Foundation · Unit 4
The parts
In short
One lesson per part: its terminals from the Manual, wired onto a spur, worked, most of them proved with the meter, and how it fails. The programmers, the thermostats, the two cylinder stats, the frost stat, both valves, the boiler and its pump overrun, the combi, the pump, the wireless pair, the wiring centre and the 3-way connector.
Comes after Safe isolation and the earth.
In this unit
The dual channel programmer
The clock of a heating system: two switches inside, one for hot water (HW) and one for central heating (CH). Each sends a live out of its ON terminal or its OFF terminal, never both.
The single channel programmer
A clock with one switch inside. On this simulator's part, ON puts a live on 2 and 4, and OFF puts it on 1. It often times a combi boiler.
The room thermostat
A switch worked by the room's warmth: live (L) in, switched live (SL) out. It calls for heat when the room is cool, and is satisfied when it is warm.
The digital room thermostat
A thermostat on batteries with a volt-free switch: it carries whatever you feed its common, 1. On this simulator's part, 2 is the call and 3 is satisfied.
Room Thermostat C
A thermostat with mains electronics and a volt-free switch: 1 is the common, 3 the call and 2 satisfied. Its common must be fed, or nothing switches.
The cylinder thermostats, A and B
Two cylinder thermostats with the same switch: B prints NO and NC beside its terminals. Both call on 1, the normally open (NO) contact, and both feel the same water.
The frost thermostat
A thermostat in the coldest unheated room. Fed from a permanent live, it can call for heat even when the programmer is off, so the pipes do not freeze.
The 2-port valve
A zone valve: its brown drives the motor, its grey waits at the end switch, and its orange comes out once it is fully open. A spring shuts it.
The 3-port valve
One valve with three ports, for the heating and the hot water. White drives it toward the heating, grey lets it get there, and the orange is live only at A.
The boiler, and the pump overrun
A live on its switched live (SL) fires it. Its pump live (PL) runs the pump, and keeps it running after the burner stops: the pump overrun.
The combi boiler
A boiler that heats the radiators and makes hot water at the tap. From outside it needs only a supply, and one switched live for the heating on its room thermostat terminal (RT).
The pump
A motor that moves the water round the pipes. On a system it is fed from the boiler's pump live (PL), and its flow keeps a heat-only boiler from locking out.
The wireless pair
A battery thermostat and its receiver. The thermostat sends its call by radio; the receiver is the switch, wired like a dual channel programmer.
The wiring centre and the 3-way connector
Twelve numbered blocks with nothing joined inside, and a 3-way connector whose ways are each their own junction. A loose screw is a fault.
In the simulator, you practise how to
- Read every terminal of each part from its Manual and wire it onto a spur
- Work each part and prove its outputs with the meter: demands, calls, oranges, the overrun
- Tell the two cylinder stats apart and read a valve, a programmer and a volt-free stat
- Say how each part fails on site and what to test first
Checks and credit
- A timed check at the end of the unit, its questions drawn at random. The score goes on your record.
Certificates of completion and credentials are DrawWarm’s own. They record practice and tests in the simulator. They are not a formal (regulated) qualification, and they do not show that anyone is competent to work on a heating system.
Where it connects
The parts it uses
- Fused Spur
- Dual Channel Programmer
- Indicator Lamp
- Single Channel
- Room Thermostat
- Digital Room Stat
- Room Thermostat C
- Cylinder Thermostat
- Cylinder Thermostat B
- Frost Thermostat
- Changeover Switch
- 2-Port Valve
- 3-Port Valve
- Heat-Only Boiler
- Circulation Pump
- Combi Boiler
- Wireless Thermostat 2Ch
- Wireless Receiver 2Ch
- Wiring Centre
Foundation
- Safe isolation and the earth
Unit 3, before this one
- The Draw Warm Method: S-Plan
Unit 5, next
Back to the Programme.
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.
Runs in the browser on a laptop or desktop. Cancel any time.