Failures of SUNVIC motorised valves including those with actuators SZM 1801 and SDM 1901 Unishare valves also SZMV2305 (and spring return types 2301 and 2701 and variants). Updated November 2016.
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.
Historical perspective: 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, these pages have become perennially popular.
Initially, these pages were 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. These 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.
Whilst much of the original website is being left 'as is' a new page of photos showing common problems is now available.
A recent addition to the market 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 new page ordering will include:
ACL (DRAYTON) motorised mid-position actuators especially type MA1 (often type MA1/679-3) and similar types as shown in this photo usually fail owing to a very small and unusual internal microswitch burning out. This can be replaced, please enquire.
More photos are here.
Email address: stevewozniak42 AT hotmail DOT com
These actuators can usually be repaired.
So predictable seem to be the failures of some Sunvic actuators that their design may be called a minor environmental disaster. Tens of thousands have probably been manufactured, all may fail within a few years (some within 18 months) and most will be scrapped by plumbers who simply fit replacement units. If properly designed, valve actuators should last 20 to 30 years - as Sunvic/Satchwell Minivals used to do. An increasing number of failures are now being seen where the circuit boards have exploded. The reason is that the 1801/1901 range of 'electronic' actuators have proven to be extremely sensitive to even the slightest water ingress via leaking valve spindles and the circuit boards are poorly designed. Older designs of actuators are unaffected by slight leakage, as is further explained below.
This section of the SeeRed website details why Sunvic motorised central heating valve actuators of a particular design often fail to operate correctly. The main text applies to both modern two-port and three-port (unishare) valves. These all have a similar design of electronic circuit and regularly fail in a similar manner. These valves are not sold only under the Sunvic name. For example, they are found in expensive Worcester-Bosch systems, as this email has highlighted. These 'badged' actuators are also sold at curiously high prices.
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.
Supply of spare parts for Sunvic motorised valve actuators. Updated November 2016.
I have a few spares available. Capacitors for MoMo valves (1801, 1901 and similar types) can be supplied for £1.50 each, plus a P&P charge of £1.00 (to UK) - so one capacitor will cost you £2.50 and two will cost you £4.00. Except for zener diodes and a few microswitches, NO OTHER SPARE PARTS ARE USUALLY AVAILABLE. I may sometimes have a second-hand part available if your actuator has an unusual fault.
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.
For 1801/1901 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 still have a few of these (same prices as above) - the actuator will operate reliably for less time but no extra diode is required. Quite recently (2013/14) it has been discovered 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. 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.
||It is curious that even in 2008
and indeed also in 2010 and beyond, Sunvic were still apparently telling customers who
complained that they 'have had no complaints about these actuators' and are not aware of
any design faults. The original design dates to around 2001/2 but as early as 2003/4
Sunvic had changed the design to incorporate a larger capacitor and a zener diode.
The only reason for undertaking this design change would be to try to prolong the life of the actuators before (premature) failure occurred. As it happens, the design modification seems not to have improved matters very much, especially for the Unishare actuators.
Here are some failed capacitors, together with a couple of faulty microswitches. Probably thousands of actuators are simply thrown away by plumbers. What a waste! Sunvic have used capacitors made by at least two major manufacturers. Both types seem to fail in the same way - so a specific design or manufacturing problem seems to be ruled out. However the problem may be related to inadequate capacitors as well as to problems with circuit design, such as absence of a series resistor.
The detailed emails I sent to Sunvic on 27 Jan (resent on 9 Feb 2007) are here. These date from before the time I had ascertained the exact reasons for failure.
In an email to me dated 30 October 2007 (click here) Sunvic claimed never to have received my emails. They were certainly received by the persons to whom they were sent "bcc" (including to my own in-box). It would therefore be truly remarkable if Sunvic didn't receive at least one of them - as can be seen, each was sent to two valid Sunvic email addresses.
A selection of emails received from consumers who have suffered failures of Sunvic valves is here. Some make interesting reading. I have dozens more!
Important safety notice: Motorised valves will most often be found as a component of traditional central heating systems that include a hot water cylinder. Increasingly, combination boilers are being used - especially in smaller houses and flats - and these are both more complex and more potentially dangerous. Never alter any part of a combination boiler system or a pressurised hot water system without seeking advice. The usual use for motorised valves with a combination boiler will be for zoning of a large central heating system (and using two-port valves). These can readily be operated without the actuators in place.
