We are about to connect a district heating scheme supply into a large existing domestic central heating system. It has two linked 45KW boilers via a common manifold with four zones off it - 2 ground floor circuits, 1 first floor circuit and a separat supply to the DHWC. The whole lot comes back through a common return which t's to both boilers. There are a couple of antique gate valve bypasses and all of this makes sense and seems to work reasonably well. The thing we cannot understand is why there is an additional bypass with a 15/50 grunfoss pump from the common flow manifold to the common return. We need to take this out in order to plumb in the district heating supply as to do so we will isolate the boilers from the system leaving them as a back up. However we don't want to do this until we understand the necessity for a pumped bypass. Anyone got any ideas on this? 
