Boilers & Pump
Heat-Only Boiler
In short
A heat-only boiler heats the water for the radiators and the hot water cylinder. Its pump and its controls are separate parts, outside it. Its terminals: L, N, E, SL, PL. Units based on makers’ products sit under it in the parts list, such as the Vaillant ecoTEC plus system and the Worcester 8000 Life System.
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 HEAT-ONLY BOILER heats the water for the radiators and the hot water cylinder. Its pump and its controls are separate parts, outside it.
It FIRES, lighting its burner, when its switched live (SL) is live and it has its live (L) and neutral (N).
While SL is live, and for a while after, it puts a live on its pump live (PL) terminal to run the pump. That run-on is the PUMP OVERRUN: it carries the last heat out, so the boiler does not overheat. In this simulator it lasts 8 seconds; a boiler on site runs it for minutes.
It needs water moving through it. With no flow it KETTLES, boiling inside, and LOCKS OUT: it stops itself.
Terminals
| Terminal | What it is for | On the S-Plan example | On the Y-Plan example |
|---|---|---|---|
| L (Live) | The PERMANENT LIVE (L) for its electronics: live all the time the spur is on. | Wiring centre block 1 | Wiring centre block 1 |
| N (Neutral) | The neutral (N). | Wiring centre block 2 | Wiring centre block 2 |
| E (Earth) | The earth (E). The boiler runs without it, but it is never left without it. In this simulator it then wears NO EARTH. | Wiring centre block 3 | Wiring centre block 3 |
| SL (Switched Live) | The switched live (SL) in: the signal to FIRE. On an S-Plan it comes from the valves' oranges. On a Y-Plan it comes from wiring centre block 8: the call of the cylinder thermostat (stat), and the 3-port valve's orange at A. | Wiring centre block 10 | Wiring centre block 8 |
| PL (Pump Live) | The pump live (PL) out: live while SL is live, and during the pump overrun after it. | Circulation Pump L | Circulation Pump L |
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
A live on the switched live (SL): the pump live (PL) goes live at once. The flame shows after a short start-up: 2 seconds in this simulator.
SL dead: the flame goes. PL stays live for the overrun, then goes dead.
Units based on makers’ products
In the parts list these sit under the Heat-Only Boiler. 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 Vaillant ecoTEC plus system
- Based on the Worcester 8000 Life System
- Based on the Worcester Greenstar 27/30Ri
- Based on the Worcester Greenstar 18Riolder
DrawWarm’s own: Heat Only, System Boiler, System Boiler + Cylinder.
Call-outs with this part on the path
- Boiler will not fire: the controls work, nothing heats
“The programmer's on and I can hear things whirring, but the boiler never fires - no heating and no hot water.”
- Boiler fires with everything off, radiators cold
“The boiler keeps firing with everything off at the timer, but the radiators stay cold.”
- Boiler fires, kettles and cuts out; radiators barely warm
“The boiler fires, makes a terrible kettling noise, then cuts out. The radiators hardly get warm.”
- Boiler overheats and cuts out; the pump hums, radiators cold
“The boiler fires, gets really hot and cuts out. I can hear the pump humming but the radiators stay cold.”
- Boiler cuts in and out on its own
“It keeps cutting in and out. Works for a bit, the boiler drops off, then it comes back on its own.”
Where it connects
Lessons that use it
- The parts list and the canvas
Foundation · Using the simulator
- Wires: draw, colour, bend, erase
Foundation · Using the simulator
- The words, and why they are what they are
Foundation · Reading a circuit
- Reading a manufacturer's diagram
Foundation · Reading a circuit
- The fused spur
Foundation · Electricity for heating controls
- Every load needs N and E
Foundation · Electricity for heating controls
- Why we work dead
Foundation · Safe isolation and the earth
- Prove dead
Foundation · Safe isolation and the earth
Wiring diagrams it is part of
- S-Plan
S-Plan
- S-Plan with a digital room stat
S-Plan
- Wireless S-Plan
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
Practise it in the simulator.
Runs in the browser on a laptop or desktop. Cancel any time.