Heating controls
UK boiler wiring diagrams — S-plan, Y-plan, C-plan, W-plan & combi
Almost every UK wet heating system is one of a handful of standard control layouts. Learn the five, and any airing-cupboard wiring centre becomes readable. Below each layout you'll find the wire-by-wire table, and at the end, how each one converts to wireless or smart control from the popular manufacturers.
- Permanent live
- Switched live / demand
- Neutral
- Earth (cpc)
- Wireless / bus (no mains)
Read this first
- These diagrams are generic teaching diagrams. Always work to the actual wiring card supplied with your valve, programmer, boiler and thermostat — terminal numbering varies between manufacturers and model years.
- Anything touching the gas side of an appliance is Gas Safe registered work. The 230 V control wiring is electrical work, but the appliance itself must not be worked on by an unregistered person.
- Safely isolate before opening a wiring centre — see safe isolation. Heating systems very often have two sources of supply (boiler spur and a separate controls spur).
- Boiler control circuits are typically fed from a fused connection unit at 3 A, wired in 1.0 mm² or 1.5 mm² flex/T&E, on its own way or a dedicated circuit.
The five standard layouts
What determines the layout is simply: is the hot water pumped or gravity fed, and how many motorised valves are there?
| Plan | System | Valves |
|---|---|---|
| S-plan | Fully pumped, separate HW and CH zones | Two 2-port zone valves (e.g. V4043H) |
| S-plan+ | Fully pumped with extra heating zones (upstairs/downstairs) | Three or more 2-port valves, one stat per zone |
| Y-plan | Fully pumped, single valve diverts or shares flow | One 3-port mid-position valve (e.g. V4073A) |
| C-plan | Gravity hot water, pumped central heating | One 2-port valve on the CH circuit |
| W-plan | Fully pumped with hot water priority | One 3-port diverter (changeover, not mid-position) |
| Combi | No cylinder, instantaneous hot water | None — just a room stat / programmer to the boiler |
S-plan
The most common modern layout and the easiest to fault-find. Two independent 2-port valves. Each valve has an auxiliary microswitch that only makes when the valve is fully open — that switch is what actually calls the boiler and pump. Both valves feed the boiler through a common switched live.
S-plan — two 2-port zone valves
Hot water demand: programmer HW → cylinder stat → HW valve. Heating demand: programmer CH → room stat → CH valve. Either valve, once open, sends live to the boiler and pump.
| Wire (V4043H 2-port) | Role | Where it lands in S-plan |
|---|---|---|
| Green/Yellow | Earth (cpc) | Earth bar in the wiring centre |
| Blue | Neutral | Neutral bar |
| Brown | Live in — drives the motor open | From the cylinder stat call terminal (HW valve) or the room stat switched live (CH valve) |
| Grey | Feed to the auxiliary microswitch | Permanent live at the wiring centre |
| Orange | Switched live out from the aux switch | Common boiler/pump live terminal — both valves' oranges join here |
Key point: the orange only becomes live once the valve is physically fully open. A boiler that fires with no heat output, or a system that never fires at all, is very often a stuck valve head or a failed aux microswitch rather than a boiler fault.
S-plan+
Identical logic to S-plan, just repeated. Each additional heating zone gets its own 2-port valve, its own room stat and its own programmer channel (or a multi-channel programmer). All the valve oranges still land on the same common boiler live. This is the layout smart multi-zone systems such as evohome sit on top of most naturally.
| Zone | Control | Valve |
|---|---|---|
| HW | Programmer HW channel + cylinder stat | 2-port on the primary to the coil |
| CH 1 (downstairs) | Programmer CH1 + downstairs room stat | 2-port on that heating leg |
| CH 2 (upstairs) | Programmer CH2 + upstairs room stat | 2-port on that heating leg |
| Common | All oranges joined | One switched live to boiler + pump |
Y-plan
One 3-port mid-position valve serves both hot water and heating. The valve can sit in three positions: hot water only, heating only, or mid-position (both). It is more compact than S-plan but harder to fault-find, because a single failed valve motor or synchronous head can take out both services.
Y-plan — one 3-port mid-position valve
The valve's grey, white and orange cores carry the logic. Grey = hot-water-satisfied signal, white = heating demand, orange = switched live to the boiler and pump.
| Wire (V4073A 3-port) | Role | Notes |
|---|---|---|
| Green/Yellow | Earth | Earth bar |
| Blue | Neutral | Neutral bar |
| Grey | Hot water OFF signal | From the cylinder stat 'satisfied' terminal / programmer HW-off. Tells the valve it may move fully to the heating port. |
| White | Central heating ON demand | From the room stat switched live via the programmer CH channel. |
| Orange | Switched live out | To the boiler and pump — made whenever either service is calling. |
Classic Y-plan symptom: radiators get warm when only hot water is selected. That is almost always a mid-position valve that is no longer seating fully on the hot water port — a worn ball/paddle or a tired synchronous motor.
