Worcester 350 CH (central heating) cuts out after 2 mins.

[nv_london]

Dear all,

When I switch on the CH on my Worcester 350 combi boiler, the boiler fires up and all 6 radiators (over 2 floors) heat up within a matter of minutes. However, after 2 - 3 mins, the boiler switches itself off but then never fires up again. (Hot water is always available on demand throughout this process). All the indicator lights remain 'on' and if you switch the central heating off and then on again, the whole process is repeated i.e. the boiler fires up and the central heating comes on for a few minutes before the flames die now but never come on again. (It's not the overheat thermostat as this is fine and never 'pops up'). Two things which have been mentioned on several other similar threads which may be relevant are, 1) thermistor or 2) divertor valve ?! Not sure how near the mark these two are!

Any help or suggestions would be greatly appreciated.

Many thanks.

 

[nickso]

testing the thermistor is always a good start with problems like this.

iirc 350's can have problems with the anticycle delay stopping the boiler restarting the heat after its reached temp. you having to switch the heating off and on again to get immediate heat may confirm this. if it is the pcb its a bit unfortunate as they are pricey iirc. you may be better getting a company in for a fixed price repair.

 

[Agile]

I have encountered several similar Worcester boilers with a pcb fault causing the boiler to not restart after a heating cycle.

It could often be prompted into repeating the cycle by momentarily disconnecting the heating sensor.

Measuring the resistance of the sensors is a first step and simulating a sensor by replacing it with a fixed resistor is often a useful tool.

Tony

 

[nv_london]

Seeing as though one can buy thermistors for under £ 10, I think I'll start here and see what happens once I've replaced it!!

Thanks for your suggestions and comments.

 

[Agile]

But a resister is only 3p and so far cheaper!

Whilst I can have different sensors in stock, you dont need a spare sensor unless its failed!

Tony

 

[nv_london]

To be fair, at cost, resistors may well be 3 pence, however, unless you buy in bulk or have very cheap delivery, the price of a resistor will not be 3 pence to the laymen. In addition, it will also involve measuring it's resistance (assuming the perosn has a multimeter or equivalent) and so from a complete novice's point of view, it's surely cheaper (with regards to equiptment and labour) to replace a relatively cheap part and see if the boiler works.

I.e. imagine, I buy a resistor, replace it and check it's resistance, only to find that I need to replace one of the boilers cheapest parts (the thermistor), I'm back at square one and have just gone through all that....is it not therefore, more economically viable and justified to just buy and replace a thermistor (if logic and others are suggesting this is a more likely solution - which is what this site is all about) and then monitor the consequences?!.

If this startegy doesn't work I've spent under 10 pounds (including my labour (as I'll do it in my spare time - yes my spare time is cheap!) - which is why I'm on this site to start with)), however, if it's not the thermistor then I can move on with my diagnosis, because conversly, if I have to order in resistors (which I don't have) only to measure their resistance (which I can't to without a multimeter) it all becomes more time and, in the long run, more money consuming than replacing a (relatively cheap part) - the only time I would agree with you would be if the part and if the potential solution were in the same, grander, order of magnitude i.e if a part cost £ 100 but a potential colution cost £ 10, then I'd consider it, however in this situation, I believe that a thermistor, at cost, would be £ 3, while a resistor would cost 3 p and so the difference in price/time, to me, isn't justified.

 

[heatingman]

Very well written, a good logical laymans response.

 

[nickso]

hmmm i got waffled out there but to be fair to the OP if he keeps his 350 long enough he will probably need the thermistor for the dhw side eventually, so his money probably wouldnt be wasted.