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Boiler Kettling And Cutting Out

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AKMild | 19:28 Wed 13th May 2020 | How it Works
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I have a Viessman Vitodens 100 combi boiler which is about 2 years old. Today I noticed that the domestic hot water was only lukewarm and on looking at the boiler'd digital display see that when hot water is demanded, the boiler ignites, quickly shows a temperature of 91c accompanied by kettling noises, then cuts out. When it has cooled to about 70c it reignites and the same thing happens over and over, leaving me with lukewarm water at best. The central heating is working fine. I have a call in to the qualified plumber/Gas Safe engineer who installed it, but I'd be interested to hear of any opinions as to what might be wrong. Given the relative youth of the boiler and the fact that it has a stainless steel heat exchanger, I wouldn't expect a furred up heat exchanger to be the problem.

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As you say, AK, the boiler really isn't old enough for anything major like a heat exchanger problem. Most likely a little "thermistor" that clips onto a pipe in the boiler. It's a kind of thermostat. If it's faulty, then it might let the water circuit overheat... until a maximum temperature sensor switches the boiler off. When things cool down, it starts all...
19:52 Wed 13th May 2020
As you say, AK, the boiler really isn't old enough for anything major like a heat exchanger problem.

Most likely a little "thermistor" that clips onto a pipe in the boiler. It's a kind of thermostat.
If it's faulty, then it might let the water circuit overheat... until a maximum temperature sensor switches the boiler off.
When things cool down, it starts all over again.... ad nauseam.

I hope that's all it is. Very cheap and easy to replace.
What is the internal water pressure gauge( bar) showing ? It should be between 1 and 1.5 and if it is less you need to turn on the valve that will raise it. Some set ups need you to fit a little link between the mains input and the boiler. Only the link that is designed for this will fit and it usually has an armoured or metal flexible casing.
Good point Togo, but if the pressure was low, then I would expect an Error Code to be flashed up, and the boiler to refuse to start at all.

When the boiler was installed was the system power flushed and do you have a magnetic scale catcher installed in the system ?
If water which is nominally at 70C is actually only lukewarm, there definitely seems to be something wrong with the thermostat or its associated circuitry. A 'hot wash' in most washing machines is typically only at 60C, so 70C should be hotter than that. (It's roughly the temperature at which bubbles start to form in a pan of water when you heat it).
To answer Chris (Buenchico),

The 70 degrees is only the temp. of the water within the boiler. It's a long way to the tap. The 70 degrees is conveyed to the tap only when the boiler is running.
The problem here is that it keeps switching off before it gets a chance to travel ;o)
Question Author
Thanks for your helpful suggestions everyone. The boiler was installed properly by a trusted and qualified engineer and has been trouble-free up to now. The pressure is fine and the Central Heating is working OK, so it's something on the DHW side I think. Hopefully the engineer will call soon.
Question Author
Well the engineer has finally been. What seems to have happened is that when we had a new radiator installed recently, the CH system was drained by the builder and after its refill some small pieces of debris got into the secondary heat exchanger on the primary side. With the flow impeded the primary heat exchanger overheated and the safety cut-out operated, hence the 'kettling' and cut-out/cycling that followed. The gas engineer cleaned up the secondary heat exchanger whilst servicing the boiler and all is now working.

Thanks for all your input.
More often than not we never hear the end result, after someone has posted a question on AB.
So it was a blockage after all .. thanks for letting us know.

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