Programmers & Timers
Dual Channel Programmer
In short
A programmer is the clock of a heating system: it decides when heat is allowed. Its terminals: E, L, N, 1, 2, 3, 4. Units based on makers’ products sit under it in the parts list, such as the Danfoss FP715Si and the Drayton LP722.
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 PROGRAMMER is the clock of a heating system: it decides WHEN heat is allowed. A DUAL CHANNEL programmer has two CHANNELS: hot water (HW) and central heating (CH).
Each channel is a CHANGEOVER switch, fed from the live (L) inside. While a channel is ON, it puts a live on its ON terminal. While it is OFF, it puts the live on its OFF terminal instead.
The programmer only ALLOWS the heat. The thermostats (stats) after it decide whether heat is needed.
Under its screen is a GRAVITY / PUMPED switch; a branded unit has a commissioning switch on its back instead. Click it with Select to change it.
PUMPED, as it comes, keeps the two channels apart. That suits a fully pumped system, or a C-Plan, whose valve gives the hot water its own control.
GRAVITY is for an older system with NO valve on the cylinder's gravity circuit. There the cylinder heats whenever the boiler fires. So HEATING ON brings HOT WATER ON with it, and HOT WATER OFF turns the heating off too.
Terminals
| Terminal | What it is for | On the S-Plan example | On the Y-Plan example |
|---|---|---|---|
| E (Earth) | The earth (E) PARKING SCREW. It holds the cable's earth core, in green-and-yellow sleeving. The unit itself does not use it. | Wiring centre block 3 | Wiring centre block 3 |
| L (Live) | The live (L) in. It feeds the clock and both channel switches. | Wiring centre block 1 | Wiring centre block 1 |
| N (Neutral) | The neutral (N) for the clock. | Wiring centre block 2 | Wiring centre block 2 |
| 1 (HW OFF) | Hot water off (HW OFF): live while the hot water is off. On a Y-Plan the 3-port valve's grey comes from here. | — | Wiring centre block 7 |
| 2 (CH OFF) | Central heating off (CH OFF): live while the heating is off. It is rarely used. | — | — |
| 3 (HW ON) | Hot water on (HW ON): the hot water DEMAND. It goes to the common (C) of the cylinder thermostat (stat). | Wiring centre block 6 | Wiring centre block 6 |
| 4 (CH ON) | Central heating on (CH ON): the heating demand. It goes to the live (L) of the room thermostat. | Wiring centre block 4 | Wiring centre block 4 |
Read from the wired example circuits: S-Plan, Y-Plan. A wiring-centre block carries whatever is landed on it; the numbers are the habit the lessons teach.
What the meter reads
With its live (L) and neutral (N) on, the clock runs. Each channel's switch feeds its ON terminal or its OFF terminal, never both. Without them, each switch rests on OFF.
The terminal a switch is not feeding can still read live from the rest of the circuit: a BACKFEED. On a Y-Plan with the hot water off, HW ON reads 230 volts (230 V) through a satisfied cylinder thermostat (stat).
Press HOT WATER or HEATING to move that channel's live between OFF and ON. While the unit has its supply, the screen shows each channel ON, with a flame, or OFF.
Set to GRAVITY, HEATING ON turns HOT WATER ON too, and 3 goes live with 4. HOT WATER OFF turns the heating off. Set to PUMPED, each button works its own channel.
Units based on makers’ products
In the parts list these sit under the Dual Channel Programmer. 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 Danfoss FP715Si
- Based on the Drayton LP722
- Based on the ESi ES2247B
- Based on the Honeywell ST9400C
- Based on the Secure / Horstmann H27XL
- Based on the Drayton LP522
- Based on the Salus EP210
- Based on the Siemens RWB29
- Based on the Honeywell ST6400Colder
- Based on the Honeywell ST699B (fully pumped)older
- Based on the Horstmann Diadem 425older
Call-outs with this part on the path
- No heating: hot water fine, radiators cold
“Hot water's fine but the radiators are stone cold, whatever I do with the thermostat.”
- No hot water: heating fine, cylinder cold
“Heating's lovely and warm but there's no hot water - the cylinder's cold.”
- Everything dead: no heating, no hot water, the controls dark
“Nothing works. No heating, no hot water, and the controls are dead.”
- 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.”
- Hot water scalding: the cylinder keeps on heating
“The hot water's scalding - far hotter than it used to be, whatever the dial on the cylinder says.”
Where it connects
Lessons that use it
- Under the lid: boxes and fronts
Foundation · Using the simulator
- The Manual, History and the examples
Foundation · Using the simulator
- The words, and why they are what they are
Foundation · Reading a circuit
- The back of a programmer
Foundation · Reading a circuit
- Boiler Plus and the interlock
Foundation · Electricity for heating controls
- The dual channel programmer
Foundation · The parts
- The Draw Warm Method: the parts on the wall
Foundation · The Draw Warm Method: S-Plan
- Block 1: the permanent lives
Foundation · The Draw Warm Method: S-Plan
Wiring diagrams it is part of
- S-Plan
S-Plan
- S-Plan with a digital room stat
S-Plan
- S-Plan with a frost thermostat
S-Plan
- S-Plan Plus
S-Plan
- Y-Plan
Y-Plan
- Y-Plan with a frost thermostat
Y-Plan
Also in programmers & timers
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.