These webpages deal primarily with modern Sunvic motor-on motor-off (MoMo) actuators including the popular Unishare type.

To operate your system without the valve actuator in place can be a little daunting if you are not experienced in electrical work and central heating. Some guidance on how to operate simple systems without the valve actuators in place is given below. However....... don't meddle with electricity unless you know what you are doing. You could kill yourself or set fire to your home. Also, do not open any actuator unless you are competent to do so.

In 2007 I became interested in why a range of Sunvic motorised valve actuators seemed prone to very early failure. I had previously enjoyed a career as a scientist with much of my work centred upon domestic and industrial heating and cooling systems. I knew from personal experience that older designs of motorised actuators used in housing often worked reliably for 20 or more years, yet the new 'improved' designs failed within a few months or years. When writing up my findings I envisaged that what I had discovered would be of interest to only a small number of technically minded people. In the event, this section of my website has become perennially popular.

Initially, it was devoted only to the 'new' designs of actuators utilising electronics in lieu of the more conventional spring return mechanisms and indeed the new designs did, in theory, offer quite a few advantages. The webpages are now being reordered to illustrate the most common faults not only in the new designs but in those that have remained substantially unchanged for decades. Some evidence has been collated showing the degree to which the micro-switches often used within motorised actuators may overheat and the propensity of the circuit boards within the 'new' Sunvic designs to explode. To my knowledge no house fires or similar events have resulted from a failed motorised actuator.

A page of photos showing common problems is now available.

A recent addition to the market, a few years ago now, is the Sunvic SZMV2305 - internally it looks almost identical to the 1801. It has a different make of microswitch - but these very rarely gave problems anyway. The reason for adopting a new part number is therefore unclear - maybe it is just an effort to distance the design from bad publicity surrounding types 1801 and 1901.

I have had personal experience of the explosion of a circuit board from a failed 1901 unit - this occurred when the unit was connected under test, having been sent to me for examination, having already blown a boiler system fuse. The flash was quite spectacular and accompanied by a large 'bang'  - and this with the circuit board having had several days to dry out whilst in the post and stored at my home. This second explosion (following the first which blew the system fuse) serves to emphasise the importance of always operating Sunvic 1801/1901 units with their protective covers in place. The photographs are at the bottom of this page.

The popular Unishare three port valves are particularly prone to failure but can usually be repaired quite easily. The two port designs simply fail to open reliably a few months before they fail completely. Again, repair is usually straightforward. Spring return valves (typically Sunvic 2301 and 2701 types and many variants of the same basic design) can also often be repaired.

Fortunately, it is not usually necessary to drain the heating system to effect repairs - all that needs to be removed is the valve actuator.

Failure can lead to low room or domestic hot water (DHW) temperatures. This in turn can lead to other components such as thermostats and programmers being suspected of being faulty. Hundreds of pounds are sometimes wasted in replacing components unnecessarily.

More seriously, faulty central heating controls can leave elderly people vulnerable to the cold. Failure of these Sunvic actuators may be expected to be intermittent, at least for several months if not longer, as electronic components gradually degrade. Old people may not be believed when they complain about being cold - because when relatives or health visitors call, the heating system may be working satisfactorily again.

A less common fault in Unishare three-port valves can lead to excessive hot water temperatures - sometimes the motors run backwards! This is owing to gearbox grease becoming 'sticky' with age and heat, and this is usually followed by complete actuator failure - photos are on this supplementary webpage. Again, it is a problem that should not occur, and especially not after only a year or so of operation.

I have never routinely sold the microswitches needed to mend many of these valves because proper testing can be necessary to establish whether the switch needs replacement. In particular, the internal circuitry of the SD2701 and related 3 port designs is quite complicated and many faults stem from sticking motors and gearboxes. Two port 2301 and similar types are simpler. Both types are prone to microswitch failure but usually only after many years' service. It is often not worth mending a 2701 with multiple failed parts.

For 1801/1901 and similar MoMo types, the capacitors must be X2 rated of a similar spec to the originals. They will be 330 nF rather than the 220 nF that Sunvic used in earlier batches of actuators - before they discovered that many of their actuators were failing so prematurely. Be aware when fitting these that the old units may have retained some charge - therefore discharge them before unsoldering. Also, if repairing an early valve with a 220nF capacitor, you will need to fit a 3.25 watt 24 volt zener diode to limit the relay coil voltage if you fit a 330nF replacement. If you just want a 220nF capacitor I  have a few of these (same prices as above) - the actuator will operate reliably for less time but no extra diode is required. In 2013/14 it was suggested that widespread premature failures of X2 rated capacitors as used by Sunvic may have been owing at least in part to poor quality manufacture of the capacitors themselves. A report from ERA is available.

In purchasing these spare parts you MUST be aware that repair of these valve actuators is (of course) not recommended by Sunvic. However, for anyone well versed in soldering of electronic components,  DIY repair is a feasible proposition, provided you are wholly competent in dealing with mains electricity and hobby electronics. If you don't meet these criteria then get someone who is a 'competent person' to remove the actuator for you and make the system safe.

Sometimes, the zener diodes that are connected across the relay coils have been found to be almost 'short circuit' - in this case the DC voltage across the relay coils may be as low as 0.2 - instead of 24. Failure of the smoothing capacitor (which sometimes goes with failure of the rectifier) can be a factor also, but this is uncommon.