Whilst repairing some types of Sunvic valve actuators (and many other makes) is an easy job for anyone versed in electronics, inexperienced people should not attempt this work. Some skill in soldering is required - and remember these are mains operated devices. The usual precautions should be taken when removing a unit from the mains wiring - including switching off the isolating switch-fuse that should control the entire heating system and removing the fuse. In addition, check that the system really is 'dead' after removing the fuse - I have known houses in which live and neutral lines have been reversed even at the main distribution board (fusebox)!
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.
TWO PORT ACTUATORS: operating a heating system without an actuator in place.
If removing a two port valve actuator, the central heating can be left in operation by turning the valve spindle to the 'on' position and using the time-switch or programmer to control room temperature in a crude manner. However, it is also necessary to 'con' the boiler into thinking the valve actuator is still in place and ordering heat. This can be done by temporally linking together junctions within the wiring centre of the heating system - but again this may be fraught with problems because (in my experience) so many are wired to a poor standard. Non-technical householders should not attempt this work themselves. It is a matter of linking together the two terminals to which the BROWN and ORANGE wires of the valve actuator were originally connected, if the thermostat is a type that can handle the pump current directly. This has the effect of telling the boiler electronics that the system is asking for heat. If you want to control the system manually, link the terminals to which the ORANGE and permanent live (GREY) wires were connected and simply switch the boiler on and off as required at the isolator switch. This avoids possibly overloading the thermostat (BROWN wire).
The way two-port actuators work is as follows: The BROWN wire supplies live (240 volts) from the relevant thermostat to command the valve to open when the thermostat requests heat. The valve actuator should respond by opening the valve and then signalling to the boiler and pump to switch on - it does this by supplying 240 volts (live) to the ORANGE wire, taking this live supply from the GREY wire.
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:
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.
To operate the system without a three port actuator in place can sometimes be accomplished by very carefully turning the spindle of the three port valve a little bit with a small pair of locking (molegrip) pliers and wedging it in an open position. In its 'closed' position it will supply only dhw and the system should work without the actuator in place at all - because none of the internal circuitry is utilised for dhw-only operation.
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.
|A view of the motor
unit SZM 1801 as used on a Sunvic two-port valve. Similar problems occur within Sunvic
three-port Unishare actuators. Indeed, the Unishare design is even more prone to early
failure than are two-port designs. Similar motor control electronics is used within
three-port Unishare valves, but some of the components are 'live' for longer, and so fail
This is disappointing because the basic design that lay behind this innovative range of valves is sound - they are a direct wire-in replacement for spring return valves that are a common feature of many domestic central heating circuits. Spring return valves can fail via motor burn-out because the motors are energised all the time the valve is required to be open - but they generally last much longer than these newer Sunvic designs!
You can easily tell if a Sunvic valve of this type is 'open' or 'closed'. Just look for the 'flat' on the red gear wheel - a good design feature. With Unishare valves, the flat denotes the mid (mixed) position.
Removing the actuator head (the part shown here) could not be simpler - just follow the instructions on the Sunvic leaflet (click here)
|The motor unit of a
two-port valve: gear wheels (red) and the light blue capacitor which has proven to give
problems. This is X2 rated and replacements MUST be of a similar specification.
X2 capacitors are usually used for suppression of high frequency voltage peaks across 240 VAC lines. They are often used in situations where failure of the capacitor might cause a fire - these capacitors are designed to fail-safe, although some problems apparently still occur because their inherent self-healing characteristics can become compromised.
Where failure of a capacitor would lead directly to an electrical shock hazard then an even higher specification Y class capacitor must be specified.
Failure of a microswitch can lead to these motors running for hours instead of for a few tens of seconds. They then overheat and burn out. Sometimes hot grease leaks out and jams up both microswitches! This is one of the more unusual modes of failure for these supposedly reliable devices.
Gearboxes also fail - see below.
|View of the two
microswitches that are operated by the red gearwheels. The electrical relay is on the left
- it is a 24VDC type which needs at least 17 to 18 volts to operate reliably. When the
large capacitor begins to degrade, the available operating voltage drops below that
necessary for reliable switch-on, hence the valves exhibit all the symptoms referred to in
my emails to Sunvic. I first suspected that either the microswitches or the relay itself
was at fault.
The equivalent three-port designs are more complicated
having two relays and two sets of C1/C2 capacitors and rectifiers, three microswitches but
only one motor - all quite clever!
|This is the component
that causes most of the trouble. The circuits used in these actuators are arguably a
poor design because partial failure of either of the two capacitors (or both of them) can
lead to a lower than acceptable DC voltages. This is a 220 nF unit from an early valve
One of the mysteries is why Sunvic used a DC relay especially for the two-port design. Using a slightly more expensive AC relay would have avoided all the problems. There may be good reasons why the circuit was designed with a DC relay - but I cannot think what they are! Except maybe - that in the Unishare design - using ac relays would have imposed significant cost and size disadvantages on the finished product. Low profile relays of the type employed simply do not seem to be available with 240vac coils.
