Sunvic motorised valves for domestic central heating.

A selection of emails received by the SeeRed author, reproduced usually without comment.

Hi Steve, just wanted to drop you a mail to say, thanks for the information on your website, superb. I have a SDM1901 not yet 2 years old and it's basically decided to stay in the mid-position regardless of what the CH or DHW are demanding. Marvellous! So I used to be into electronics and of course, you always have a tendency to take things apart (as you do). First obvious problem, one of the "after thought" diodes isn't even soldered on at one end and is merely making electrical contact, by touching the solder terminal on the PCB... Nice... Not sure if that is the problem/only problem just yet, but hey ho. Then I thought I'd search the web and here I am. So just thought I'd show my appreciation by dropping you a mail.

Kind Regards, Mark

16 Feb 2008

Hi Steve

Fascinated by your website which I discovered from a Google search on Sunvic.

I thought you might be interested in a problem I have had with my Sunvic SDM1901 3-way valve actuator (a couple of years old). My problem has been excessive hot water temperatures, and I tracked it down to the valve actuator stopping slightly off position when CH but no HW was demanded. This leads to hot water from the boiler leaking into the hot water circuit even when the boiler thermostat is switched to 'satisfied'. I worked through the electronic issues you described on your Sunvic pages, but I could find nothing wrong. The relays/microswitches seem to work as designed. (Incidentally I think that the Sunvic MoMo design is, in principle far superior to the traditional spring return design for these valves).

As far as I can tell, the problem seems to be that the actuator motor rotates randomly in the wrong direction. When the actuator rotates anti-clockwise (looking from top) all is well, but when the motor rotates clockwise, it stops in the wrong positions. This would seem to be a result of the asymmetric design of the cam (depression actually - is this a cam?) acting on the two lower microswitches.

I'm not familiar with these synchronous motors. I would not expect much of a response from Sunvic so I wondered whether you had any thoughts? Have you seen this before? Should they rotate unidirectionally? What causes the motor to rotate in one direction of the other? Does this just sound like a new motor job? Can they be sourced or would I need need a complete new actuator?

I appreciate that you probably get loads of email generated from your website, but any thoughts much appreciated.

Kind regards, Andy

Nice to see you sticking it up Sunvic, I have replaced the capacitors in mine & all is ok, very informative website, good luck from Paul in Hull.

Hi Steve,

I came across your website while searching for information on 2-way Sunvic MoMo valves. How interesting to find that I am not the only one to experience both failure of these units and Sunvic's reluctance to accept that there is anything wrong with the design; "We have had no problems with these - don't know why two of yours have failed".

They accepted one from me for investigation which they initially 'threw away', then after some further correspondence sent me a new actuator unit as a gesture of goodwill and finally I received the original actuator which they had 'found' which on inspection had had "no problems" (what?) but to which they had fitted a "new capacitor" (why, if there were no problems?).

Armed with the 'new capacitor' info I changed the small can capacitor in a couple of units which had subsequently packed in, but didn't twig that the blue brick was the cap to be changed, so the repair failed to fix the problem.

So I am quite buoyed by the prospect of being able to fix the (four) actuators that are currently languishing in a box in my workshop.

Thanks for taking the time to put the info on the web and for sourcing suitable capacitors.

November 2008

Thanks very much for the excellent info on your website.

As you may guess I encountered exactly the problem you describe. I agree with you that the design is seriously flawed. I also had the version with a zener diode amateurishly “dobbed” onto the back of the circuit board. I had no intention of buying a replacement which is likely soon to fail in the same way. Nor was I much enamoured of the idea of making a repair which may also be short-lived.

Once again, thanks for your information which led me in the right direction.

Regards, PJP
January 2009

Hi Steve,

I somehow wished I'd done a quick search before embarking on my investigation in to my lack of mid position operation on my Unishare.