C-plan and W-plan
C-plan — gravity HW, pumped CH
Older layout on open-vented systems. Hot water circulates to the cylinder by thermosyphon with no valve at all; only the heating is pumped and valved. A single 2-port valve sits on the heating flow, driven by the programmer CH channel through the room stat. The cylinder stat still switches the boiler for hot water.
A C-plan cannot meet current Part L boiler-interlock expectations without modification — a gravity primary is normally converted to S-plan when the boiler is replaced.
W-plan — hot water priority
Uses a 3-port diverter (changeover) valve rather than a mid-position one — it can only be in one position at a time. Hot water always takes priority; heating resumes when the cylinder stat is satisfied. Useful on undersized boilers or where cylinder recovery time matters more than continuous heating.
Wiring is simpler than Y-plan: no mid-position logic, just a changeover contact and a single switched live to the boiler.
Combi boilers
No cylinder, no valves, no wiring centre. The only external control is a room thermostat and/or a timer, landing on the boiler's own stat terminals. There are two conventions, and mixing them up is the single most common combi control fault.
| Boiler terminal convention | What it wants | Do not |
|---|---|---|
| Volt-free (dry contact) | A pair of terminals bridged by a link out of the factory. Remove the link and connect a volt-free switch — the thermostat provides no voltage at all. | Never feed 230 V into volt-free stat terminals — it will damage the PCB. |
| Switched live (230 V) | Permanent live out, switched live back. The thermostat needs L, N (if mains powered) and returns the live when calling. | Do not fit a volt-free-only stat here without a permanent live source for the contact. |
| OpenTherm / manufacturer bus | Two-core low voltage, polarity free, modulating control. | Do not run the bus pair in the same multicore as 230 V cores unless the cable is rated for it and segregation is maintained. |
Converting to wireless or smart control
Every smart heating control does one of three things: it mimics a room stat (a switch), it mimics a programmer (one or two timed channels), or it talks to the boiler on a modulating bus. Identify which of the three you're fitting and the wiring answers itself.
1. Stat replacement
A relay/receiver replaces the room stat's switching contact. Existing programmer, valves and cylinder stat all stay. Simplest conversion.
2. Programmer replacement
A two-channel receiver replaces the HW + CH programmer, so the app controls timing for both services. Needs the receiver to be dual channel (e.g. Hive SLR2, Wiser 2-channel, tado° Extension Kit).
3. Bus / modulating
OpenTherm, EMS-2 (Worcester) or eBUS (Vaillant). Two low-voltage cores to the boiler, no relay at all. Best efficiency — the boiler modulates rather than cycling on and off.
Wireless conversion of an S-plan system
The wall thermostat becomes battery powered and talks wirelessly to a mains-powered receiver. The receiver's contact sits exactly where the old room stat's switched live was made off — the valves, cylinder stat and boiler wiring are unchanged.
Manufacturer notes
Google Nest
Learning Thermostat 3rd gen (Heat Link), Thermostat E, Nest Thermostat (4th gen, EU)
- Power
- Heat Link / receiver needs permanent L + N + E. The wall unit is battery or Heat Link powered.
- Output type
- Volt-free changeover contact (common + normally-open). OpenTherm supported on the 3rd gen Heat Link with a compatible boiler.
- Fitting into an existing system
- Heat Link replaces the room stat and normally lives at the wiring centre or beside the boiler. In S-plan the volt-free pair goes in place of the room stat's live-in / demand-out. On a combi it links the boiler's own switched-live or volt-free stat terminals.
Hive (British Gas)
Hive Thermostat SLR1 (heating only) / SLR2 (heating + hot water) receiver + Hub
- Power
- Receiver needs permanent L + N + E. Wall thermostat is battery powered (AA).
- Output type
- Volt-free changeover per channel. SLR2 gives a second channel for hot water — the direct S-plan/Y-plan replacement for a two-channel programmer.
- Fitting into an existing system
- SLR2 is the easiest swap for a fully pumped S-plan or Y-plan: it drops straight onto the old programmer backplate footprint and drives the HW and CH channels. SLR1 suits combis and heating-only systems.
tado°
Wireless Smart Thermostat + Extension Kit, or Wired Smart Thermostat
- Power
- Extension Kit needs permanent L + N + E. Wireless stat is battery powered.
- Output type
- Two relays (heating + hot water) with both volt-free and switched-live wiring options. Supports OpenTherm on compatible boilers.
- Fitting into an existing system
- Extension Kit replaces the programmer; the wireless stat replaces the room stat. On S-plan wire HW to relay 2 and CH to relay 1, and keep the cylinder stat in the HW leg.
