How To Connect A Central Heating Boiler

by Editor on January 19, 2012

This article explains how to connect a boiler to a new or existing central heating system with pump over-run. The boiler wiring diagrams show some of the most commonly found domestic boiler control panel electrical connections.

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Electrical Connections to Boilers

Please Note- Only competent and trained persons such as qualified electricians, plumbers and gas fitters should attempt to connect electrical supplies and controls to central heating systems.

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A domestic boiler and heating installation should be electrically supplied via a single double pole fused spur unit and protected by a 3 amp fuse.  The fused spur would ideally be sited near to the boiler or wiring center junction box which is often in the cylinder cupboard.

Many new type boilers and including Combination and System boilers need a permanently live supply for time and temperature control and for pump over-run features (see below). For this reason it can be a good idea to site the supply fused spur next to the boiler and use a 3 core flex for the boiler feed and another flex out from the boiler to any controls. This is also a safer set up
when it comes to boiler servicing as the engineer can isolate the whole system from the boiler.

Note that all boiler supply and control wiring which enters the boiler casing must be heat resisting flexible cable. This should be routed away from hot pipework and enter the boiler’s connection panel through the correct cable entries and clamps.

Central Heating Control Wiring

Where control wiring is needed from the boiler to a room stat, programmer or motorised valves etc. you can use 3, 4 or 5 core cables as shown in the wiring diagram above and described below.

Pump Over-Run

Many boilers now have a pump over-run feature. This is where the system pump continues to run after the boiler has shut down. Pump over run makes use of the excess heat in the boiler as it cools down by circulating the water for a preset time or temperature controlled period. Where a pump over run is required it is usually pre-installed as part of the boiler control circuitry and will require a permanent electrical feed. Additional wiring may be needed for a remote pump which isn’t fitted inside the boiler casing. Find more info on Pump Over run at the bottom of this page.

Boiler Connections

Older fully pumped central heating systems (and some newer ones) often pick up an electrical power supply from the existing immersion heater circuit and the boiler is wired by running a 4 core cable from a Y or S plan wiring centre. This includes a LNE permanent supply and a switched supply to fire the boiler. A 3 pole switch s often positioned next to the boiler to electrically isolate it for servicing.

Remember that this only isolates the supply and switched feed to the boiler (if it is connected correctly!). I would always prefer to find the main system electrical supply and switch off there before removing the panels.

A 5th core (or a seperate 3 core) is needed where a remote pump is installed.  The diagram below shows typical boiler connections used in this type of installation:




Modern boiler connection panels layouts and terminal identifications differ and can at first be confusing. You need to remember that most conventional system boilers control the pump over run and they are also the source of the electrical feed to any controls. For this reason the main heating system supply would go to the boiler first. The wiring terminal examples shown below are typical of those used by modern manufacturers such as Worcester Bosch where the main electrical power supply feeds into the boiler via LNE terminals and control wiring runs out from the boiler to a room thermostat or programmer.

Note: You will need to consult the wiring diagram for your particular boiler to identify how your particular system requires connecting.



L N E terminals– Electrical input supply for the whole system. Use 3 core heat resisting cable from a double pole switched 3 amp fused spur unit

The System Controls terminals
These 2 terminals normally have a factory fitted link between them. When controls such as a room stat, programmer and
motorised diverter valves are installed, this link needs to be removed to allow
them to be connected.



Here are some more typical boiler control terminal connection identifications:

LS / L1 / RT / Supply Out terminal– This is normally a 230 volt output to the controls. It is usually fed from within the boiler and does not require any other external or auxiliary supply. This could be used as the live supply to a roomstat, programmer or wiring centre.

LR / L2 / RT / Return terminal– This connection needs a supply back from the controls to fire the boiler. This could be a 230 volt switched live signal from a roomstat or programmer or even from the orange wire connection of a zone valve via a wiring centre.

