Worcester 350 combi - No hot water

20 Oct 2009
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United Kingdom
Can anybody please advise to the likely cause of the following problem on my Worcester 350 Combi boiler?

The central heating sideof the boiler works fine but there is no hot water (may get a bit of lukewarm at beginning).
The boiler does not ignite when demand for hot water is initiated (demand light does come on though).
The boiler also goes out when central heating is on if hot water tap is turned on.

Is it possible that it is just a faulty hot water overheat thermostat?

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I have just removed the cover and the domestic hot water sensor has a white powder scaling on and below it. Could this be a problem (possible leak and therefore air blockage?)
the white paste might be heat sink compound, if so it's not a water leak causing it.
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350's use a wet pocket thermistor, it will be corrosion form water weeping out of it for the last 20 years.

if the demand light comes on when you turn the tap on its not the flow switch at fault.
Cheers Nickso, is it possible that the thermister has been damaged by the corrosion and therefore the fault?
hard to say, thermistors are a well known HW fault on 350's so its a possibility though not necessarily due to the corrosion.

easy to change if you fancy a pop at it yourself. not expensive either.
main driver board zagas 125. had same problem with hot water.you will get one on ebay cheap
how cheap? the price of a thermistor cheap?

i'd be wary of having a go with a pcb with little knowledge, i'd be happier having a go with a cheap thermistor with little knowledge.

if you are going down the route of buying a pcb without knowing for sure its the fault i would recommend you just forget it and phone worcester now for a fixed price repair. especially if said pcb is a recon.
Cheers guys. Changed the thermistor (£6 from ebay inc postage!) Hot water restored, happy days!
well **** me, two correct guesses in a week. i must be good or something. :LOL:

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