Honeywell Home
T6R / T4R wireless, Y87RF Round, evohome (multi-zone)
- Power
- BDR91 relay box needs permanent L + N + E; stat is battery powered.
- Output type
- BDR91 is a single-channel relay (volt-free or switched live). Add a second BDR91 for hot water; evohome adds one relay or valve actuator per zone.
- Fitting into an existing system
- The BDR91 binds wirelessly to the stat and simply sits where the old room stat's switched live was made off — usually at the wiring centre. evohome converts S-plan+ into true multi-zone with individual TRV heads.
Drayton Wiser
Heat Hub (1-channel), Hub-R / Heat Hub 2-channel, Wiser Room Thermostat & smart TRVs
- Power
- Hub is mains powered at the boiler or wiring centre (L, N, E).
- Output type
- Volt-free or switched-live relay output(s); the 2-channel hub replaces a HW + CH programmer directly.
- Fitting into an existing system
- Designed as a drop-in programmer replacement — the 2-channel Hub-R fits S-plan and Y-plan; smart TRVs then add per-room control without extra wiring.
Worcester Bosch
EasyControl CT200, Comfort+ II
- Power
- Low-voltage two-wire EMS-2 bus from the boiler — no mains at the stat.
- Output type
- Modulating bus control (not a simple on/off relay).
- Fitting into an existing system
- Two-core bell wire from the boiler's EMS/BUS terminals. Because it modulates, the boiler runs at lower return temperatures and better efficiency than an on/off stat. Requires a compatible Worcester boiler.
Vaillant
sensoCOMFORT / sensoHOME / vSMART with VR interfaces
- Power
- Two-wire eBUS from the boiler — no mains at the stat.
- Output type
- Modulating eBUS control, weather compensation with an outside sensor.
- Fitting into an existing system
- Polarity-free two core to the boiler's eBUS terminals. Remove any 24 V/230 V link across the boiler stat terminals when converting.
OpenTherm generally
Ideal Halo, Viessmann ViCare, Nest 3rd gen, tado°, Salus, Honeywell
- Power
- Two-wire OpenTherm bus, low voltage, polarity free.
- Output type
- Modulating — the boiler is told what flow temperature to produce instead of just on/off.
- Fitting into an existing system
- Only works if the boiler has OpenTherm terminals. Do not mix an OpenTherm pair into a 230 V zone-valve circuit — keep the bus separate and correctly segregated from mains cores.
Smart control compatibility matrix — UK-available products only
Every product listed is sold and supported in the UK on 230 V wiring. Yes = suits the system as supplied. With extras = needs a second receiver, a bus/OpenTherm bridge, a zoning kit or a retained programmer. No = not suitable.
| Product | Channels | S-plan | Y-plan | Combi | Hot water | OpenTherm | Smart TRVs |
|---|---|---|---|---|---|---|---|
| Google NestLearning Thermostat 3rd gen (UK, Heat Link) | 1 or 2 (2nd Heat Link for HW) | Yes | Yes | Yes | With extras | Yes | No |
| Google NestNest Thermostat (4th gen, UK) | 1 | Yes | Yes | Yes | No | No | No |
| HiveThermostat SLR1 receiver + Hub | 1 | Yes | Yes | Yes | No | No | With extras |
| HiveThermostat SLR2 receiver + Hub | 2 (CH + HW) | Yes | Yes | Yes | Yes | No | With extras |
| tado°Wireless Smart Thermostat + Extension Kit (V3+/X) | 2 (CH + HW) | Yes | Yes | Yes | Yes | Yes | Yes |
| tado°Wired Smart Thermostat V3+ | 1 | Yes | Yes | Yes | No | Yes | Yes |
| Honeywell HomeT4R / T6R wireless + BDR91 relay | 1 per BDR91 | Yes | Yes | Yes | With extras | With extras | No |
| Honeywell Homeevohome (multi-zone) | Up to 12 zones | Yes | With extras | Yes | Yes | With extras | Yes |
| DraytonWiser Heat Hub (1-channel) | 1 | Yes | Yes | Yes | No | With extras | Yes |
| DraytonWiser Heat Hub 2-channel / Hub-R | 2 (CH + HW) | Yes | Yes | Yes | Yes | Yes | Yes |
| Worcester BoschEasyControl CT200 | Bus (mod.) | With extras | With extras | Yes | Yes | Yes | Yes |
| VaillantsensoCOMFORT VRC 720 / sensoHOME | Bus (mod.) | With extras | With extras | Yes | Yes | No | Yes |
| IdealHalo RF / Halo Combi | 1 (+HW on system models) | Yes | Yes | Yes | With extras | Yes | No |
| ViessmannViCare Thermostat / Climate Sensor | Bus (mod.) | With extras | With extras | Yes | Yes | Yes | Yes |
| SalusRT310i / iT600 with KL08RF or RXRT510 | 1 or 2 with additional receivers | Yes | Yes | Yes | With extras | With extras | Yes |
| HeatmiserneoStat + neoHub (+ neoStat HW) | Multi-zone | Yes | With extras | Yes | Yes | With extras | Yes |
| Tapo / TP-LinkTapo H100 + KE100 TRV heads | TRV only | No | No | No | No | No | Yes |
Google Nest — Learning Thermostat 3rd gen (UK, Heat Link)
Output: Volt-free changeover / switched live, OpenTherm
Heat Link sits at the wiring centre or boiler. Hot water needs a second Heat Link or retaining the existing programmer channel.
