This article explains how to connect a boiler to a new or existing central heating system with pump over-run. The wiring diagrams show some of the most commonly found domestic boiler control panel electrical connections.
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.
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.
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 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.
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.
More info about Central Heating systems wiring & connection in the ERA