Worcester 350 overheat cutting out with DHW

[ChyAnGwel]

Hi Everyone

I hope you can give me some help/advice. I have a Worcester 350 combi. The problem I have is this. It works fine when on central heating, but when I use DHW the boiler fires up and hot water from the taps is fine. It’s only when I turn the tap/taps off the problem occurs, the boiler starts overheating and boiling and the overheat switch on top cuts out, it does not to this every time only occasionally, not done it for over a week then twice this evening. I had it serviced by Worcester Bosch 2 years ago and they replaced loads of parts Gas valve, circuit boards, fan, air switch and one pump not sure which one. I had a new expansion receiver fitted just before Christmas. Any help would be gratefully received.

ChyAnGwel (Ross)

Sorry should have said this is when the boiler is in DHW & central heating mode

 

[diamondgas]

Well now .... :welcome: to the forum Ross.....!

Worcester 350,,, would that be the one with the two pumps inside? ....Hmmmmmmm hot water off then overheat .......... Initial quiery would be is this in an open system and fitted upstairs? ....... Does the issue seem to happen when drawing a lot of hot water? Intermittent faults are not the easiest to resolve and can incurr a lot of 'guessw'work alas 🙂

 

[ChyAnGwel]

Hi and thanks for your reply.

It’s a seal pressurised system Fitted it a cupboard in the bathroom. Yes it does have two pumps, and it makes no difference if it draws large or small amounts of water.
When you shut the tap off you hear the boiler start to boil the water then it starts to rattle and you hear water rushing around then it cuts out. Also there is no drop in the system pressure when this happens.

Hope this helps & thanks again for you reply
Ross

 

[Mike Jackson]

I would gues at sticky hot water flow switch.

 

[diamondgas]

As above or you could even have a dead leg somewhere, air trapped in the hot water pipes somewhere?

 

[ChyAnGwel]

Hi and again thanks for your replies.

This morning I took covers and dropped down the front panel. I have the original installation manual that came with the boiler so I carried out all the DHW test & central heating test that are in the book and it all worked OK. When you turn the tap on the boiler ignites on low then increases to high if you then start to shut down the water flow the boiler cuts back until eventually stopping when the water is turned off the fan runs at full power, until it cools down.
The only thing I noticed is when in central heating mode everything works as it should but for one thing. One the circuit born on the left there are some relay switches and one must control the fan when in heating mode the lower one “about 6 o’clock position” buzzes and arks and the fan stutters and does not always seem to come on correctly could this be the fault.

It looks quite easy to change and I don’t think it would be a problem for me to do. I am an Auto technician chargehand for the ambulance service and we change circuit boards on ambulance systems and blue light control regularly and they look far more complicated than this.

Again thanks for all your help
Ross

 

[diamondgas]

The fan is two speed and it sounds like the relay on the board is knackered 🙂 I doubt it'll cause the overheating you describe but does sound like it needs replacing 🙂

 

[ChyAnGwel]

Hi again and thanks once again for you help

Just had a look at the DHW pump took the little screw out of the back and the pump seems to be running in central heating mode OK, starts when water switched on and stops when water switched off. On some occasions it keeps running when the water is turned off and on others it doesn’t but when it doesn’t the boiler does not overheat. If I disconnect the pump every time the water is turned off the boiler overheats. If I then reconnect the pump and press the reset button the pump does not start but the fan is working at full speed for some time. Going to get a new sequence board and replace it and see what happens. Will keep you posted .

Thanks again for your help and if you have any more suggestions they would be very helpful
Ross

 

[diamondgas]

The pump should go off as soon as the hot tap is turned off, if it isn't that could indicate a sticking or slow to close water flow switch. The fan will stay on if the water in the main heatex is above 75C.

 

[ChyAnGwel]

Thanks again for your replies

Rang Worcester today and they were not interested boiler too old for them to come and repair standard call out is £230. So priced up some parts today with Plumbase I have a friend who works there. The parts alone are over £200 plus fitting the electronic programmer is playing up won’t keep its settings whenever the power is off, this would cost about £125 as already cost £89 for an expansion vessel and that’s just this year. As the boiler is around 20 years old wondering do I get a new one, Would like another Worcester and looked at a Worcester Greenstar 37cdi.He also said about a Viessmann Vitodens 100-W saying they are a good boiler, and sell well and are liked by a lot of the local installers. I have never heard of them before and did a search online and they get mixed reviews. What do you think of them here, or should I stay with Worcester.

Again Thanks for your help
Ross

 

[southcoastboile]

viesmen are bad boilers in my experience, stay well away. Worcesters are reliable but parts can be a headache to change on the new ones. Badly designed to do repair work on them. I would go with a glowworm or valiant personaly if you are going to spend top money on a new boiler.