Thermostats
Cylinder Thermostat (B: NO and NC printed)
In short
The same cylinder thermostat (stat) switch as A. Beside its terminals it prints NO, normally open, and NC, normally closed, as some makers do. Its terminals: C, 1, 2.
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
The same CYLINDER THERMOSTAT (stat) switch as A. Beside its terminals it prints NO, normally open, and NC, normally closed, as some makers do.
Its common, C, goes to 1 while the water is cool. That is the CALL, printed NO. C goes to 2 when the water is hot. That is SATISFIED, printed NC.
Read the label: NO is the call, whatever number a maker puts beside it.
Terminals
| Terminal | What it is for |
|---|---|
| C (Common) | The common (C): the hot water DEMAND in, from the programmer's hot water on (HW ON). |
| 1 (Calling) | Normally open (NO), the CALL: live while the water is cool. It goes to the hot water valve's motor. |
| 2 (Satisfied) | Normally closed (NC), SATISFIED: live when the water is hot. On a Y-Plan it feeds the 3-port valve's grey. |
What the meter reads
Cool water: 1, normally open (NO), is live. Hot water: 2, normally closed (NC), is live. It switches as thermostat A does: the printed words name the contacts, and the numbers are the maker's.
Like thermostat A, it switches 2 degrees either side of its setting: a 4 degree DIFFERENTIAL.
In this simulator the cylinder warms only while the thermostat (stat) calls, so it stops just past the setting. So a thermostat linked out cannot overheat the water here. On site it would.
Where it connects
Lessons that use it
- A thermostat is a switch
Foundation · Reading a circuit
- The cylinder thermostats, A and B
Foundation · The parts
Practise it in the simulator.
Runs in the browser on a laptop or desktop. Cancel any time.