Sunvic Unishare three port valves - why they fail after only a year or two in service.

This supplementary webpage details why some designs of Sunvic Unishare three port central heating valves fail after only a year or so in service. The detailed design of these valves makes them more likely to fail prematurely than two port actuators based on similar electronic circuits. In three port Unishare actuators there are two X2 rated capacitors, two relays and three interconnected microswitches.

As in two port designs, these valves depend crucially for correct operation on the operating voltage across relay coils. In Unishare circuits, one of the capacitors is connected directly to the grey wire - which in three port circuits is live whenever domestic hot water is NOT called for by the programmer (HW OFF) and when it is called for by the programmer (HW ON) but the cylinder thermostat senses that the water is already hot enough. This is most of the time in typical houses! Therefore, one of the X2 capacitors is 'live' for most of the time and subject to gradual degradation owing to voltage spikes that are an inherent feature of domestic supply lines. It is this factor that seems to cause such premature failure in Unishare valves. In actuators that have failed, it is usual to find one capacitor giving 12 to 15 volts across one of the relays, and for the other one still to be supplying 20 to 22 volts across the other coil. Replacing just one of the capacitors can therefore be adequate. If the voltage on the second relay is less than 20 volts, both X2 capacitors should be replaced.

Earlier types of 3 port Unishare valves - typically the Sunvic SD2700 series with spring return motors - are also prone to problems - but different ones! It is important that these are wired up correctly - the circuit diagram is shown below. In particular, the HW off connection must be made between the programmer and the cylinder thermostat and the grey wire to the valve. The same wiring is used for the more modern 1901 unishare actuators. Note that three port actuators MUST be fitted correctly to the valve - with the cable from the actuator to the PORT A SIDE (central heating side) of the valve.

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