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…

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:



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

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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.


{ 6 comments… read them below or add one }

Simon April 18, 2014 at 19:34

My background is electrical and electronic engineering and I’ve been intrigued by these valves for years. They seem to be known commonly as ‘spring return’ (S/R) valves – as this great article has explained, the design uses a quirk of ac synchronous motors (or synchron motor) which is that you can stall them by injecting dc into the ac feed.
The reason for this comment is to pick up on other comments about the inefficiencies involved in having a design where the motor is normally running but stalled – either by being at the limit of the travel allowed or by use of the dc control signal. This obviously eats energy.
The more elegant and energy efficient (and hence more expensive up front) approach is to use a 2- or 3-port valve which is motored in both directions, rather than using the motor to drive one way and a spring return to drive the other way.
An example is the Sunvic MoMo valve

I can’t comment on how good they may be as I’m a diy’er not a trade central heating installer, but if I have to spend my own money on replacing a 2- or 3-port valve I’m definitely going down the MoMo route. Basic engineering principles tell you that (a) they should last longer and be more reliable in operation and (b) they should save a lot of energy. I’ll leave it to someone else to work out what the energy saving in a typical year could be.

John Pilkington April 12, 2014 at 20:47

Very helpful explanation. This may not be the right thread, but I am trying to work out a weird anomaly on my Y-Plan system.

For info, it is about 24 years old. A Potterton Kingfisher 2 cf60 boiler, a new Wilo Pico pump and a new Honeywell Motorised valve. The Timer controller is the original Honeywell Honeywell ST699.

I have checked the wiring many times now and can’t see anything wrong.

When the Central Heating (wall) stat calls for heat, the system works as one would expect. IE the pump and boiler both switch on until the room warms up, then the wall stat is satisfied and switches it off. So far so good.
However, looking at the Hot Water, it does not behave in the same way. It seems as if the pump and the boiler act separately. Firstly, I switch the CH to off on the Timer Controller to make sure that that does not complicate the issue.
Then I activate the Hot Water (HW) by moving the Hot Water slider (on the Honeywell Timer Controller) to ‘continuous’ I can now hear the pump switch on and a visual check of the pump shows it running at 40W (this modern pump has an led readout). So it sounds on and looks to be on.
However, the boiler only comes on if I nip upstairs and turn the cylinder stat to a higher temp required to ‘call’ for heat. If I turn the cylinder stat down, to below the water temp, then the boiler does the right thing and turns itself off. However the pump keeps on running all of this time, regardless of whether the boiler is on or off.

I tested the boiler terminal that comes from the live through the boiler stat (which I have also recently replaced) and on to the gas controller and it reads 240v when the boiler is on and zero when it is off (as one would expect).

Completely at a loss! My working solution is to just have the hot water timer set to 12 hours of daylight so I dont have to listen to the pump all night, but it means it is running needlessly all day and not just when it needs to heat the water!

Any ideas much appreciated!

In most new central heating systems the pump is controlled from the boiler. This is called a pump over run. If the pump is running all the time then there could be a problem with the pump timer control unit inside the boiler. Best to get your system checked and tested by an expert.

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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