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, and for learners under the supervision of one. It explains how the wiring works; it is not training to do the work. In England and Wales, electrical work in a home must meet the Building Regulations (Part P), and some of it must be notified to building control or done by a registered electrician; Scotland and Northern Ireland have their own rules. For the wiring, call a registered electrician or a heating engineer competent in controls. Work inside a gas boiler is for a Gas Safe registered engineer only. If you smell gas, leave the building and call the gas emergency number: 0800 111 999 in Great Britain, 0800 002 001 in Northern Ireland.
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
Live tests. On site, any reading taken with the supply on is work on or near live parts. Regulation 14 of the Electricity at Work Regulations 1989 allows it only when working dead is unreasonable, as when a fault shows only while the controls work, and only with suitable precautions: test leads made to the HSE’s guidance note GS38, fingers behind the guards. Regulation 16 allows it only for a competent person, or a learner under the supervision of one. Wiring and repairs are always done dead. In the simulator, nothing is live.
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.
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