Wiring Diagram of S Plan Central Heating System

by Editor on February 22, 2012

This article shows a typical S Plan wiring diagram and gives an overview of the electrical control wiring connections for a Sundial / Honeywell SPlan or Drayton Twin Zone Central Heating system.

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S Plan Twin Zone Central Heating Wiring Diagram

S Plan Twin Zone Central Heating Wiring Diagram

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

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System Operation

S Plan or SPlan Plus are fully pumped central heating systems using individual motorized zone valves to control domestic hot water and heating.

The wiring connections of SPlan heating systems may at first appear complicated because they use more than one motorized valve. Despite this, the interconnection of the thermostats and valves is relatively straight forward compared to single valve Flow Share and Biflow (mid position) or diverter valve systems. The switching operation of each side of the system follows the same sequence of; Programmer to Thermostat to Valve. The wiring connections are made even simpler because there are no ‘HW OFF’ connections at the programmer or the thermostats.

Motorised Zone Valves

Typical Motorised Actuator Valves: 2 x Sundial / Honeywell 4043 or Drayton Twin Zone 2 Port Spring return zone valves.

The Hot Water valve (located nearest to the cylinder) energizes to open and allow flow to HW.
The Heating valve energizes to open and allow flow to CH.
Each valve de-energizes to close by spring return.

Typical Wiring Colours:
Brown- Live to motor
Grey- Permanent live feed
Orange- Feed out to Boiler/Pump
White- Not required
Blue- Neutral
G/Y- Earth

In addition to an actuator motor (brown wire) the 4043 motorized valve contains an auxiliary micro-switch which turns on (clicks after 18 seconds) when open. This switches the (grey) permanently live input to the output (orange) which brings on the boiler/pump.

Note- White Wire. When you come to connect an SPlan heating system you may find that some  4043 2 Port motorised valves will have a white wire. This is used when the valve is installed as part of a C Plan system. The white wire is not required for S Plan and should be made electrically safe by terminating it on its own in the wiring centre junction box.

S Plus Plan- Extra Zones

The Honeywell Sundial S Plus Plan allows for additional motorized zone valves to be installed and controlled from local room thermostats for independent control of additional zones in larger systems.

Installation considerations

Key considerations for SPlan electrical installation wiring and connecting are:

  • Overview of the system’s operation- How is it required to work in that particular installation?
  • Identification of the Hot Water and Heating Valves
  • 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

Room Thermostat

Room stats are used to enhance temperature control and energy efficiency. They should be used in addition to thermostatic radiator valves. They 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 S Plan wiring diagram above, the room stat ‘calling’ opens the heating motorised valve when calling for heat (brown).

Cylinder Thermostat

When the cylinder stat calls for heat it opens the hot water motorised valve (brown).

Cylinder Stats 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!

Frost Protection

Frost Thermostats are used to protect vulnerable pipe work from freezing and are best sited where they exposed to outside 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 S Plan or Twin Zone installation would normally connect between the permanent ‘live’ of the system (grey) and the boiler feed connection from the valves (orange). Frost Stats don’t usually require a neutral connection.


Time control is from a 2 channel programmer for independent control of hot water and heating.
Programmable cylinder stat and room stats 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. More info about Connecting Boilers.

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More Info
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.

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{ 6 comments… read them below or add one }

darren August 30, 2015 at 18:05

Need some advice with regards to wiring s plan heating from the valves to a Klover bio mass boiler.
Do you just run the orange wire from the two valves to the terminal connection on the back of the boiler to switch the boiler on, or the orange and grey; in addition, does it matter which side the orange and if requred the grey enter the connection block on the back of the boiler.

muke October 7, 2013 at 18:10

the return pipe on the hw is the problem it should connect to the return nearest to the boiler the heating circuit

heating supplies Ireland November 1, 2012 at 06:17

This is great post about S Plan Heating System which is really very must expressive and descriptive.
The Block Diagram is clearly understandable with system operation defined.

Rob February 23, 2012 at 20:20


Had me scratching my head for a while, told customer to call heating engineer so hopefully wont hear back for her.

Editor February 23, 2012 at 19:03

Hi Rob
Yes I agree it sounds like a pipework problem. If room stat is controling CH valve and cyl stat is controling HW valve and you can feel the valve actuators moving accross I would call in a heating engineer to check plumbing.

Rob February 22, 2012 at 20:32

Hi need some help.

S plan heating system with 2 valves, one for central heating the other hot water.
Called out because heating comes on when HW only option selected on 2 channel time clock. With HW off and CH on, CH works fine through room stat. However as soon as HW is selected radiators become red hot to touch and don’t cool down with CH in off position.
Both valves have been checked for correct operation / wiring, every thing looks ok.
CH 2 port valve is closed when CH is off and remains closed when HW is turned on but yet radiators heat up?
Thinking it could be a problem with pipe work as customer has not known it to work correctly.
Any ideas?

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