Yes as I thought, that is not a drain pipe as such , it is the condensate drain and as @Razor900 says to get some condensate even in HW mode is perfectly normal with your boiler, and wouldnt affect the water pressure in the system, the valve in your pic with the red handle is the flow to your radiators, this valve is closed you need to open it, there is another one at the other end make sure that is ope too then see what happens, your heating wont work with either of these closedHere you go pal
This is the amount of water from the pipe in the past day:
This is my fear and the only likely conclusion at the minute. I'm really hoping a leak sealer will sort it out, otherwise it means breaking up a newly tiled floor and sawing through freshly laid plywood sub floor!
I really don't know how the plumber has commissioned the system after capping 2 radiators and fitting a new radiator and there wasn't a problem then! It seems crazy.
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