Nevertheless, I spent an enjoyable two hours pulling it apart, reverse engineering the circuit (as I'm sure you know, the mind gets in knots when you have 5 different changeover switches to deal with, and try and draw in a way that makes sense!!), and trying to trace the pathway that was not intact.

Eventually, still sure I had traced the schematic wrong, because I couldn't work out what kind of idiot would design a 24V circuit using a series capacitor as a voltage dropper in this way, I satisfied myself that despite a small click on the secondary relay, it was not closing, and the applied voltage ~12V) was insufficient.

Thinking it was unlikely that the bridge was at fault, I concluded that the cap must have broken down. It was then that I did a quick Google, and found your page! How I wish I'd done that first. Fortunately, it's the cap on the supply to the smaller relay that fails, so HW ONLY and CH ONLY work fine, just not mid position.

This means that CH doesn't come on until HW is satisfied, which is not the end of the world…….but might be in a couple of weeks when the temperature drops outside!

I have a Sunvic 3-port motorised valve fitted to my central heating (I think that it was a retro fit in 2002) which is now exhibiting intermittent working when the central heating switches on - it works fine for the hot water. When it stopped working I called a local CH engineer who advised that the complete valve should be replaced which would involve draining the system (and me parting with 170!). Being a 'tinkerer' with all DIY stuff, I loosened the screws which attach the actuator to the valve body, and lifted it slightly and hey presto the pump kicked-in so the valve must have opened I thought. However the next day the CH did not come on so I repeated the above with the same result. Now, I need to know what should I do - get the whole valve assembly replaced, get a new actuator or get it repaired? I did notice that the actuator cover is constantly warm/hot to the touch - is this right or the first sign of full failure?

Excellent website, thanks Steve, Relays were showing ~9v and ~11v before repair. No wonder they weren't switching! Now both 23.2V as expected. At least my previous electronics soldering skills haven't deserted me, component removal and replacement was easy.

Many thanks again Best regards

Thanks to the info on your website I've saved myself the time and expense involved in replacing yet another motorised valve to my central heating system. The Sunvic 3 port one I have at the moment has lasted 4 years. One of the 330nF capacitors has gone down and I would like to order 4 replacements so that I can replace both and have 2 spares. (The zener diodes are already in place ).


Hi, I just had a smz1801 which failed.

I diagnosed a faulty capacitor or relay. Called Sunvic. They'd never heard of them failing. . . . I asked for tech support to call me back; they never did!

No spares, nothing to do except return to vendor. So I was amused to find your webpage detailing EXACTLY this failure since 2007!

Thought you might like to know that they're still in denial.


Date: Sun, 12 Dec 2010

Subject: Sunvic motorised valve

I was scouring the web hoping to find an answer to our intermittent CH problem when I found your site. 

We have a Worcester-Bosch 24i boiler which was fitted 2 years ago.

Shortly before its 2nd birthday the CH failed for no apparent reason.  We changed the batteries in the room thermostat 'just in case' and after a couple of days it gradually returned to normal.  This lasted about 2 weeks during which it had its 2nd service - but was working perfectly at the time.

After another couple of weeks it failed again.  Typically it will be working well and then, late on a very cold the evening, suddenly stops for no apparent reason. Often the next morning there is no heating and then for a few days the heating will be intermittent until normal service is  suddenly and unpredictably restored for a few more days.  We are presently on the third such circuit and the heating failures seem to be lasting longer and recurring more frequently.

I am not sure whether the HW is affected.  The cylinder is warm and the water when drawn is hot but we had an extra large cylinder fitted in case we wanted to opt for solar heating and as most appliances now are cold-fill it would take some time to discharge all the stored water as we (OAPs aged 66 & 75) are the only occupants.

Worcester Bosch sent someone to look at the boiler but he said there was nothing wrong, the problem seems to be with the controls.  I was trying to find a way to check individual controls when I came across your article.

