Instead of a 90-degree fitting that was used in a partition wall, glue a coupling on the spigot of the mounting plate. Attach this assembly to the wall as described above in the partition wall section. Then, from inside the cupboard, insert a short piece of tubing into the coupling. Dry fit a tight 90-degree fitting pointed in the direction of the trunk line* below or above. Cut a 2 1/2" hole below or above the fitting and feed a length of tubing through to the trunk line. When you are satisfied with the fit, permanently glue the fittings in place.
[* an inlet extension is also available: part no. M399]