Boilers & Pump
Circulating Pump
In short
A circulating pump is the motor that moves the water round the system: through the boiler, the radiators and the cylinder. Its terminals: L, N, E. The parts list also has it as Pump (grey).
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 CIRCULATING PUMP is the motor that moves the water round the system: through the boiler, the radiators and the cylinder.
It is fed from the boiler's pump live (PL). So it runs while the boiler fires, and through the PUMP OVERRUN after.
With no flow there is no heat anywhere, and a heat-only boiler LOCKS OUT.
Its terminals are in a TERMINAL BOX. To wire it, click the pump with the Wire Tool, or right-click it and choose Open the terminal box.
Terminals
| Terminal | What it is for | On the S-Plan example | On the Y-Plan example |
|---|---|---|---|
| L (Live) | The live (L), from the boiler's pump live (PL). It may go through a wiring centre block that carries only PL, if that is tidier. Only a boiler with NO overrun may have its pump on the same switched live as the boiler's own switched live (SL). Check the boiler's own manual. | Heat-Only Boiler PL | Heat-Only Boiler PL |
| N (Neutral) | The neutral (N). | Wiring centre block 2 | Wiring centre block 2 |
| E (Earth) | The earth (E). | Wiring centre block 3 | Wiring centre block 3 |
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
L live, and N there: it spins up over about a second, then runs.
It has no overrun of its own: it stops the moment its live goes. The run-on after the burner stops is the boiler keeping its pump live (PL) terminal live. That is 8 seconds in this simulator, and minutes on site.
Dead, across L and N the meter reads its WINDING: about 120 ohms in this simulator. Many pumps today are electronic, and their makers give no figure.
Call-outs with this part on the path
- 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.”
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
- Reading a manufacturer's diagram
Foundation · Reading a circuit
- The CPC and earth continuity
Foundation · Safe isolation and the earth
- The boiler, and the pump overrun
Foundation · The parts
- The pump
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
- 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
Read the explanation
- Prove a part with two readings before you condemn it
A motor needs a live and a neutral. Read live to earth, then live to the part’s own neutral, before you change a valve, a pump or a fuse. Then prove the repair.
Practise it in the simulator.
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