Wiring of Y Plan Biflow Central Heating Systems

by Editor on February 22, 2012

This article shows a Y Plan wiring diagram and gives an overview of the electrical control wiring connections for a typical Sundial / Honeywell YPlan or Drayton Biflow fully pumped central heating system which use a mid position motorised valve.

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Y Plan Biflow Wiring Diagram

Y Plan Biflow Wiring Diagram

(To enlarge image click once, and then double click in new window)

Please note that this is a guide only and the terminal numbers and wiring colours of various heating systems may vary.

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The Y Plan or Biflow is a fully pumped hot water and heating system. It shares the flow between the hot water or heating, depending on the demand, using a single mid position motorized actuator valve.

From a connection point of view, the difference between the wiring of a Y-Plan system and most others is that it requires a HW OFF connection from the cylinder stat and programmer to the motorized valve (normally a grey wire). This connection enables the boiler and pump to operate for heating whilst the hot water is ‘off’ or ‘satisfied’.  The Flow Share or Mid Position valve allows the motor driven actuator to operate in either direction as well as to a mid position unlike a spring return diverter valve.


Typical YPlan Motorised Actuator Valve: Honeywell Sundial 4073 Flow Share or Drayton Biflo 3 Port Mid-Position valve.
Function: Shares flow between Hot Water and Heating.
Typical Wiring:
Brown- Live to motor
White- CH ON
Orange- HW ON & feed to Boiler/Pump
Grey- HW OFF
Blue- Neutral
G/Y- Earth

More info:  How does a Mid Position Valve Work?

Installation Considerations

Key considerations for Y-Plan electrical installation wiring and connecting of a Y Plan or Biflow system are:

  • Overview of the system’s operation- How is it required to work in that particular installation?
  • Siting of the main power double pole 3 amp fused spur isolator near boiler or wiring centre junction box
  • Siting of the Room Stat- away from radiators and heat sources
  • Boiler over run may require additional wiring to Wiring Center
  • Frost Stat protection- pipework outside or in loft
  • All new domestic central heating electrical wiring installations should be notified to Local Building Control under the Part P of the Building Regulations.

Room Thermostats

Room thermostats are used to enhance temperature control and energy efficiency. They should be used in addition to thermostatic radiator valves. Room stats must be sited so as to measure an average room temperature and normally at a height of 1500 mm. Be careful not to fit them near to radiators or other sources of heat or in cold draughts. As can be seen from the Y Plan wiring diagram above, the switched output from the room stat connects directly with the heating (white) connection to the motorized valve.

Cylinder Thermostats

Cylinder Stats are normally mounted on the bottom half of a hot water storage cylinder or tank. They must be connected using heat resisting cable due to high ambient temperatures surrounding central heating plumbing and their close proximity to insulation and hot surfaces. 3 core and 4 core flexible cable should be used depending on the system requirements. Never use the earth green and yellow core as a live conductor even if it is colour sleeved!

Frost Protection

Frost Thermostats are used to protect vulnerable pipe work from freezing and are best sited where they exposed to exterior temperatures. The function of most Frost Stats is to cause the boiler to fire and operate a circulation pump via a direct feed when the temperature drops to a preset low level. A frost stat in a Y Plan installation would best connect between a permanently ‘live’ connection of the system and the boiler ‘feed’ from the motorized valve (orange). Frost stats don’t usually require a neutral connection.


The YPlan time control is from a 2 channel programmer for independent control of the hot water and heating sides of the system.
A programmable room thermostat stat and cylinder stat can also be used (hard wired or wireless options).

Boiler Connections

Many boilers may have a pump over run feature where the pump is fed from it’s own connection at the boiler control connection panel. This may involve additional wiring to a remote pump where it is not installed within the boiler casing. Find more info about Connecting Boilers.

More Info
You can find more 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.

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