What is an S-Plan Heating System?
An S-Plan heating system is a system that uses zone-valves (also known as 2-ports)
S-Plan heating systems are now the most common type of system installed in the UK after Combination boiler systems (which do not have any external motorised valves).
The S-plan is popular because it is versatile (you can add an unlimited number of zones), simple to wire, and easy to fault find on.
It also allows you to satisfy the need for a positive shut-off of water flow to an unvented cylinder (in order to comply with G3 regulations) if the cylinder were to overheat.
How does a 2 port valve work?
A 2-port (or Zone valve) has a permanent 230v supply (usually on the grey wire) plus a Neutral and an Earth wire.
There is then also a Motor Live wire (usually the brown) and a Switch Live wire (usually the orange).
When there is a call for heat from which ever circuit the 2-port is connected to (the timer and thermostat are both calling for heat) 230v is sent down the Motor Live (Brown) wire.
The motor inside the valve turns, and opens the body of the valve, to allow the heating pump to push water through it.
When the motor has pushed the valve all the way open, it also pushes down a microswitch which makes a circuit between the Permanent Live (Grey) wire and the Switch Live (orange) wire.
Once there is 230V on the Orange wire, the boiler will fire up and run until the demand is satisfied.
Once the demand is satisfied (either the timer is turned off, or the thermostat reaches the desired temperature) the power to the Motor Live (Brown) wire is lost, so the valve body springs back and shuts off the flow through the valve.
At the same time, the microswitch making a circuit between the Permanent live (Grey) wire and the Switch Live (Orange wire) is opened, so no power is now sent through the orange wire and the boiler will stop running.
How do you wire up an S-plan heating system?
To learn how to wire an S-plan System, you can either watch our video, or follow a wiring diagram such as the Honeywell wiring diagram below.
Common Faults on S-Plan Heating Systems
There are a few faults that we regularly come across on S-Plan systems..
Heating / Hot Water will not come on –
– Can be caused by faults with the external controls (not sending power to the Motor Live (Brown) wire on the 2-port.
– This could also be caused by either the Microswitch inside the valve being damaged (not making a circuit between the Permanent Live (Grey) and Switch Live (Orange Wires).
– Or by the body of the valve being seized and not allowing the motor to turn (meaning the valve will not allow water to flow through it, and the microswitch wont be operated).
Heating / Hot Water will not turn off –
– This is most often cause by the microswitch sticking down. So even when there is no power on the Motor Live (Brown) wire, there is still a circuit made between the Permanent Live (Grey) and the Switch Live (orange wires).
– You will also find the same fault if the valve body becomes stuck in the open position, and the spring is unable to pull the body shut once the Motor Live (Brown) wire loses power.
Radiators heat up when Only the Hot water is turned on –
– This fault is usually due to system dirt getting inside the body of the 2-port valve. The dirt prevents the valve from closing all the way and allows some water to flow through it even though it’s in the Closed position.
– You can also have this fault if an S-Plan system is not plumbed in correctly. If the Cylinder Return is not the last pipe to tee into the Primary Return before the boiler, you will get reverse circulation through the radiator pipework.