Wiring of C Plan Central Heating System

by Editor on February 22, 2012

This article shows a typical C Plan wiring diagram and gives an overview of the electrical control wiring connections for a Sundial / Honeywell C Plan Central Heating system.

C Plan Central Heating Wiring Diagram

C Plan Central Heating 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 systems may vary.


The C Plan is a gravity hot water system with pumped heating.
A single motorized actuator valve (spring return to close) is normally installed on the domestic hot water pipe work.  The valve closes when the cylinder thermostat turns off as the HW is satisfied. This allows priority to the CH radiator circuit. A room thermostat controls the CH pump and a mirco-switch inside the valve sends a feed to the boiler (normally the orange wire).

Motorised Valve

Typical Motorised Actuator Valve: 1 x 4043 2 Port Spring return motorised zone valve.

Valve energizes to open and allow flow to HW & de-energizes to close and allow flow to CH

Typical Wiring:
Brown- Live to motor from cylinder stat
Grey- Permanent live feed
Orange- Feed out to Boiler
White- Feed out to CH Pump/output from Room Stat
Blue- Neutral
G/Y- Earth

The 4043 motorized valve contains an auxillary 2 way switch, the ‘Common’ of which is the Orange (Boiler).
In the HW (energized / open) position the switch connects the Grey (perm feed) to the Orange (Boiler).
In the CH (non-energized / closed) position the switch connects the roomstat output & pump supply to the Orange (Boiler).

Installation Considerations

Key considerations for electrical installation wiring and connecting of a C Plan Gravity hot water 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 central heating electrical wiring installations should be notified to Local Building Control under the Part P scheme.

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 C Plan wiring diagram above, output from the roomstat connects directly to the heating system pump and the white of the 4043 motorised valve.

Cylinder Thermostat

The Cylinder Stat connects from the HW on terminal of the programmer / timer and switches to open the motorised valve when hot water is needed.

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!

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 C Plan installation would normally connect between a permanently ‘live’ feed of the system and the ‘heating’ signal to the valve (white). 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.

More Info
Find wiring diagrams and info for CPlan, SPlan / Twin Zone, W Plan & YPlan / Biflow Central Heating Systems and Boiler Connections with Pump over-run in the ERA- Electrician’s Resources Archive.

Electrician’s Blog.co.uk


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