I am having further thoughts on this. I have spoken to my plumbing chap who has the unvented ticket.
I asked him about curling the discharge pipe close by the wall exit so that it points back into the wall. His preference is for a pipe that runs down the wall, stops near the bottom, and now with a mesh guard to protect people getting too close to the pipe in case it vents boiling hot water.
I have now flipped. Let us study this from a health and safety point of view. You either have boiling water spewing out at high level (curled into wall), or spewing out close onto a pathway running down the side of the house. That same pathway running down the side of the house that my 2 toddler boys play. So in the event of a fault, there is chance my 2 boys will get scalded. Crazy, dangerous, short sighted and irresponsible. The alternative? close off the pathway running down the side of my house to pedestrians? Utter madness.
Let me calm down, press rewind, and approach this from first principles, and not from narrow minded interpretation.
The discharge pipe is there to safely dispose of discharged water. There is also a requirement to know if water has been discharged to alert to a fault with the system.
The safest way is to discharge into the stack, as described by muggles above, in the loft.
QUESTION 1: can I use a water sensor in the discharge pipe to signal when water is present? The sensor would trigger an audible alarm, and backed up by a visual LED. The trigger would be latched so that a small burst of discharged water would cause a permanent alarm (until manually cancelled). The alarm and LED would be mounted on the ceiling of the upstairs landing so that it would be more readily noticeable (like a smoke alarm).
QUESTION 2: Unvented cylinders appear to require a 2 port motorised valve on the feed pipe from the boiler. Is the 2 port valve to be fitted in addition to a conventional 3 port mid position motorised valve, or can the unvented cylinder be fed directly from the 3 port valve from the boiler?
Or should I adopt a 2x 2 port valve system in place of a 3 way mid position?