mid position valve

Posted on Feb 7, 2014

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

mid position valve
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up, and its operation is thus not easy to understand. Here`s how it works. The spring pulls the valve to open the flow through port B (traditionally connected to the hot water cylinder`s heating coil), while the motor winds it towards opening port A (feeding the radiator circuit). If the motor is left continuously powered, it will stall in with port A open, 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. The switches SW1 and SW2 are shown in the valve rest position (port B open). 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 port A open, and blue is neutral. In the central heating and hot water "both off" state, the system wiring results in grey being live. If the valve happens to be in the A+B or A port open position, SW1 will have been operated, the motor will be fed with AC, and the valve will wind to open port 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...

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