Thermostats
Digital Room Stat (volt-free)
In short
A digital room thermostat (stat) runs on batteries, so it needs no live (L) or neutral (N) of its own. Its terminals: 1, 2, 3. Units based on makers’ products sit under it in the parts list, such as the Siemens RAA20 and the ESi ESRTM.
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 DIGITAL ROOM THERMOSTAT (stat) runs on batteries, so it needs no live (L) or neutral (N) of its own.
Inside is a VOLT-FREE changeover switch: nothing inside feeds it. It carries whatever you put on its common, 1. While it calls, 1 is joined to 2, the normally open (NO) contact. While it is satisfied, 1 is joined to 3, the normally closed (NC) contact.
The numbers are this simulator's own. A branded thermostat prints its own, but the call is the NO contact whatever its number. That is one volt-free contact, for any voltage, with no satisfied contact at all.
It comes set at 21 °C, so in the 17 °C room it calls at once.
Terminals
| Terminal | What it is for | On the S-Plan example |
|---|---|---|
| 1 (COM) | The common (COM). Feed it with the heating demand. | Wiring centre block 4 |
| 2 (NO) | Normally open (NO): the CALL out. It is joined to 1 while the thermostat calls. | Wiring centre block 5 |
| 3 (NC) | Normally closed (NC): joined to 1 while the thermostat is satisfied. | — |
Read from the wired example circuits: S-Plan with a digital room stat. A wiring-centre block carries whatever is landed on it; the numbers are the habit the lessons teach.
What the meter reads
Feed 1 from the programmer's central heating on (CH ON), and take 2 to the heating valve. It then switches the heating the same way as the room thermostat (stat).
Nothing comes out of it that you did not put in.
Units based on makers’ products
In the parts list these sit under the Digital Room Stat. 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 Siemens RAA20
- Based on the ESi ESRTM
- Based on the Honeywell T4360A
- Based on the Danfoss TP5000 Si
- Based on the ESi ESRTP4+
- Based on the Salus RT510
Where it connects
Lessons that use it
- The digital room thermostat
Foundation · The parts
- Job: fit a digital room thermostat
Foundation · Jobs
- Swap the room thermostat for a digital (volt-free) one
Quick Jobs
Wiring diagrams it is part of
Read the explanation
- How to read the wiring diagram on the back of a programmer or thermostat
The terminal map on a programmer or thermostat: L and N, the numbered outputs, COM, NO and NC, mains-switched against volt-free, and the installer’s link.
Practise it in the simulator.
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