Size of pipe is 25mm for the main flow with it going down to 20mm in places. The WC drain point is where I can attach a 20mm hose for WC, just move a few valves and full flow to drain. There is hoses here and there but allowed for good flow on all. The heater section I also added a 20mm pipe to allow for the reduction in flow Ian was talking about. A single 25mm pipe has less resistance than two hydor heaters in parallel, the hose may be 16mm internal measurement for the hydor but the narrowest diameter on the hydors is more like 12mm