N / NS terminal– This is a 230 volt Neutral output connection. Use this for
room stats, programmers,wiring centres and any controls that need a neutral

Pump L N terminals– The boiler controls the running of the pump from these terminals. A pump within the boiler casing may already be connected here.  A remote pump will need to be wired back to here.

Frost Protection
Frost Thermostat terminals usually have a factory fitted link between them. When a frost stat installed this link needs to be removed to allow connection of frost controls.
FP1 / FS terminal– This is the supply out to the frost stat.
FP2 / FR terminal– This is the return back from the frost stat.



Other systems may not have a dedicated wiring connection terminal for a frost stat. In this case you could simply connect the frost stat between the system’s permanent live and the boiler switched supply to directly fire the boiler in freezing conditions. See diagrams below.

Note: Some Heating Controls and Frost Stat connections may only be suitable for a 24 or 12 volt devices or require an auxiliary supply. Be sure to check your boiler instruction manual.

Here are some more examples of boiler wiring connections used in other systems such as Ideal Heating:



I hope this info on boiler wiring was useful.  Please feel free to leave your comments below.

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Some more useful info from Mike The Boiler Man about the reason for Pump Over-Run:

Condensing boilers have a device built into them to control the pump, keeping it running for a few minutes after the burners have shut down (to distribute the residual heat and protect the fragile high-performance heat exchanger, in case you were interested). This means running an extra cable from the boiler location to the pump location (usually next to the hot water cylinder upstairs in the airing cupboard). Installing this extra cable can be time consuming and disruptive to carpets and decorations. Fitting certain non-condensing boilers instead sidesteps this requirement, or the customer probably wouldn’t notice if the installer didn’t bother to connect up the pump over-run (until the heat exchanger fails a year later, that is).

Possible cost of not installing pump over-run wiring: £100 to £1,000.


{ 17 comments… read them below or add one }

Lewis January 11, 2018 at 14:03

I am having a new WB condensing regular boiler fitted in the garage. I currently have the wiring centre and the pump in the airing cupboard with the usual 5 core (LNE, S/L and pump) cable between the two locations. The new WB boiler wiring diagram suggests that I need a separate neutral for the pump.
There seem to be two schools of thought:
1. pick up the neutral from the wiring centre (as now)
2. Run a separate neutral which is required for the boiler electronics to operate correctly. Not doing so would invalidate the warranty.

Is there a definitive answer?
Competent DIYer
The new boiler may have an integral pump. Any addditional pump could pick up neutral from the boiler or wiring centre. But best to run separate neutral if warranty requires. But I’m not sure why that would be?

Editor August 23, 2016 at 05:47

Hi Steve. I had a new external Worcester boiler fitted 2 years ago. Now it has had it’s 1st service, by a local engineer, he has pointed out that plastic coated domestic electrical wiring has been used, not heat resistant. The electrician now says that this is ok and the wires he has used are more expensive than heat resistant anyway. Do you think it is safe to leave it wired in this manner. Thanks Roger.

Hi Roger
Any cable should be suitable for the conditions in which it is installed. The cost of the cables is irrelevant. Strictly speaking it’s “best practice” to use heat resisting cables for boilers and heating installations. This is because cables often need to run close to hot pipes and within boiler casings where heat can build up.
The most commonly used “plastic coated domestic electrical wiring” (known as PVC sheathed flat twin and earth) is not heat resistant and will be affected by continuous excessive heat. This means that it will break down or melt at temperatures that it is not designed for. Although Twin and Earth is rated at 70°C, that’s it’s maximum safe operating temperature. You (or a qualified person) will need to assess whether this is a risk by inspecting your installation.
If your cables are running near to pipework or subject to any heat above 30°C I would suggest they are re-routed or replaced with suitable heat resisting cables.
I hope this helps.

