Now this is a bit of wild guess but it could well be a drain waste heat recovery unit. For preheating incoming water to cylinders. It has two connections that look like they'd take the water you want to preheat through it and a connection big enough for a waste pipe too. The only problem is that the outlet looks too small for that purpose?
Whatever system it's supposed to go on to it is surely some kind of heat exchanger/heat recovery device.