How does a Y Plan Central Heating 3 Port Mid Position Motorised Valve work?

by Editor on November 2, 2011

Find Mid Position Valves on EBAY… How does a Y Plan Central Heating 3 Port Mid Position Motorised Valve work?

This interesting article is from DIYFAQ.ORG.UK with links to WIKI for further info.

It shows a circuit diagram and explains the internal workings of the 3 Port Mid Position Central Heating Valve commonly used in Honeywell Sundial Y Plan (4073A) and similar Drayton Biflow systems. There’s also some useful info on faulty and stiff motorized valves.

How the Mid-Position Three Port Valve Works

By Matthew Marks with thanks to Geoff Drage for data.

The three port valve is a masterpiece of clever engineering, in that it manages to move to one of three positions using only a cheap non-reversible AC motor, a spring, a couple of micro-switches, a resistor and a diode, and act as a relay for the boiler into the bargain! However, it has obviously required quite a bit of lateral thinking to conjure up, and its operation is thus not easy to understand. Here’s how it works.

The spring pulls the valve to position B, while the motor winds it towards position A. If the motor is left continuously powered, it will stall in position A, but if it is fed with DC (produced with the resistor and the diode), then it will stall in any position. Two micro-switches, operating just either side of the ‘A+B’ point, are used to define this position.

This is the circuit diagram of the innards of the valve:

Mid Position Valve Diagram1 How does a Y Plan Central Heating 3 Port Mid Position Motorised Valve work? Mid-Position-Central-Heating-Valve-Diagram1

The switches SW1 and SW2 are shown in the valve rest position (B). As the valve moves over towards the A position, SW1 changes over just before the A+B point, and SW2 just after. The white and grey wires are the control inputs, the orange is the output to the boiler for position A, and blue is neutral.

In the “both off” state, the system wiring results in grey being live. If the valve happens to be in the A+B or A position, SW1 will have been operated, the motor will be fed with AC, and the valve will wind to A and stay there (although the orange boiler output will not be live). This is a fly in the ointment for this valve configuration: the motor can be left consuming power and wearing out its hot windings unnecessarily (the spec says the valve consumes 6W). This will not happen in the summer though, when heating is never selected: SW1 will be at rest, and the valve will sit at B un-energised.

The 270K resistor supplies a small AC current to de-magnetise the motor from the effects of the rectified DC that is used to hold it in the mid position. Without this, there is some risk that the return spring will not be able to overcome the residual magnetic stiction to return it to the end position.

In the “water only” state, neither grey nor white are energised. The spring will therefore pull the valve back to B, where it will sit.

In the “water and heating” state, white is energised. If the valve is at B, the motor will wind it until A+B, whereupon SW1 changes over, DC is applied to the motor via SW2, and it will stall. If it overshoots, or if it is at B, SW2 will be operated as well, removing all power from the motor, and allowing the spring to pull the valve back to A+B. It is fun to watch this happening: as the spring pulls the valve back from B to A+B, the motor acquires quite a momentum and overshoots. It then winds forward a little, and stops in the correct position.

In the “heating only” state, both white and grey are energised (hence the need for a changeover tank stat, and a “hot water not required” output from the programmer). Regardless of the position of either switch, AC will be supplied to the motor, and it will wind to position A. In addition, SW2 will connect white to orange, switching on the boiler. (The boiler is switched externally to the valve in the other situations.)

As has already been mentioned, a common failure mode is the motor burning out: hence the provision of replaceable heads. In this case, the valve will sit in position B and the motor will be cold. The valve can stiffen up, though, if water gets in between the two O-rings that seal the actuating shaft. This will manifest itself as the valve sitting in a random position, or in position B but with a hot motor. Applying silicone grease to the operating shaft can cure this: the heating will need to be drained, the valve head and cover removed, and a cir-clip taken off the shaft. The actual valve consists of a freely-rotating rubber ball which is swung on an arm between the two ports.


Thanks again to Matthew Marks and Geoff Drage for this article.

For more on central heating valves and full schematics and diagrams of how they are included in complete systems, please see the Central Heating Controls & Zoning article at


More info to help with installation and fault finding

Here’s an overview of the Honeywell 4073A Mid Position Motorised Valve’s operations from the spec sheet:

No power (on valve) = HW only (port B open)

240V on white wire = HW + CH (mid position)

240V on white & grey wires = CH only (port A open) & 240V output on orange wire

240V on grey wire = Valve held in last position (but if last position was CH, approx 100V output on orange wire)

Honeywell 4073A Mid Position Motorised Valve

Installation-Guide and Spec Sheet & Spare Parts.

Find wiring diagram for Y Plan central heating system.


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  • more How does a Y Plan Central Heating 3 Port Mid Position Motorised Valve work?

{ 4 comments… read them below or add one }

wayne December 10, 2013 at 22:28

Interesting and well explained article. Jim i noticed the motor getting hot, thought they wood designed a better valve, but i would say the valves or brilliant by design. maybe the people who make the electronic timers should program it to switch to “hot water on” for a brief moment after a coming off heating so the valve rests in B position?

Simon Frankland October 20, 2012 at 10:37

Is there a more sophisticated timer available that would allow the 3 way valve to reset at the end of each heating cycle so it doesnt hold continuously?Looking at the diagram for ST699 a 3rd relay controlling the supply to pins 5 & 8 operated for 5-10 seconds at the end of each heating cycle should reset the valve?

Bob Rea October 6, 2012 at 14:55

Well spotted Jim, mine also stays hot (fully powered and in a stalled position).
Solution (but not satisfactory): After the programmer has switched the system off, you would need a time dely before the HW off connection was made (enough time for the valve to return under spring tension) or turn the main switch off for 10 seconds and then turn on again.

Jim Simmonds January 11, 2012 at 11:26

Great explanation, but rather worrying. It appears that whenever the valve it left to rest in the C/H position, it will get very hot, even though no heating of any type is ‘ON’ and all pipes are cold. This means that electricity is constantly being used and the motor slowly but surely burning out. This must explain why the one on my domestic installation is always too hot to touch. At 6w, this is a lot of electricity. Are there no other types of this valve that operate in a more sensible and less costly manner?

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