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Honeywell 3 Port Valve Actuator How it Works

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I recently had a problem with a 3 port valve, mine is a Honeywell but there are many similar ones by different manufacturers. Being electronically minded, I decided to check the actuator’s function. I found a couple of sites that "explained" how it worked and a video. None of them made complete sense so I decided to check out the circuit myself.

I came to the conclusion that the explanations that did not make sense were based on analysis of the circuit diagram only. My explanation is based on analysis of the circuit diagram and observation and measurements of the valve actuations. There are 2 areas where I’ve made what I hope are intelligent guesses.

So for those who want to fault find their 3 port valve, here is my explanation.

The first none electronics action after removing the actuator is to check that the valve has not fully or partially seized up. You should be able to move it easily with you fingers, note how small the movement is, only a few degrees. If its stuck, out with the spanners, new valve needed.


Using the diagram above as a reference.

The valve actuator moves the valve to the correct position and turns the pump and boiler on or off for central heating (CH). It does not turn on the pump and boiler for hot water (HW).

The actuator has 5 wires, 2 are earth and neutral. The white wire supplies 220vac to the valve motor and Sw1 & SW2, at the appropriate time the orange wire receives 220vac from SW2 which is passed to the pump, boiler and to the valve motor via 2 resistors. The grey wire delivers 220vac to SW1 from the HW Thermostat when it is above temperature i.e. HW OFF

Valve Inactive
With no power from the programmer to CH or HW, springs in the valve actuator pull it to the Hot Water (HW) only position. The diagram above shows the micro switches in this position.

HW On only
If HW is required, the pump and boiler are turned on by 220vac from the Programmer via the HW thermostat. When it is below temperature, 220vac is removed from the Grey wire to SW1 and used for pump and boiler. When it is up to temperature Grey is put back to 220vac.
The Valve has not moved.

Valve Operation CH On HW ON
This is the valve’s mid position.
The valve will start to move when 220vac from the CH thermostat is sent to the White wire. At this point 220vac is applied via SW1 and SW2 to to the actuator motor. Since the path from SW2 includes a resistor and a diode the supply takes the easy route via SW1 and powers the motor. The motor pulls against the springs and continues to turn until it operates SW2 (not SW1 as stated elsewhere) at this point the valve is in the HW & CH position when power is applied via SW2 to the pump and boiler. The motor stops when SW1 is actuated soon afterwards. Since HW is ON, 220vac is not present on SW1 (Grey). The motor is supplied from the Orange wire via 2 current limiting resistors so that it can hold against the pull of the springs.
The valve is in mid position. Hot water can flow to HW and CH.

CH On HW Off from Mid Position
If hot water is no longer required i.e. the HW thermostat is actuated, the Grey wire to SW1 will be at 220vac The motor is now supplied with power and moves the valve to the CH only position.

Intelligent Guess No. 1
It would seem that the motor is held at full power in this mode.
This would explain why the Motor is a commonly available spare and why it can get very hot.
See Quess 3 below for how to go to MID or HW only

CH On HW On from CH only
When the valve is held in the CH only position, and HW On is needed, the 220vac at the SW1 Grey is removed. The motor now has reduced holding power via the resistors from SW2 but the springs are stronger so it is pulled back until SW1 is deactuated, 220vac from White is applied and the motor moves the valve to Mid position. Since the boiler and pump are already on (CH), power from the HW thermostat doesn’t do much.

Intelligent guess No. 2
Up until now the diode has not had a mentioned. Some explanations talk about demagnetising or reducing magnetic striction? by supplying rectified AC to the motor.. This does not make sense to me and sounds like techo-babble, the reason I say this is because:
The motor is supplied with AC most of the time, I would have thought this was a better demagnetiser than half wave. The only time it is not (HW+CH mode) the diode is not even in circuit.
The only time the diode is in circuit with anything is when CH is turned off and SW1 and SW2 are deactuated. The diode + resistor make a circuit with the motor, since a synchronous motor can act as generator and produce an EMF, I think it might be a braking system when the springs pull it from CH+HW to HW only, but I’m not 100% convinced. It does supply Orange via a resistor but I can't find any reason why the pump and boiler would needed a current limited halfwave supply. I've seen people mention finding 100V on Orange, this must be it, as to its purpose?

If you know qny better please let me know, preferably facts, but fact based opinions are fine.
I guess I’ll get what you give me :)

Update 1/5/2017
I was not overjoyed about the part of the explanation for moving from CH only to HW only (now removed) so I did some checking.

On my system HWoff was on i.e. 220vac at Grey when HW is not required either up to temperature or off at the programmer. I've looked at a few programmer wiring diagrams and some of them use HWOn and HWoff. When the water is up to temperature HWon is applied to grey. When the HW is turned Off at the progrmmer HWOff is applied to Grey. So it does make sense.

The circuit diagram above was not my creation so I checked it against the circuit in my old valve actuator. It is correct.

I had noticed that the actuation levers on the micro switches SW1 & SW2 had an unusual shape. On the very end was a V shape. If the cam is run past this point the SW1 deactivates. SW2 could but I don’t think it does, see below.

Intelligent guess No.3
For CH only the sequence is as before, but the motor moves the cam up the V and over the other side until SW1 is released. 220Vac from CHon White supplies the motor keeping it at CH only. The cam physically cannot go any further the valve acts as a stop.

At this stage SW2 cannot be released otherwise the pump and boiler would lose supply, it must be held on by the cam. I was not able to check this because the micros switches on my old actuator had been bent. Anyway so far so good.

CH off HW on
White is at 0V, power to pump, boiler and motor is lost. Grey is also at 0v so the springs pull the motor and valve all the way back to HW only.

CH off HW off
The valve now needs to be pulled back to HW only but this time HWoff at Grey is at 220vac so as soon as SW1 is made it would supply the motor 220vac which we do not want.

The springs in the CH only position are pulling at their strongest. When 220vac is lost on White the strings pull the cam over the SW1 “V” so quickly that the motor cannot overcome the momentum and continues to the HW only position.

This would also support the theory of the diode being part of a brake circuit. It is only needed between MID and HW only. The springs need to pull past SW1 during CH only to MID so applying a brake would not be a good idea.
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