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Y and W type plans can be used, but the need for additional relays is not practical. It is much easier to convert Y and W type plans to S types from the start. 8.8 Bivalent Systems For bivalent systems, it is necessary to disconnect the internal immersion element from the relay inside the heat pump control panel. Taking the live and neutral outputs from this relay extend this wire to the boiler. Isolate this cable through a fused double pole spur switch and mark on the switch that the supply com