How S-plan and Y-plan controls create a boiler demand, how to supply and isolate them safely, and how to test and fault-find them in a logical order.
How a demand reaches the boiler
In a conventional S-plan, each thermostat controls its own spring-return two-port valve. A demand drives that valve open; removing the motor feed lets it close. The separate auxiliary contact then provides the boiler’s switched demand once the actuator has opened. The contacts from both valves join at the wiring centre, so either zone can request heat. A basic circuit also feeds the pump there; a boiler requiring pump overrun uses its specified pump connection and permanent supply. The contact indicates actuator travel, not proven water circulation.
A Y-plan mid-position valve rests on its hot-water port when unpowered. A heating demand drives it fully to the heating port when hot water is not wanted (signalled on its own core) or to mid-position when both services call. With hot water alone, the cylinder thermostat fires the boiler directly.
In the Resideo guide’s V4073A arrangement, white carries the heating demand. Grey is energised by the programmer’s hot-water-off output or the cylinder thermostat’s satisfied contact. With a heating call present, that grey signal drives the actuator to the heating-only position; without it, the actuator stays at mid-position and does not make its normal heating-only boiler output. An open water path does not itself prove that the boiler fires or the pump runs. Orange provides the heating-only boiler demand. Use the diagram for the exact valve and demand state before relying on any core colour.
Supply, isolation and frost protection
Follow the boiler and controls manufacturers’ supply instructions. The Resideo Honeywell Home May 2024 guide shows 3 A fuses for its gas-appliance diagrams and specifies at least 3 mm separation in all poles of the mains isolating switch. It also requires fusing appropriate to the appliances connected. Where one common supply is specified, bring the boiler and connected controls under that isolation arrangement. A boiler that needs pump overrun must retain its specified permanent live during normal operation. Frost and pipe thermostats wired in series need a supply the programmer does not switch off, following the chosen system diagram.
Identify all supplies before work, including any separately fed controls. Isolate securely, control the means of reconnection and post a notice. If isolation relies on removing a fuse, keep it under your control or secure the enclosure as required. Then neither the fuse nor a replacement can be put back while work is in progress. Prove dead at the work point with a suitable two-pole detector, proving the detector before and after use on a compatible proving unit or known live source. Account for stored energy and any supply that could enter through another connection.
Site a frost thermostat in the coldest part of the unheated space it protects, set to call for heat at about 5°C. Wire a pipe thermostat in series with it to end the demand once the pipework has warmed. Its position and setting vary by maker. Worcester Bosch puts it on the pipe furthest from the heated area at 15°C; Resideo puts it on the boiler return at 25°C. Follow the instructions for the controls fitted.
Testing after wiring
Follow the appliance’s prescribed tests after wiring. The Greenstar Ri manual, for example, lists earth continuity, short circuit, polarity and resistance to earth after electrical connections have been remade. Use suitable dead tests before energising and protect electronics as the instructions require. Then prove the operating states: heating, hot water, both together and frost protection. When normal demands end, check that normal firing ends while required pump overrun and protective functions remain available.
Check the equipment class in its instructions. The Resideo guide’s Class I zone valves need their protective conductor connected. Its Class II thermostats and programmers do not rely on protective earthing of the device. A provided earth parking terminal holds the sleeved conductor safely and maintains any onward continuity. A parking terminal does not mean a double-insulated device needs an earth connection to its case.
Fault-finding along the demand path
Use the wiring diagram for the selected operating state. In a conventional S-plan, follow the programmer, thermostat, valve feed, actuator position, auxiliary output and boiler demand input. An unexpected reading narrows the next checks; wiring, connections and other feeds can affect it. Y-plan follows different paths, and its V4073A can cause reduced back-fed readings. Follow the model’s test sequence rather than condemning a part from one voltage. Any disconnection or reconnection requires secure isolation and proving dead. Necessary live measurements belong to a competent person using suitable GS38 test equipment and precautions.
OpenTherm controls
An on/off thermostat opens or closes a demand circuit, which may be mains voltage or a lower-voltage input. Volt-free contacts supply no voltage themselves; the connected circuit determines their working voltage. OpenTherm instead exchanges control information over its specified pair, including a requested heating-water temperature. A boiler can also modulate internally under an on/off demand, so on/off does not mean continuous full-rate firing.
Power arrangements depend on the model. A T4M takes power through its OpenTherm link. The T6 family uses a separately supplied receiver and its specified thermostat power arrangement. Connect the OpenTherm pair only to compatible OT terminals, keep it separated from mains as specified, and follow the actual product instructions.