Our controls which look identical to the one in your article EXCEPT that the word SUNVIC is not impressed into the top.  The top has the raised bands as shown on your photo. On the side of the cover the design and lettering are identical including the description SZM 1801.

An apparently identical part is shown on the official Worcester-Bosch Instruction Manual which accompanied the cylinder.  This part,  like ours does not appear to have the Sunvic designation but is otherwise identical. It is listed as:  Two port valve part no: 8-716-113-410-0

From what I have said does it seem likely that the problems we are having could be caused by this control?  And, so far as you are aware, is it possible that Sunvic are making these controls for Worcester Bosch?  We have a 5 year extended guarantee on our system - which hopefully extends to the cylinder - the approved fitters we employed have since been taken over by another company who will accept no responsibility.

I am sorry to impose on you but we cannot get anyone else to look into this problem except at risk of incurring huge additional costs.  We had rather hoped that this system would 'see us out' with a certain amount of comfort.  Currently we are without heating. .

Thank you for any advice you may be able to offer

A quick search of the Internet shows these Sunvic valves being sold at high prices as spares for Worcester Bosch systems. Some are also badged as RANCO:

sunvic used by bosch.jpg (32621 bytes)

Here is part of one advertisement for part number 8-716-113-410-0 - with a price tag of 96!

These valves typically cost 35 to 40 at discount stores.

The main part of the image is from a Worcester Bosch heating system manual. These boilers and systems may be well designed. But they are expensive and should be engineered for a long trouble free life - or else maintenance costs may outweigh the value of any possible energy saving.

Hello Steve,

Thank you for creating your website about the weaknesses of Sunvic 3 port valves.

My experiences are as follows: replaced a 15 year old 3 port valve about 6 years (April 2004) ago with a Sunvic valve purchased from B&Q. The electronics failed about 2.5 years later (Dec 2006) so I bought another complete valve from B&Q and installed the new actuator on the existing valve body. Old actuator head and new valve body stored in the garage.

Last month noticed the valve getting noisy; it could be heard turning when elsewhere in the house. On closer inspection also noticed a slight drip from the valve.

Removed the actuator and attempted to tighten the 4 screws that hold the valve plate to the valve body. Only one screw could be turned and then only slightly. Swapped actuator motors with the one from the previously removed actuator and the valve's quiet again. So the valve is dripping slightly - I'm debating whether to swap valve assemblies, swap complete valve bodies or buy a new brand of 3 port valve!

Thanks again for the resource - has been very useful.


Hi There!

Pulled your interesting article re: above rubbish actuators!

Have just removed mine its on perpetual revolving motion (limit switches not responding) this is the 2nd of these actuators I have installed on my system in the last 3yrs as you state in your valve failures precise " they are not the most reliable" I have strapped my system out at the moment doing a shared valve mid-position, at least it keeps HW and CH somewhat available!

I had thought about going back to a spring return Honeywell when the cold snap dissipates somewhat, may still do that!

Yours sincerely

Hi Steve,

To be honest my heating is a nightmare. It is a new build house completed in 2004 with underfloor heating upstairs and down.

We had an actuator fail (short circuit) in the downstairs system after about 12 months, the builders sent an electrician to sort it out as the original installer was not available. He didn't seem to know what he was doing and all he did do was blow up the controller in the upstairs circuit.

It would appear that this is when the damage occurred to the 1801. The original installer then was able to attend and trace the fault to the valve actuator downstairs. He didn't have a replacement but left that circuit isolated so we could have the heating on.

He then replaced the upstairs controller and the 1801. 5 years later I started to get problems again, the breaker started to trip, then the controller went dead, so I got a new one which lasted 2 weeks. I then bypassed the controller feeding the upstairs heating and water with the 1801 removed.

The heating now worked so I concluded the 1801 had failed again.

My wife has just called me to say the breaker has tripped again.

I will need to try and find a competent heating engineer to get to the bottom of the overload.

Thanks very much for your website and I'll wait for your advice.


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