John Schofield October 12, 2015 at 20:39

Hi. My RCD has been tripping intermittently for over a year now and I am 100% sure that it is the boiler that is causing the tripping. The tripping only occurs seconds after the hot water and/or heating comes on and the boiler has fired up. It happens only intermittently – sometimes it will trip 2 or 3 times in a day and sometimes it will not trip for several days. It did not trip at all during the summer months when we shut the boiler down completely and relied on the immersion heater for hot water. That is why I am sure the boiler is causing the problem. However, the boiler has its own dedicated power source and is not connected to the RCD! When the RCD trips, various appliances (TV, dishwasher, fridge etc) switch off but the boiler keeps firing and the control panel is still lit. Has anyone come across a similar issue and, if so, can they explain how something that is not connected to the RCD might be causing the RCD to trip? Many thanks.

william September 10, 2015 at 10:19

Hi. I want to have a temporary power supply (perhaps a generator) that I can plug my oil fired heating system into so that it will still work during power cuts. I have a supply with 400w output but this doesn’t seem to be enough. What wattage do I have to go up to.

BUD BARKLEY August 13, 2015 at 10:49


Old Codger July 31, 2015 at 11:47

Hi. I am trying to wire up a Worcester 19/24CBi boiler to a y plan system, but am struggling to understand the vague wiring diagram in the boiler instructions regarding where the switched live and boiler wiring actually goes on the remote 10-way terminal block and if any links need to be made across this block etc.
The clock/programmer is a Randall 4033.
A drawing to follow plus plain English would be very useful
an urgent response would be appreciated -thanks

You may find some help here:|ga|5|HI%20-%20HVAC%20%20-%20USClone%20-%20Search|Worcester%20-%20Broad&JPKW=boiler%20worcester&JPDC=S&JPST=&JPAD=82890576168&JPMT=b&JPNW=s&JPRC=1&JPAF=txt&JPCD=20150624&JCLT=&JPOP=Oscar_URLMigrationDuplicates_20150624&mkwid=szEbyRAZ2_dc&pcrid=82890576168&pkw=boiler%20worcester&pmt=b&plc=&gclid=CJijluXph8cCFQYOwwodSg0C6Q

Bob Spalding July 30, 2015 at 17:23

Had a log burner with back boiler fitted to heating system about 3 years ago. I thought no problems. At The last service, the engineer told me the oil fired boiler fires up when not called to by the controller. Never gave it much thought until today. Symptoms are ,when controller is off, boiler will fire up by adjusting boiler temperature thermostat and will stop when lowering thermostat. I’ve boiler being controlled by the thermostat. Problem is no circulating pump running. I would think it was rewired wrongly when the new log burner control were fitted. Am I right?

If the system was working fine before the new control was installed then I’d have the wiring connections checked.
All the best.

David January 12, 2015 at 12:24

I’ve just had a Ferroli Modena 32C HE boiler installed and I would like to fit a honeywell wireless thermostat, the diagram for the BDR91 relay box/receiver that’s fitted close to the boiler shows two wires live and neutral coming from the boiler, can I use a separate power supply instead of the one coming from the boiler.
Reply- No. Use the supply which comes from the boiler. This ensures that the system and controls are all protected by the same fuse for safety.

Editor January 4, 2015 at 19:27

Hi Paul
Other possible wiring from the connection box on your boiler could be to a thermostat (3 cores & earth) or maybe to a ‘wiring centre’ type junction box to connect with any valves etc (5 cores & earth).
Good luck

paul madderson December 27, 2014 at 17:55

Hello, its my first question on your blog. I’m installing a Vaillant eco tec 624 VU system boiler in my home extension. I’m not gas safe registered or electrically qualified either but I just want to do some prep work before the pro’s come in to complete the job. All I want to do is to run the water side of the piping and to route the cables to the necessary controls in the neatest way possible while I have the opportunity before the build is complete. All I need to know is how many cables will enter the boiler electrics box so I can route them in the wall without them being externally unsightly. All the new electrics are dead at the moment as I’m still heating my home with my old Myson Apollo. I’ve fitted a double pole fused switch isolation which will be fitted with a 3 amp fuse when in operation to isolate the boiler. I can see from the installation manual where the main power cable needs to go but I can’t fathom out what else needs to be wired into this box. If other cables need to be routed into the boiler electronics box I would be grateful if someone could point me in the right direction. If this request is possible could someone let me know how many cores the cables should have, how many cables need to enter the box and what controls the cables actually feed. As I said I have no intention of doing the pro’s work I just want to route stuff the best way possible while i have the opportunity. Thanks, paul

