Mid Position Motorized Valve - how it works

A domestic mid position motorised valve is a fairly simple device and uses some very basic components in an interesting way.

A small synchronous motor is used to move the internals of the valve and a spring pulls it back when de-energised.
When it is in the fully energised position, the cogs simply butt up against the stops inside the valve, Power is still applied, but the motor isn't strong enought to cause a problem.
Try doing that to some other motors, and they will just go up in a puff of smoke.

Also, a syncron motor has a very useful property, which helps the mid position valve to fulfil it's function. That is, when fed with a Direct Current (see below for a fuller explanation), the motorized valve will actually stick in position. The spring cannot pull it back, since the DC Current is holding it there. and cannot move forward, as it needs AC for that

These valves usually have 5 connecting wires, sometimes only 4 if the earth is missing. For the purposes of simplifying the diagrams below, I am only going to show the live wires. This type of diagram is called a functional flow wiring diagram and are MUCH easier to follow. Neutral (blue) and earth (green/yellow) are implied. Manufacturers wiring diagrams , while often very good, usually make it difficult, if not impossible to understand what's going on, as they show so many wires. Thus making fault finding difficult.

There are actually three Live wire connections to the mid position motorised valve.

A white wire, which is energised by the external time and temperature controls, If white is live, the controls are calling for Central heating.
A grey wire, also energised by the external controls, only this time, when it is energised, it means we do not want hot water. ( and if the grey is de energised, we do wand Hot water).
and an orange wire, this is a bit tricky to explain, it has a dual method of energising, and you need to know how the Valve and controls work in combination. ( explained fully below).
For now, note that the external controls toggle the grey and orange wires. So If hot water is required, orange is made live and gray is not , if hot water is not required, the grey is live and orange is not.

Refer to the wiring diagram of the external controls. The diagram shows the controls in the off position.
this makes it a bit clearer. see, when the heating and hot water are off, the grey wire is still live. The white and orange should be dead.

Wiring of the external controls

Now, you will see from this wiring diagram, that the orange wire is also connected to the boiler and pump, so when it is live, the boiler will fire up so we can actually get the heat we are trying to control.

From the wiring diagrams above, you will see that this is fine when the controls are calling for hot water, but how does this get energised when we only want Central heating ? and if we do energise it, which obviously we have to, to get the boiler to come on, how come we don't get hot water as well ? Well read on and refer to the internal wiring diagram below.

Internal Wiring of the mid position Zone Valve



Inside the motor box, is an arrangement of two, change over micro switches, two resistors and a diode to convert the AC to DC, so we can stall the motor.
Note, we are only using one diode, so when the motorised valve is in the mid position,

the current to the motor is not really DC, but rather, a modified AC, The wave looking something like this. This is enough to stall the motor.

So the valve can be in three states, as follows:-

1. De-energised:- Where the spring pulls on the valve, closing off the Central heating port thus corresponding to a hot water only position, if the power is on, but we are not calling for hot water or central heating, the gray wire will be live, via the controls as above. When we are calling for hot water, then the controls energise the orange wire and de energise the gray, thus firing up the boiler and pump. Hot water from the boiler is pumped to the valve and as the HW port is open, it all goes to the calorifier in the HW cylinder.The motor will get a small current, through both resistors, but the two resistances combined are enough to ensure the motor does not turn.
2. Fully energised:- where the motor has wound up tight and closed off the hot water port, corresponding to the Central heating only position. It should be noted, that in this position, the motor is still energised with AC, but it is not turning, as it has stalled. This is usually very bad for an electric motor, try jamming the motor on your vacuum, and you will soon see smoke, as the motor tries to draw more power to turn. The Syncron motors in these valves , which only consume 6 watts of power at 240Volts, are designed to cope with this condition.
   The control arrangement makes the White live, the gray live and the orange off. The orange wire is only temporarily off however. As the motor opens the valve and throw the switches, first, sw1 changes over, from the white to the gray, Following the wiring diagrams, you will see the motor still gets full power via the gray wire, and hence continues on, tripping sw2 thus energising the orange wire, via the white.
3. Mid position:-the controls make the white and orange wires live and the gray off . If the valve is already closed (HW only ), then it the motor will be fed power initially via the white wire, when it switches sw1, Power comes via the white supply through SW2,, half rectified through the diode and reduced by R1 via the white wire, effectively stalling it at this mid point, corresponding to both heating and hot water. Again, it should be noted that this is normally a bad thing for an electric motor.and it is one reason why these motors burn out regularly. Note, that since the orange wire is live by the controls,
   Note that the controls are supplying power to the orange wire, which is how the boiler and pump are energised. this power is fed through resistor r2. the value of which has bee chosen to minimise the magnetic stiction effect you get when a motor like this is fed DC. So the motor is actually being fed a modified AC Signal, enough to have a stalled motor, with minimal magnetic stiction.
   Just to complete the explanation, the valve could have been in the Central Heating only position, when we called for both hot water and Central heating. From this position, both switches were already changed over and the motor was being fed full power via the gray wire through sw1. Now the controls have de-energised the gray, and energised the orange, so the motor looses power, and the spring will start to pull it back. Checking the internal wiring diagram again, you will see that since the orange wire is live, the motor is in fact being fed reduced power via the resistors r1+R2. this allows the motor to produce some force, which prevents the spring from pulling the valve back to the mid position too quickly and overshooting. When SW2 swaps over, the motor is again fed through the diode, stalling the valve in the mid position as explained above.