BAXI 105e Instant Combi - DHW Cutting Out, CH OK

Discussion in 'Plumbing and Central Heating' started by lejonnieboy, 22 Apr 2015.

  1. lejonnieboy

    lejonnieboy

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    I have a problem with the hot water on my 105e Instant Combi.

    When I turn a hot water tap on, the boiler fires up and hot water flows, but not as hot as normal (I can easily keep my hand under it). If I watch the temperature LEDs, they climb to maximum and then the boiler cuts out with the flame and safety thermostat LEDs flashing - this takes around 20-30 seconds.

    I can reset it and repeat.

    Central heating works fine - temperature LEDs look good and no cut-out.

    So it seems like the boiler thinks the hot water is hotter than it actually is.

    I've checked the hot water thermistor resistance (at the PCB connector end after disconnecting) - it's 16 kohm with cold water running through and drops to 3-6 kohm when the water is hot. This seems reasonable. The manual says 11 kohm when cold so although there's a difference I suspect the thermistor is OK (if it was faulty a higher resistance would mean the boiler detects a colder-than-actual temperature).

    For good measure I checked the safety thermostat and that is OK (close to 0 ohm). And anyway, if it was faulty I presume the CH would not work?

    Any ideas? My best guess is a PCB problem but as that is quite expensive I thought I'd just check with those more knowledgeable than me!

    Thanks in advance!
     
  2. ollski

    ollski

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    Is the hot water going to the radiators?, probably be looking at a diverter valve to begin with.
     
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  4. muggles

    muggles

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    Unlikely to be the PCB. I'd say either diverter or blocked plate heat exchanger
     
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  5. lejonnieboy

    lejonnieboy

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    Thanks! I'll do a bit more investigating...
     
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