Google Nest — Nest Thermostat (4th gen, UK)
Output: Volt-free / switched live via Nest Temperature Control base
Heating only. Keep the existing programmer for stored hot water.
Hive — Thermostat SLR1 receiver + Hub
Output: Volt-free changeover
Heating-only receiver. Hive Radiator Valves add per-room control.
Hive — Thermostat SLR2 receiver + Hub
Output: Volt-free changeover per channel
Direct two-channel programmer replacement — the easiest S-plan / Y-plan swap.
tado° — Wireless Smart Thermostat + Extension Kit (V3+/X)
Output: Volt-free or switched live relays, OpenTherm
Extension Kit replaces the programmer; wireless stat replaces the room stat. Keep the cylinder stat in the HW leg.
tado° — Wired Smart Thermostat V3+
Output: Volt-free / switched live, OpenTherm
Wires in at the existing room-stat position where a permanent live is present.
Honeywell Home — T4R / T6R wireless + BDR91 relay
Output: Volt-free or switched live relay
Add a second BDR91 for hot water. OpenTherm needs the R8810A OT bridge instead of the relay.
Honeywell Home — evohome (multi-zone)
Output: BDR91 relays / HR92 radiator actuators, OpenTherm via R8810A
Converts S-plan+ into true multi-zone. Y-plan needs converting to zone valves for real zoning.
Drayton — Wiser Heat Hub (1-channel)
Output: Volt-free or switched live, OpenTherm on Hub-R
Drop-in single-channel programmer replacement, smart TRVs add zoning without rewiring.
Drayton — Wiser Heat Hub 2-channel / Hub-R
Output: Volt-free or switched live, OpenTherm (Hub-R)
Best-value two-channel conversion for a stored hot water system.
Worcester Bosch — EasyControl CT200
Output: EMS-2 two-wire bus, OpenTherm mode available
Bus control only — no mains at the stat. Zoned stored-water systems need the Worcester zoning kit or retained programmer.
Vaillant — sensoCOMFORT VRC 720 / sensoHOME
Output: eBUS two-wire, polarity free
Weather compensation with the outside sensor. Remove any factory link across the boiler stat terminals.
Ideal — Halo RF / Halo Combi
Output: OpenTherm or volt-free
Modulates on Ideal boilers with OpenTherm; falls back to on/off elsewhere.
Viessmann — ViCare Thermostat / Climate Sensor
Output: OpenTherm / Viessmann bus
Best paired with a Viessmann boiler and the Vitoconnect module.
Salus — RT310i / iT600 with KL08RF or RXRT510
Output: Volt-free / switched live, OpenTherm on selected models
Common trade choice; iT600 handles UFH manifolds as well as radiators.
Heatmiser — neoStat + neoHub (+ neoStat HW)
Output: Volt-free / switched live wiring centre outputs, OpenTherm via neoStat-hw/OT
Strongest option for multi-zone and underfloor heating projects.
Tapo / TP-Link — Tapo H100 + KE100 TRV heads
Output: No boiler switching
Radiator control only — it cannot fire the boiler, so keep an existing stat/programmer and maintain the interlock.
Practical conversion checklist
- Photograph the existing wiring centre and programmer backplate before disturbing anything, and label every core.
- Confirm whether the boiler stat terminals are volt-free or 230 V switched live, and whether a factory link needs removing.
- Check the receiver has a permanent live and neutral available — old room stat drops frequently have no neutral at the stat position, which is why most kits put the relay at the wiring centre instead.
- Keep the cylinder stat. Smart controls rarely replace it, and it is your scald and Legionella safeguard on a stored-water system.
- Don't lose the boiler interlock: the boiler must switch off when no zone is calling. A permanently linked-out stat terminal breaches Part L.
- Record the alteration — a change to fixed wiring warrants a Minor Works Certificate with the relevant tests.
Disclaimer
Provided for guidance and educational use only. Verify everything against the manufacturer's instructions, BS 7671:2018+A4:2026 and site conditions before carrying out work. All Cabling© Limited accepts no liability for loss, damage or non-compliance arising from use of this page.