Editor November 8, 2014 at 15:43

Hi Sarah

The boiler doesn’t need to have its own circuit from the fuseboard but the supply to the boler and any heating or hot water controls must be protected by a 3 amp fuse and an isolation switch.

The boiler takes a small amount of electrical load so is very unlikely to cause overload. The flickering of your lights and fan could be a loose connection somewhere.

If you have any doubts about your installation, I would recommend getting it checked by an approved electrician as soon as possible.

All the best


Householder November 5, 2014 at 07:22

Hi there – I’ve just had my boiler installed by a NIeCEC qualified electrician who is also my builder. During the works he was running the electrical supply from three pin plug in a power socket in my landing. He has now supposedly installed the boiler on its permanent wiring but it is now causing the lighting and extractor fan in my bathroom to flicker and cut out so I suspect the circuit is now overloaded. Should the boiler have its own circuit?


Tony Woollard November 2, 2013 at 11:20

Regarding the rcd being tripped by the heating circuit. This is a very common problem, mainly due to the heating controls,and pumps, having capacitors. I have found many problems after changing a consumer unit where every time the central heating is switched on it causes the rcd to trip. I have also found many heating systems where the insulation resistance is below the recommended minimum. I would, however, be very concerned if the system had been running ok for a while and then started to trip, that points to a fault somewhere. Having said that, it could be that one of the new components has a surge capacitor built in!
The only way to really know is to get a professional to check it, as we will have not only the correct test equipment, but also the knowledge to interpret the results correctly

Andrew walton April 30, 2013 at 18:20

Most interesting – and what I expected, but can anyone tell me why Honeywell’s installation instructions for the DT92E digital wireless thermostat instruct you to reroute the live wire to the boiler via the BDR91 receiver box? I have asked them, but cannot get a sensible reply.

Bill Jarvie January 22, 2013 at 11:53

RCD or Non-RCD?
A few weeks ago, my 8 year old Oil-fired boiler decided to continually trip out all my RCD circuits less than a minute after firing up. However, my local firm of heating engineers, after tweaking and/or changing out everything possible, and being unable to trace the reason, simply moving the electrical supply from RCD to Non-RCD in my consumer unit. Seven trouble-free weeks later, while pleased with the result, should I be concerned with the change i.e. does this meet with current regulation?
Hi Bill. My view is that there is always a reason for an RCD to trip (if it’s not faulty). There is obviously a leakage current flowing to earth. This can commonly be caused by a pump, gas valve, motorised valve or a trapped or damaged wire. Changing the boiler to a non RCD protected circuit isn’t solving the fault. Certain circuits are recommended to be RCD protected to prevent people from getting electricuted from live parts or from earthed parts, such as pipes, which may become live should an earth fault occur.

George May March 25, 2012 at 01:59

Thanks for having this very informative site you have. I just got a fon call from my sister that her heating would not switch off, even with the controller switched to off, this seemed really daft to me, but reading your bit about the live going to the boiler first kinda explains that, seems like a relay or whatever controls the pump over run has stuck! So I told her just to switch off the breaker on the consumer unit for the heating, and get the mannie out on Monday…….
So thanks again for this info and peace of mind at 3 o clock on Sunday morning.
Dod May, Bonnie Scotland.

electrical control panels February 29, 2012 at 09:18

Wow……very informative diagram explain it briefly and clearly.
Thanks for sharing brilliant post…….:-)

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