On a 3-port mid-position valve, which is Port A and which is Port B ?
The Honeywell FAQ clearly states Port B is for HW and Port A is for Heating, and with no power to the valve it is set for HW-only (Port B).
However the uk.d-i-y FAQ says "The spring pulls the valve to position A, while the motor winds it towards position B.", and "In the water- only state ... the spring will therefore pull the valve back to A, where it will sit."
The uk.d-i-y Wiki seems to concur with the Honeywell FAQ, so I guess the wiki entry is correct and either i) the uk.d-i-y FAQ is misleading, or b) I'm reading it wrong.
In fact, let's check this makes sense (* for each change):
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:
[snip piccie]
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.
*** are we happy with SW1 and SW2, or do we want to swap them on the diagram:
formatting link
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.
another problem. What happens if the programmer calls for "CH On" together with "HW Off" ?
What is wanted is for the valve to motor fully to B position (heating only), but it looks like the valve would motor to the mid-position and stay there. Moreover there would be no feed on the orange wire to fire the boiler & pump. I believe what's missing is a link from valve Grey to the programmer HW Off, as shown here:
CH On, HW Off, should drive the valve to the A position (A when the text on the FAQ is corrected as is is now ;-). That should close SW2 and make the orange wire live, which fires the boiler.
Y plan diagram. However the link is in the system wiring for the Y plan setup, not internal to the three port valve.
CH On, HW Off, should drive the valve to the A position (A when the text on the FAQ is corrected as is is now ;-). That should close SW2 and make the orange wire live, which fires the boiler.
Y plan diagram. However the link is in the system wiring for the Y plan setup, not internal to the three port valve.
Except it won't do that with the wiring as shown at the above url.
As drawn, the valve would motor to the mid-position and stay there, and there would be no feed to fire the boiler or pump. Adding a wire from valve Grey to the programmer HW Off should allow the valve to motor fully to A, and then fire the boiler.
You're right. *sigh*, it's designed to confuse. ;)
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