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Not sure if this helps or not, but I've just taken a couple of photos of our UFH expansion vessel: The bit with the cap on at the front is the fill loop connection. I've added red and blue max and min markers on the pressure gauge, just so I can see at a glance if it's in the right zone. The fill loop is the flexi that can be seen at the bottom (I use the same flexi to fill the UFH and the buffer tank, they are separate). The feed down to the UFH is the copper pipe right at the back. The bit of grey pipe is the overflow from the PRV, OK in this case as the maximum temperature is restricted by the ASHP, so plastic is OK in this case. If there is a risk that the temperature could get very high then the overflow needs to be in copper. The black plastic tundish can just be seen at the bottom of the pic. Here's another view showing the tundish fitted to the waste pipe: The drain from the tundish feeds down to a large diameter drain that runs through a trap downstairs. This trap is kept topped up by the water that is drained from the water softener, so cannot dry out. The same drain is also used for the PRV outlet on the buffer tank (that one is set to 1.5 bar, because of the tank limit) and again plastic is OK as the buffer cannot ever exceed the ASHP maximum temperature (the ASHP cannot ever get hotter than about 60°C even under fault conditions). For Willis heaters I think you would have to assume that, under fault conditions, the water could exceed 100°C (because of the pressure) and so make sure that the overflow, tundish and drain followed the requirements in the regs for the safe discharge of boiling water and steam. Details are in Part G3:

The plans were validated, with a target date of March 30th for approval.