What is a Y-Plan Heating System?
A Y-Plan heating system is a system that uses one motorised valve, which has 3 ports for water to flow through.
The Water flows from the boiler through the bottom of the valve, then the valve either sends the flow of water through Port A (to Heating), Port B (to Hot Water) or through both ports in order to heat the radiators and hot water together.
Y-Plan heating systems were widely used in older heating systems. But due to the fact that you can not add any additional zones and that you can not usually use a 3 Port on an unvented cylinder installation, they are now far less common on modern systems.
How does a 3 port valve work?
The motorised valve has 2 micro switches inside, switch 1 moves over when the valve reaches mid position, switch 2 moves over when the valve reaches Heating Only position.
When the power is sent through the white wire (Heating on) there is a 230v supply to the motor. The motor turns until it reaches mid position and switch 1 moves over (no longer sending 230v directly down the white wire to the motor).
The power from the white wire is now running through switch 2 only – which then passes through a diode and is converted to a DC voltage before it reaches the motor.
There is also voltage from the Orange wire running to the motor – but it is run through 2 resistors before reaching the motor.
This DC and Reduced AC voltage is enough to hold the motor in Mid Position, but not enough to move the valve any further (towards Heating Only)
If the Hot water demand is satisfied, or if there was never any Hot Water demand in the first place, then there is also a 230v supply on the Grey wire (Hot water Off).
The grey wire will then power the motor with a direct 230v when Switch 1 is moved over (when the valve reaches mid position) and the valve will now move all the way over to the Heating Only position.
The valve will now stay in this position, as there is a constant 230v Supply on the grey wire (until the next time that a Hot Water demand is made).
How to Wire a Y-Plan Heating System
In a Y-Plan Heating system, you must have a wire connecting the ‘Hot Water off” terminal at the programmer (Usually terminal number 1) and the ‘Satisfied’ Terminal on the Cylinder Thermostat. These terminals are joined together with the Grey Wire from the Motorised valve.
The Switch live wire to the boiler is connected with the Orange wire from the motorised valve and the ‘Call’ wire from the cylinder thermostat.
The motorised valve rests in the last position it was in before the demand was satisfied. So if the last demand was for Hot Water the valve will sit in Hot Water position, with 230v on the Grey wire from the motorised valve, but the motor will not be powered.
When there is a demand for both Heating and Hot water 230v is sent through the white wire (Heating on) and powers the motor.
The motor will move into mid position and hold there until 230v is sent down the Grey Wire (Hot water off) and the valve moves all the way over to Heating Only – or until the power on the White Wire (Heating on) is turned off and the valve springs back to Hot Water only.
Common Faults on Y-Plan Heating Systems
One common fault that you may find with Y-Plan systems is that the boiler keeps running and Heating the radiators (even when the controls are not calling for heating).
This can be due to the reduced voltage running through the motorised valve from the Grey wire (Hot water off) to the Orange wire (Switch Live).
In some boilers this reduced voltage is enough to fire up the boiler.
This can usually be fixed by fitting a wiring suppressor or capacitor between the Orange (switch live) and Blue (Neutral) wires in the wiring centre.
There is a full explanation and demonstration of this fault in the above videos.