Wiring of W Plan Central Heating Systems

by Editor on February 22, 2012

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

W Plan Central Heating Wiring Diagram

W 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 connection numbers and wiring colours of different systems may vary.


The W Plan is a fully pumped hot water priority system. It’s single motorised valve diverts to either the hot water or heating but not both at the same time. This can cause problems for central heating systems in winter if there is a high demand for domestic hot water.

Electrician’s Comment

When at first you look at this system’s wiring connections you may think that the cylinder stat is wired incorrectly. In most other system configurations the ‘C’ terminal would normally connect directly to the ‘hot water on’ terminal from the programmer (or timer), but in this case it connects in series with the output from the room thermostat and also directly to the boiler/pump. When hot water temperature is satisfied, the cylinder stat turns off and switches the output from the room stat to energize the motorized diverter valve which then sends water to the heating system.

Diverter Valve

Typical Motorized Actuator Valve: Honeywell Sundial 4044- 3 Port Spring return diverter valve.
Diverts flow to Hot Water when de-energized / Diverts flow to Heating when energized.
Typical Wiring:
Brown- Live to motor opens valve for ‘heating on’.
Blue- Neutral
G/Y- Earth

Installation Considerations

Key considerations for electrical installation wiring and connecting of a W Plan 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 close to 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 where exposed to cold draughts.

As can be seen from the W Plan wiring diagram, the output from the roomstat connects to the cylinder stat and to the boiler/pump but does not directly operate the diverter valve.

Cylinder Thermostats

The cylinder stat is at the center of the W Plan system and is the main component in the opening and closing the spring return biflow 3 port. Whether in the HW or CH position, the ‘Common’ terminal is always live and will send a feed to the pump/boiler. Because this is a hot water priority system, a ‘HW calling’ situation will always cause the diverter valve to de-energize and send hot water to the cylinder.

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 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 signal and cause the boiler to fire and operate a circulation pump via a direct electrical feed when the temperature drops to a preset low level. A frost stat in a W Plan installation would ideally connect between a permanently ‘live’ connection of the system and the boiler/pump feed. Frost stats don’t usually require a neutral connection.


Time control can be from a 2 channel programmer for independent control of HW and CH or from programmable cylinder stat and room stat (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 out more about Connecting Boilers.

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.

Electrician’s Blog.co.uk

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