| 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 microswitches, 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 A, while
the motor winds it towards position B. If the motor is left
continuously powered, it will stall in position B, but if it
is fed with DC (produced with the resistor and the diode), then
it will stall in any position. Two microswitches, operating
just either side of the 'A+B' point, are used to define this
position.
In the "both off" state, the system
wiring results in grey being live. If the valve happens to be
in the A+B or B position, SW1 will have been operated, the motor
will be fed with AC, and the valve will wind to B 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 A unenergised.
In the "water only" state, neither
grey nor white are energised. The spring will therefore pull
the valve back to A, where it will sit.
In the "water and heating" state,
white is energised. If the valve is at A, 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
B. 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 A 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 A 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.
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