Thermostats
Frost Thermostat
In short
A frost thermostat (stat) is a switch worked by the air where it is fitted. It is never fitted outdoors. Its terminals: L, N, SL.
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 FROST THERMOSTAT (stat) is a switch worked by the air where it is fitted. It is never fitted outdoors. It keeps the pipes and the boiler from freezing.
It has two usual places. One is the coldest UNHEATED room with pipes, such as a loft, a garage or a boiler room. There it is set at 5 °C, with a pipe thermostat (pipe stat) in series. The other is a room with a radiator, set at 12 to 16 °C, to protect the whole system.
Below its setting it CALLS for heat, whatever the programmer says. It comes set at 5 °C, and its dial runs from 0 to 10 °C.
It is fed with a PERMANENT LIVE: a live with 230 volts (230 V) on it all the time the spur is on. So it can start the boiler on a freezing night.
In this simulator it feels the Conditions card's Unheated room.
On site a PIPE THERMOSTAT (pipe stat) on the return is wired in series with it. Then a system that is already warm is not run.
Terminals
| Terminal | What it is for | On the S-Plan example | On the Y-Plan example |
|---|---|---|---|
| L (Live) | The PERMANENT LIVE (L) in: live all the time the spur is on, whatever the controls do. | Wiring centre block 1 | Wiring centre block 1 |
| N (Neutral) | The neutral (N) for the thermostat. Many frost thermostats need no neutral. This one takes one. | Wiring centre block 2 | Wiring centre block 2 |
| SL (Switched Live) | The switched live (SL) out: the CALL. It goes to the heating valve's motor, past the programmer and the room thermostat (stat). | Wiring centre block 5 | Wiring centre block 5 |
Read from the wired example circuits: S-Plan with a frost thermostat, Y-Plan with a frost thermostat. A wiring-centre block carries whatever is landed on it; the numbers are the habit the lessons teach.
What the meter reads
Set it above the Unheated room on the Conditions card, and it calls: its switched live (SL) is live. Set it below, and it is satisfied: SL is dead.
Call-outs with this part on the path
- 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 frost thermostat
Foundation · The parts
- Job: fit a frost thermostat
Foundation · Jobs
- Frost and pipe stats together
Intermediate · Protection and extras
- Combi frost protection (page 52)
Worcester wiring
- Add a frost thermostat to an S-Plan
Quick Jobs
Wiring diagrams it is part of
Practise it in the simulator.
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