Thermostats
Room Thermostat
In short
A room thermostat (stat) is a switch worked by the temperature of the room. It tells the heating when the room is cold. Its terminals: L, N, SL. Units based on makers’ products sit under it in the parts list, such as the Drayton RTS1.
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
What it is
A ROOM THERMOSTAT (stat) is a switch worked by the temperature of the room. It tells the heating when the room is cold.
When the room is more than half a degree below its setting, it CALLS for heat: its live (L) is joined to its switched live (SL). When the room reaches half a degree above its setting, it is SATISFIED: SL is dead.
In between, it stays as it was. That gap of 1 degree is its DIFFERENTIAL: it stops the thermostat switching on and off again and again at the setting.
Its neutral (N) is for its own small heater, the ACCELERATOR. It warms the thermostat a little while it calls. So the thermostat switches off a little sooner, and the room does not overshoot.
It comes set at 15 °C, below the 17 °C room, so it is satisfied until you turn it up.
Terminals
| Terminal | What it is for | On the S-Plan example | On the Y-Plan example | On the Combi example |
|---|---|---|---|---|
| L (Live) | The live (L) in: the heating DEMAND, from the programmer's central heating on (CH ON). | Wiring centre block 4 | Wiring centre block 4 | Room Thermostat #1: wiring centre block 1; Room Thermostat #2: wiring centre block 1 |
| N (Neutral) | The neutral (N), for the thermostat's own heater, the accelerator. The drawing shows it, but it is not modelled here: in this simulator a resistance test between N and the switched live (SL) reads open (OL). | Wiring centre block 2 | Wiring centre block 2 | Room Thermostat #1: wiring centre block 2; Room Thermostat #2: wiring centre block 2 |
| SL (Switched Live) | The switched live (SL) out: the CALL. It goes to the heating valve's motor. | Wiring centre block 5 | Wiring centre block 5 | Room Thermostat #1: wiring centre block 5; Room Thermostat #2: wiring centre block 9 |
Read from the wired example circuits: S-Plan, Y-Plan, Combi boiler with two heating zones. A wiring-centre block carries whatever is landed on it; the numbers are the habit the lessons teach.
What the meter reads
Turn the setting above the room, to 18 °C in a 17 °C room. The flame shows: it calls, and its switched live (SL) is live.
Turn it below the room, to 16 °C in a 17 °C room. The flame goes: it is satisfied, and SL is dead.
Set level with the room, at 17 °C, it stays as it was: the room is inside its differential.
Units based on makers’ products
In the parts list these sit under the Room Thermostat. Each is the simulator’s own unit, simplified for teaching, and may differ from the product it is based on: on site, read the unit’s own instructions. DrawWarm is not part of these makers and is not approved by them.
- Based on the Drayton RTS1
Call-outs with this part on the path
- No heating: hot water fine, radiators cold
“Hot water's fine but the radiators are stone cold, whatever I do with the thermostat.”
- Radiators hot with everything off at the timer
“The radiators get warm when nothing's asking for heat - it's all off at the timer and they're still hot.”
Where it connects
Lessons that use it
- The Manual, History and the examples
Foundation · Using the simulator
- The words, and why they are what they are
Foundation · Reading a circuit
- A thermostat is a switch
Foundation · Reading a circuit
- Boiler Plus and the interlock
Foundation · Electricity for heating controls
- The room thermostat
Foundation · The parts
- The Draw Warm Method: the parts on the wall
Foundation · The Draw Warm Method: S-Plan
- Block 1: the permanent lives
Foundation · The Draw Warm Method: S-Plan
- The Draw Warm Method: the parts on the wall
Foundation · The Draw Warm Method: Y-Plan
Wiring diagrams it is part of
- S-Plan
S-Plan
- S-Plan with a frost thermostat
S-Plan
- S-Plan Plus
S-Plan
- Y-Plan
Y-Plan
- Y-Plan with a frost thermostat
Y-Plan
- Combi boiler with two heating zones
Combi
Practise it in the simulator.
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