This was a Worcester 350 Combi boiler I repaired September 2018 – today actually in Hillsborough, Sheffield, S6. This is not the typical boiler I work on daily, in fact over the last four and a half years I have only worked on two.
About the Worcester 350
Worcester has led me to beleive that they were manufactured between 1995 and 1998. No its not a Vaillant but given this boilers age etc I thought I would give some information for anyone needing help trying to repair one of these older boilers.
The 350 is a typical 35KW negative pressure, fan assisted, standard efficiency indirect Combination boiler which has two pumps instead of the conventional single pump.
The good and the bad
The good news is they’re relatively straight forward to diagnose issues with. The bad news is that just about every single part is obsolete now and you cannot purchase anything from typical retail channels apart from what they might have left over.
This customer actually called her insurance out to have a look at the boiler when it went off. They spent ten minutes looking at it and said it needed a new printed circuit board(s). This boiler has 3 printed circuit board’s, which are all obsolete. Anyway the customer asked me to give them a second opinion which I did.
It had been a couple of years since I worked on one, and I knew back then spare parts were getting scarce but I thought I would have a look anyway.
The boiler had blown a fuse on one of the boards and the main 3 amp fuse. I replaced this and trued the central heating and they blew straight away. When this happens it generally mean a high voltage component has failed and is causing the fuses to blow. The best way to determine what is causing the problem is to disconnect every high voltage connector from the printed circuit board’s and start plugging them back in one by one until the fuse blows again.
With everything disconnected, the power light glowed – if the fuse were to still keep blowing with everything disconnected, then it would point towards a printed circuit board fault.
The fan and resistor
I suspected the fan or one of the two pumps was the problem.
So I plugged the fan back in first and tried it and the two fuses blew immediately. So I connected everything back up apart from the fan and the power led stayed lit. When you put a demand on the pumps would run.
Now the fans on these boilers are a bit unusual in that they have a resistor with a heat-sink connected to the fan housing which is part of the speed control. When I tested the coils on the actual fan these seems ok, and measured around 55 ohms I think.
So one of two components was the problem, either the fan coils or the resistor. When I spoke to Worcester technical support they suggested disconnecting the restore from the fan housing and isolating it from earth to see if the fan would run. Indeed it did run perfectly! So the resistor was shorting down to earth causing the fuses to blow! 
I was able to order the last one from a supplier online for around £20. This part is also obsolete so you can’t get them anymore unless a supplier has some stock left over.
I replaced the faulty component and tried the boiler and it fired up and worked like a dream. So fortunately with some proper diagnose and a bit of luck we were able to keep this one going just a little bit longer.
There are some companies that “recondition” some components such as the fan but if the resistor has failed it cannot be replaced.
The first 350 I repaired needed an expansion vessel which was still available at the time. Even still and external vessel can be fitted if the internal one fails and is no longer available. The rest of the parts are definitely a problem.
Your options if you have a Worcester 350
So if you have one of these boilers you’ve got two options. The first is to either keep it until it fails and be ready to replace it knowing that it might not be repairable.
The second option is to replace it while its still working, as to avoid it failing at an inconvenient time when you least expect it and needing to replace it anyway.
If there wasn’t an issue with parts I would typically recommend keeping it for a but longer.
I don’t work on these boilers anymore
I don’t service or repair these appliances anymore, for the reasons I have mentioned above with parts being the main issue. On top of that every so often you get a boiler with a printed circuit board that “dies” when you turn the power off, or on and off several times.
Im not sure what the reason is and it doesn’t happen very often but it does happen. Plus when you’re talking about a boiler that is at-least 20 years old, you feel that they’re a bit fragile. Ive worked on a much newer Vaillant ecoTEC boiler that died on me one time.
In the unlikely event you service on one of these and it decides to die on you, it could look like something you have done. Then it could put you in a sticky situation given you can’t get any parts even if you wanted to. Also if you made a mistake and water damaged the printed circuit board’s or other components its the same situation. These are situations I would rather avoid to be honest.
Im sure there are still a few out there but I don’t care to work on them.
Hi, the Worcester Bosch 350 was introduced in the early ’90s I believe and a fault would appear on the Zagas 125 driver board requiring early replacement under warranty. With time, both PCBs suffer from dry joints, the relays become temperamental and polyester capacitors C10, C11 and C12 can have problems. With repeated mis-firing C10 overheats and melts causing smoke and scorching of the board. Close by, R46, 47 and 48 can burn out.
At this time exchange boards are available, but their thoroughness in checking the soldered connections is suspect (particularly the double board Zagas 125). In one case an electrolytic capacitor was just hanging loose with many others requiring reflowing. This board has a flat ribbon connector which has 12 thin brittle wires, and as the two boards are worked on these break easily. A very strong magnifying glass is necessary for checking all the joints etc !
Thanks for the additional information it might be useful to someone.
Hello admin,
I assume you are Richard, and this is your website?
Firstly, thank you for sharing your experiences, so that people like me are able to learn and understand !
Also, thank you for providing this forum for people to share comments and experience.
I have already learnt and enjoyed reading through the posts.
All the best,
Austin
Hi Steve,
I’ve just come across your comments on the Worcester 350.
Thank you for sharing this information. Hopefully it will help me in fixing mine.
You seem to have a very good understanding of what goes wrong on these boards.
Do you repair electronics as a career?
We have one of these boilers, it was installed in 1993.
I am on my third set of ‘reconditioned boards.
They have lasted between 1 and 4 years each.
Do you know of anywhere there are schematics available for the circuit boards?
I have searched via google and not found any.
I do have the installation and service manual, however it is very limited in content – only showing simple block / function diagrams.
My electronics knowledge is quite basic, but I am slowly learning.
Would you be willing to point me in the right direction with fault finding/understanding of the driver and sequence boards please?
I completely understand if not, no problem.
I’m just attempting to maintain our old boiler instead of just replacing it.
In the same way, I’m attempting to understand the way the circuit works on these boards,
so that I don’t have to throw away – but rather restore…. it’s a philosophy which I use as often as I’m able.
Anyway, my apologies for a rambling email.
Any help would be greatly accepted, and thank you again for your sharing of knowledge in your post!
All the best,
Austin
Thanks for this explanation – one in a rental property has just had fan fail, so you have confirmed for me that I need new boiler!