Safety note: whilst replacing these capacitors is a trivial job for anyone versed in electronics, inexperienced people should not attempt this. Some skill in soldering is required - and remember this is a mains operated device.
Replacement capacitors MUST have an X2 rating with a 275VAC working voltage.
|The zener diode fitted
in modern Sunvic valve actuators to limit back emf produced by switching off of the relay
coil - and to limit forward voltage. This modification (shown rather poorly soldered in
this example) may have been introduced by Sunvic in response to early valve failures
examined under warranty.
Ideally, a diode would have been incorporated into the pcb design: this 'afterthought' is hardly an advertisement for quality manufacturing.
Other diodes found in these actuators are physically smaller and are BZX 24 volt types. If the diode fails the coil voltage may rise to about 30 - which may not be too serious - or may drop to 0.2V - which stops the unit working at all.
|A 330nF capacitor is
fitted to modern actuators.
Replacing a microswitch must be done very carefully and using a type exactly equivalent to the original - with a 3 or 5amp 250 VAC rating.
In one Unishare actuator I tested, a 330nF capacitor was so degraded the relay voltage was only 9.8 - and the unit was less than 3 years old! In another, it was 12 volts after only 15 months.
Sunvic appear to have changed the capacitors to 330nF to try to avoid premature failures but to little avail - because whole idea of using X2 capacitors to drop mains voltage may be suspect: a particular problem may be high temperatures of operation and frequency of thermal cycling. Another problem (highlighted in 2013/14) may be poor quality capacitors, and this seems to have affected many electronics manufacturers.
It is interesting that most plug-in digital timers use an X2 capacitor to drop mains voltage. It is a cheap way to produce low voltage DC if only a low current is required. These units may have a larger zener diode and be more able to tolerate degradation of the capacitor. The electronics may also be supplied via a voltage regulator - so reductions in the basic VDC output may not matter too much.
|Overheating of PCB
tracks in a Sunvic actuator that failed because of a microswitch in which the points had
become welded together.
This failure was caused by excess current: a warm air system heater had been incorrectly wired with full fan current passing through the valve actuator. A wiring fault of this type could result in a house fire - so if a component fails in this manner, find out exactly why!
The tell-tale overheating and incipient blistering of the tracks (shown asXXX) is most pronounced on the track connected to the grey wire - which takes the mains live supply into the valve circuits.
Discoloration of the track connected to the orange wire was slightly less pronounced.
|The inside of a
microswitch that had failed (with contact points welded together). The cause was
electrical overloading owing to a fault in a warm air heating system. A 3 amp fuse is usually specified for the isolator
of a conventional domestic wet central heating system.
The contacts are underneath C. Discoloration at regions X bear witness to overheating.
If the contact points were the cause of the overheating (because of high contact resistance) then discoloration would be expected primarily close to the points - yet here it is just as bad on the left hand side. This suggests excess system current as the primary cause of actuator failure.
|In this case however,
the circuit board has failed in a spectacular fashion, melting both micro-switches and
rupturing the 330nF capacitor.
Although the reverse side of the pcb is badly burned in the immediate locality of the switches, the tracks elsewhere are not discoloured, and the system was fused at 3 amps. Excessive current is therefore an unlikely cause of the failure.
In this example therefore, failure was likely owing to a high resistance contact in one of the micro-switches - leading to localised overheating.
Similar problems are sometimes found within domestic 13 amp wall sockets as well as (quite commonly) in Sunvic 2301 and 2701 actuators.
|In this example, the
relay points and push-on connectors have overheated. This illustrates why actuators have
heat resistant housings. The temperature that was attained was enough to melt and burn PVC
and to char the relay casing and pcb - yet probably a house fire was unlikely.
Despite that such pieces of 'domestic electronics' may pose a low fire hazard it is maybe not a good idea to smother them in blankets - as can happen inside airing cupboards.
It is surprising that this actuator did not go 'open circuit' and stop passing current before the overheating became so extensive - but current would cease only when one of the wires or contacts had completely oxidised or melted.
Moral of the story: never ignore a strange smell of burning from your airing cupboard, or from under the floorboards!
This all shows the consequences of poor basic design - including perhaps tens
of thousands of repairable Sunvic actuators being scrapped.
The full wiring circuits for two-port and three-port valves are included in the following pdf page. This is taken from the patent website describing these Sunvic valves.
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