Wiring Diagram for S Plan Zoned Central Heating Systems Find wiring diagrams and info for C Plan, S Plan / Twin Zone, W Plan & Y Plan / Biflow Central Heating Systems and Boiler Connections with Pump over-run in the ERA- Electrician’s Resources Archive. Electrician’s Blog . Tagged as: Central Heating, Honeywell, S Plan Y plan wiring Danfoss | Screwfix Community Forum Nov 22, 2008Hi guys, Got to wire a y plan Monday and am a litle unsure. The wiring diagram shows an external pump. Plumber has supplied the Danfoss wiring centre which seems pretty straight forward BUT I am not sure of the connection from the switched side of the boiler.