In one actuator I mended in December 2010 there was a most unusual fault - a high voltage flashover had blown the relay coil and fried one half of the rectifier bridge. In essence, a high voltage pulse had arced across from the thermostat wire to the low voltage relay coil and returned to earth via one half of the rectifier bridge. I actually bothered to clean up the pcb and renew all the components. The actuator was returned to a bemused householder - who was left with the unsolved problem of why his heating system routinely tripped out circuit breakers. In later years, the issue of exploding circuit boards became more common as more and more valve spindles started to leak very slightly. In these cases, for 1801 actuators, a new design can overcome the problem (see top of page). No such easy option exists for the 1901 three port design.

All legal liability for any incompetent repair rests solely with the person who undertook the repair. If you don't know what you are doing, then don't do it! Organisations such as CORGI (now GAS SAFE) say that only 'competent persons' should ever try and mend a heating system. In my experience, many of their so called 'qualified engineers' should never have been let out of school, so poor is both their knowledge and workmanship. Against that, many DIY repairs to houses are appalling!

It may be noted that the Sunvic company has failed to respond to emails questioning the reliability of products that are described in Sunvic sales literature as being very reliable. Unishare three port (mixer) valves can fail in under three years - I have repaired some that are between 1 and 2 years old. There is a good technical reason why these popular Unishare valves are proving to be even less reliable than their two-port alternatives.

Also, remember to make a careful note of which colour wire goes where - and if you are removing two valve actuators, which wire from each actuator goes where! Do NOT attempt this work unless you are experienced with mains wiring. With systems using two two-port valves (usually, one for central heating, the other for domestic hot water control), you may find that two orange wires, one from each valve actuator, are joined together - this is normal. The two grey wires may also be joined (again one from each actuator) - these are permanent live feeds to the two-port actuators, similarly the two blue wires (mains neutrals). The two brown wires must be replaced in their correct positions - or you will end up with the central heating controlling the dhw and vice-versa. Typical wiring diagrams are shown in the Sunvic catalogue (750k pdf file, allow time to download) - the diagrams are on page 17. Be sure to study the right one!

The wiring of many central heating systems in the UK is undertaken to an appalling standard (typical of the UK building and plumbing industries) so be prepared to find shoddy work that needs rectifying. For example, it is quite common to find three port valves wired up without a cylinder thermostat. It is all too common to find systems fitted without a bypass loop and in system designs where one should have been fitted.

If the Sunvic valve actuator controlling the dhw supply has also failed the dhw can of course be operated also but the stored water temperature will approach that of the main boiler thermostat (because the valve will no longer be controlled via the cylinder stat). In these cases, to avoid the risk from scalding, turn down the main boiler thermostat to about 55-60C. The valve can be closed manually whenever it is desired to stop heating the DHW. All these procedures require some knowledge and competence and, once again, do not meddle unless you understand what you are doing.

If the boiler and plumbing has been installed correctly (famous last words for the UK plumbing industry!) and with a bypass loop, there should be no problem in closing both dhw and ch valves manually if you wish to control the system in this way - the boiler will then cut out on its own thermostat(s) (which it may do anyway during normal operation with a light heating load) and the pump should run until excess heat has been dissipated via the bypass loop. Again, there are many examples of poorly installed plumbing systems in the UK, so ask an expert (if you can find one) to check if you are in any doubt about operating your system without the valve actuators in place.

Three-port valves operate in a similar manner but the wiring is different and some valves are never installed properly by builders and plumbers in the UK. Some other types of three-port valves (not the Sunvic type discussed here) are quite complicated internally.

Supplying 'live' to where the orange wire from the valve was connected generally enables the boiler to light up and the pump to operate whatever type of valve has been fitted, but there are issues of how hot each circuit will get if it is not properly controlled - so turn down the main boiler thermostat to about 55-60C. Generally the orange wire is made live within three port valve circuits when CH is required and the actuator is instructing the system to provide central heating. It is made live for dhw via the cylinder thermostat. But remember - don't meddle with electricity unless you know what you are doing. You could kill yourself.

My personal recommendation for standard domestic heating systems has always been to use two two-port valves, rather than one three-port valve. The much older design of Satchwell Sunvic 'Minival' two-port motorised valves were renowned for their long trouble-free life - I have some that are 30 years old and they are still working. Again, the appropriate wiring diagrams are given in the Sunvic catalogue. A common cause of failure is that the valves become stiff and this overloads the drive gears in the actuator. The drive spindle of 2 port minivals should be free enough to be turned around using only fingers. If it is not, try rotating it many times in either direction using pliers. This can 'free off' scale inside the valve.

pdf diagrams of wiring in Sunvic valves (use the zoom feature of Acrobat to view the lower diagram clearly)

Also available is the installation leaflet for Sunvic SZMV series valves and SZM 1801 actuators - strangely it states that it should not be left with the user - yet these motorised valves used to be sold by B&Q and other do-it-yourself superstores direct to end users! This leaflet includes the basic wiring diagrams needed by installers.

Other companies in the plumbing industry are more open about their products - for example, Triton showers are often easy to mend because parts can be ordered direct from their website. However, like so many UK companies, Triton are increasingly buying in components made in China or elsewhere overseas and no small spare parts are available - only complete assemblies. I once had to mend a flow stabiliser valve in an electric shower. All I needed was a simple 'O' ring to replace one that had perished. Triton offered to supply a complete valve for £40. I used an O ring of slightly the wrong size and it works perfectly. 'Make-do and mend' has a lot to recommend it - but only if you know what you are doing!

detailed explanation of failure of Sunvic MoMo valves

why Unishare three port MoMo valves are less reliable than equivalent two-port valves

water damage to the electronics of a Unishare 1901 actuator

photos of motor and gearbox failures in modern Sunvic valves

home page