Explained · updated 5 November 2026
Pump overrun: what it is and how it is wired
In short
Pump overrun is the pump running on after the boiler’s burner stops, to carry the last heat out of the boiler. The boiler times it, so a pump-overrun boiler feeds the pump from its own pump live (PL) terminal, never from the controls. It needs a permanent live too, to keep going after the call ends.
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.
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.
What the overrun is for
A HEAT-ONLY BOILER heats the water, and a separate CIRCULATING PUMP moves it round the radiators and the cylinder’s coil. When the call for heat ends, the burner goes out at once. The boiler’s heat exchanger is still hot.
The PUMP OVERRUN runs the pump on for a while after the burner stops. It carries that last heat out into the system, so the boiler does not overheat. A boiler left hot with no water moving could boil inside its own heat exchanger: KETTLING.
The pump has no overrun of its own. It stops the moment its live goes. The run-on is the boiler keeping its pump live terminal live after its switched live has gone dead.
How long it runs on is set by the boiler’s maker. On site it can be minutes, and the boiler’s manual gives the time. In the simulator it is cut to a few seconds, so you can watch it happen.
Which boilers have one
- A heat-only boiler with a pump live terminal has an overrun, and its pump is wired to that terminal.
- A heat-only boiler without one has no overrun. Its pump goes on the same switched live that fires the boiler, so the two start and stop together.
- A combi or a system boiler has its pump inside. The run-on is the boiler’s own business, and there is no pump output for you to wire.
Which kind you have is in the boiler’s manual. Read it before you land the pump.
Wiring a pump-overrun boiler
Three wires make it work, and each has its own place. On a wiring centre numbered the common way:
- The boiler’s L to block 1, the PERMANENT LIVE. It keeps the boiler, and so the pump, running after the call ends.
- The boiler’s SWITCHED LIVE (SL) to the boiler’s demand block: block 10 on an S-Plan, with the valves’ orange cores; block 8 on a Y-Plan.
- The boiler’s PL to the pump’s L. Run it direct, or through a spare block that carries the pump live and nothing else. Widely used makers’ diagrams use block 9 for it.
The pump’s neutral and earth go to blocks 2 and 3, or to the boiler’s own pump terminals where it has them.
Why nothing else on the pump live’s block? Every wire landed on a block is joined to every other wire on it. Anything sharing that block would be fed by the pump live, and would run whenever the pump runs.
Two ways to lose the overrun
Both come from wiring a pump-overrun boiler as if it had none.
- The pump on the demand block. Its live then comes from the valves, not the boiler. It stops the moment the call ends, with the burner, and the last heat stays in the boiler.
- The boiler’s L on a switched live. The boiler then loses its own supply when the call ends. The overrun has nothing to run on.
The other way round matters too. A boiler with no overrun gives no pump live, so its pump belongs on the demand block. Check the boiler’s own manual, every time.
Proving it when you commission
Watch the overrun as part of the truth table. In the row where both thermostats are satisfied, the valves shut and the burner stops. The pump runs on, then stops by itself, at the time the boiler’s manual gives. See what a truth table proves.
With the meter, these are live tests: only for a competent person, or a learner under the supervision of one, with test leads made to the HSE’s guidance note GS38. Read each terminal to a neutral you know is good.
- The boiler’s SL: 0 V from the moment the call ends.
- The boiler’s PL: still about 230 volts (230 V) through the overrun, then 0 V.
A pump that runs on well past the time in the manual, or never stops, is a call-out. Trace it with the meter like any other: the readings show where its live comes from.
Before you work on the pump
The pump’s live comes from the boiler, not from the wiring centre’s supply blocks. So it can still be live after the controls say off, through the overrun.
Isolate at the point of isolation and lock it off. Then prove dead at the pump itself: live to neutral, live to earth, neutral to earth. If the boiler has a supply of its own, find it and isolate that too. See safe isolation in five steps.
Controls work stops at the terminals the boiler’s manual gives for the controls. If reaching them means opening the boiler’s sealed case, or touching anything inside it, that is a job for a Gas Safe registered engineer.
Sources
- The Electricity at Work Regulations 1989, regulations 12 to 14 (isolation, and working on or near live conductors) and 16 (competence).
- HSE guidance note GS38, Electrical test equipment for use on low voltage electrical systems.
- The boiler makers’ installation instructions: they say whether a boiler has a pump overrun, which terminals to use, and how long it runs on.
Contains public sector information, some of it published by the Health and Safety Executive, licensed under the Open Government Licence v3.0.
Where it connects
- Heat-Only Boiler
The part
- Circulating Pump
The part
- S-Plan wiring diagram
Wiring diagram
- S-Plan Plus wiring diagram
Wiring diagram
- The parts
Foundation · Unit 4
- Commissioning
Intermediate · Unit 9
Read next
- Boiler interlock: why the boiler fires only when a thermostat calls
Boiler interlock: with no call for heat, the boiler and pump are off. How an S-Plan, a Y-Plan and a combi do it, and what Approved Document L asks.
- How the wiring centre’s numbered blocks tell the story of an S-Plan
An S-Plan wired in the order it works: the supply on blocks 1 to 3, heating on 4 and 5, hot water on 6 and 8, the valves’ orange cores on 10.
- S-Plan or Y-Plan? The wiring difference, block by block
An S-Plan has two zone valves; a Y-Plan, one mid-position valve. The blocks they share, where they part, what fires the boiler, and how to tell them apart.
Practise it in the simulator.
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