Helpful video reference. We use Drayton's product guidance video "Drayton RTS2 Room Thermostat: How is it wired to my central heating system?" as the video reference here. Drayton is a UK manufacturer with decades of experience in domestic heating controls and their RTS2 is one of the most common wired room thermostats in UK homes. The video clearly shows the three-terminal wiring arrangement and how it integrates with the S-plan or Y-plan system.
1. Understand what a wired room thermostat does
A simple wired room thermostat is a temperature-controlled switch in series with the heating circuit. When the room is below the set temperature, the thermostat closes its contact and allows a switched live signal through to the boiler (or zone valve). When the room reaches the set temperature, the contact opens and the boiler stops.
In a standard S-plan system, the programmer first sends a switched live to the zone valve. When the zone valve opens fully, its end switch connects a second live to the room thermostat. The thermostat then passes that live on to the boiler if the room is below the set temperature. All three devices must be calling simultaneously for the boiler to fire.
This guide covers replacing a wired analogue or simple digital room thermostat such as the Drayton RTS2, Honeywell T6360, or Danfoss RT52. For smart thermostats such as Hive or Nest, see our separate guide to wire a smart thermostat.
2. Choose the right replacement thermostat
Most wired analogue room thermostats are interchangeable as long as you match the voltage (230V) and the wiring interface: live in and switched live out, with or without a separate neutral. The Drayton RTS2 has a neutral terminal. The older Honeywell T6360B does not: it has just two active terminals (call and switched call) and uses no neutral, so the wiring is simpler but the replacement must also be a two-terminal type if you want to avoid adding a neutral cable.
Check what comes out of the wall before buying a replacement. Count the wires and match accordingly.
3. Isolate the supply
At the programmer or time clock, set heating to off. Go to the consumer unit (fuse box) and switch off the circuit supplying the boiler and controls, usually a 6A or 3A MCB or fuse. Then switch off the boiler isolation switch if there is one. Return to the thermostat and use a voltage tester on each terminal: all should read zero. Only then proceed.
4. Photograph the existing wiring
Unclip or unscrew the thermostat from its backplate. The backplate stays on the wall for now. Take a clear, well-lit photo of the wiring arrangement on the backplate showing each wire and its terminal number. If the cable sheaths are damaged or the colours are hard to read, label the wires with tape before disconnecting anything.
5. Remove the old backplate
Loosen each terminal screw, withdraw the conductors, and unscrew the backplate from the wall. Note which direction the cable enters from: you will need to route the cable through the new backplate the same way. If the wall has a pattress box, the new backplate must fit the same size.
6. Fix the new backplate and connect the wiring
Position the new backplate using a spirit level. Mark and drill if needed. Feed the cable through the backplate's cable entry and fix the plate to the wall. Now connect the conductors using the wiring diagram supplied with the new thermostat and your reference photo. For a Drayton RTS2 this means: blue conductor to terminal 1 (neutral), brown incoming from the programmer's switched live to terminal 2 (live in), and brown outgoing to the boiler or zone valve to terminal 3 (switched live out). Tighten each screw firmly and tug each conductor gently to confirm it is held.
If the cable has a bare earth conductor, sleeve it with green-and-yellow sleeving and connect it to the earth terminal if the thermostat backplate has one. Many plastic thermostat backplates have no earth terminal, which is correct for a double-insulated device.
Clip or screw the thermostat body onto the backplate.
7. Test the heating response
Restore the supply at the consumer unit and the boiler isolation switch. Set the programmer to call for heating. Turn the thermostat temperature above room temperature. Wait two to three minutes: the boiler should fire. Now turn the thermostat temperature to its lowest setting (5°C). The boiler should stop within a minute.
If the boiler fires but never stops regardless of the thermostat setting, you almost certainly have the live in and switched live out transposed on the backplate. Switch off, recheck against the wiring diagram, and correct the terminals before switching back on.
When to call us
If the controls are a tangle of old cables, or if the system has never worked properly since a previous owner fitted something, it is worth having the whole control wiring traced and mapped properly before fitting a new thermostat. Richard can fault-find central heating electrical supplies across Sandwich and east Kent and confirm what is actually happening before you spend money on parts.
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