|I sent you the following email
nearly two weeks ago - I look forward to receiving your initial response in the next few
From: "stephen wozniak"
Subject: failure of two-port valve actuators
Date: Mon, 29 Jan 2007 18:13:03 +0000 (date and time
when originally sent)
Sales & Marketing Director
Sunvic Controls Limited
Failure of 2 port MoMo valve actuators type SZM 1801,
purchased October 2003.
Quote: "Sunvic Controls Ltd has launched a brilliantly
innovative, totally reliable range of two and three-port motorised valves for domestic
Quote: "These patented MoMo motor-open/motor-off valves,
available as two-port or three-port in all popular sizes, provide total reliability plus
compatibility with existing and conventional wiring systems."
To begin, I cannot find the type number SZM 1801 on your website but the actuator bodies
have a label attached bearing Patent Pending GB0222324.6 which aligns with Patent
GB2393490 on the Patent Office website - so I am assuming you did make the actuators in
I am experienced in design and installation of central heating
having been a research scientist in H&V for many years. I have also personally
installed probably a dozen of your earlier type MINIVAL SM 3203 actuators. A couple of
these are still working perfectly in my own home after 10 years. I am familiar with the
problems of the earlier type spring-return valves that led directly to your marketing of
4-wire compatible MoMo designs.
On 6 October 2003 I purchased from B&Q in Exeter two complete
two port valve assemblies that incorporated SZM 1801 actuators. Both of these failed
recently and in an identical fashion.
The valves were purchased as part of an upgrade package - comprised
the usual room stat, cylinder stat, two 2-port valves etc and was undertaken with a view
to making later installation of a more modern boiler much easier. As you will be aware,
these require a fully pumped system.
Failure of the valve used on the CH was first noticed in December
2006 (although when it first exhibited erratic behaviour is uncertain) and the DHW valve
I have now turned my attention to exactly what is wrong with these
valve actuators. On test, both always switch off reliably and are 'stable' in the off
position. It is switch-on that is the problem. When power is applied to the brown wire,
sometimes the valve motor will run but more usually not - except that is when the assembly
is subjected to vibration or a sharp tap on the outer casing. This mechanical disturbance
is always sufficient to 'wake up' the motor and gear mechanism - which then operates
The intermittent operation is thus explained - vibration from the
nearby pump and the odd bit of creaking pipework could have been quite sufficient to
'nudge' a valve into operation. Indeed, it is possible that the vibration from the DHW
valve turning off under time-switch or cylinder stat control (for example) would have been
sufficient to nudge the closely adjacent CH valve to operate if it had been electrically
commanded to switch on but had failed to respond.
Once the actuators are switched on and 'open' no amount of vibration
or tapping will cause them to switch off. The only other significant observation is that
the actuators seem to exhibit worse behaviour when 'horizontal' than when 'vertical' (that
is, how they would be if installed in vertical pipework). Gently lifting a non-responding
valve from the horizontal to vertical position (with the motor shaft now horizontal) can
be sufficient to cause it to operate. In situ, the valves were in vertical pipework, which
may explain why their failure was not noticed sooner.
Other curious behaviour includes that when the valves are 'open' and
power is removed (to mimic a power cut), sometimes but not always both valves will undergo
a complete cycle when power is restored, but with one or other valve (sometimes both of
them) failing to return to the 'on' or 'open' position. It is then necessary once again to
tap the casing(s) to initiate rotation to the open position.
Before I delve further into exactly why these valves are behaving in
this curious manner I would like to know if you recognise a generic design or quality
problem with valves of this vintage. It is probably either a simple matter of poor design
of the microswitch operating 'lugs' which makes them susceptible to a very slight amount
of wear - or maybe the hysteresis in the microswitches themselves or the considerable
amount of 'free movement' in the gearing has not been adequately accommodated. The other
possibility is that the relay contacts have 'burnt' or become high resistance. I note from
your Patent for these valves the following text that describes the operating sequence for
opening a valve:
"As with a conventional
spring-return valve, the control electronics, as 5 shown in Fig. 4, has four input wires
coloured brown, blue, orange and grey. As with a spring-return valve, there is one wire
(the brown wire) that controls both opening and closing of the motorised valve. When mains
power applied to the brown wire, this is a signal to open the valve. When this power is
removed, the control circuitry provides the 10 required signal to the motor drive 10 to
close the valve.
As shown in Fig 4, the brown input is
reduced in voltage and rectified by a series capacitor C1, a bridge rectifier BR1 and a
parallel capacitor C2.
The blue wire provides a return path
for the AC input. This provides a low voltage DC signal for operation of a relay RL1. When
power is 15 applied to the brown wire, the relay RL1 operates and is held closed to cause
mains applied to the grey wire to be transferred to a first of the microswitches, labelled
S1 in Fig 4. The microswitch S1 transfers power from the grey wire to the motor drive 10,
causing it to start rotating.
This, in turn, drives the connecting
member 30 and the valve spindle in 20 the forward direction, once all lost motion has been
Movement continues until the button of
the microswitch S1 enters the corresponding cam slot 34. This turns the microswitch S1
off, and the motor drive stops. At this time, the valve is in the open condition."
It would seem that failure of relay RL1 to operate (maybe because it
is a low-spec design and the contacts have 'burnt' after only a few years' operation??)
would indeed cause all the symptoms that have been experienced. From the evidence so far
available, this seems to me to be the most likely explanation. The entire central heating
system is fused at 3 amp as per boiler manufacturers recommendations - and your actuators
should easily be able to accommodate this loading. The correct performance of these does
of course depend crucially on proper long term reliability of RL1.
I am particularly disappointed in all of this because I deliberately
chose 2 two-port motor-on motor-off valves because I had always found these to be more
reliable than the more usually installed three port designs - to say nothing of spring
return two-port types.
The date codes on the motors are 03-03 and 49-02 but this is hardly
relevant as there appears to be nothing wrong with the actual motors.
The date codes on the undersides of the actuator bodies are 04-03
and 06-03, all of which is consistent with purchase in October 2003. I have the original
I await hearing from someone competent in your technical department.
In the meantime I will continue to investigate whether any other
feedback from users or further relevant information is available on the Internet.
Dr Stephen J Wozniak
29 January 2007