Ariston Combi-boiler technical question

My Ariston Eurocombi Boiler has developed an intermittent fault, where it shuts down (flame goes out) for a second or so, then fires back up again, whether supplying hot water or the radiators. This repeats constantly, every 20 seconds or so, whatever temperature the HW or RAD temp controls are set to.
Fault finding suggests an intermittent fault in the boiler output limit thermostat, and temporarily bypassing the 'stat restores normal operation, but is obviously not a safe way to operate the boiler.
Looking at the boiler innards, the design appears to have been modified since the manual was written, in that there's a bracket on the output pipe for the 'stat, but tracing the wiring diagram the 'stat now appears to be integrated in the DHW temperature control, 'ganged' with the DHW 'stat.
Replacing the DHW control would appear to be a high cost, labour intensive task, involving draining-down and dismantling, but simply bolting on a previous-model pipe-mounted 'stat and re-routing the existing wiring would see the job done in a few minutes.
Can anyone see a problem with that idea? Would I need to use a thermal transfer gel/paste when fixing the 'stat? The DHW 'stat appears to be working perfectly, switching hi/lo at the set temperature, but is there a common failure mode which could see the DHW 'stat fail soon, making it worth replacing the control now?
Thanks in advance
--
Keith
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And you're sure it isn't the gas valve ? Have you removed the wiring from, or by-passed the wiring to, the thermostat to see if the boiler works properly ? A thermostat only works as a single pole switch to break the circuit, so connecting the wires from either side and touching them together won't cause any problems.
Try by-passing the thermostat and firing the boiler up to see if it holds on properly. If not, then it isn't the 'stat that's at fault.
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I

You followed up:-

It certainly seems like the stat is at fault, but I don't want to run the boiler with it permanently bypassed, just trusting the overheat cut-out.
I've two options, replace the whole temperature control, which incorporates the limit stat, or fit an older version stat to the unused bracket on the output pipe where it used to be fitted, and save money and time.
--
Keith
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Don't run the boiler continually with the thermostat by-passed, just by-pass it for the duration of the test. Other components can cause the same symptoms, so the each have to be eliminated from the loop before the real fault is found. Testing the 'stats is one of the easiest tests to do because it's normally done by simply by-passing them to see if the fault clears itself. Running the boiler for 30 minutes to see if the 'stat is actually causing the problem won't hurt the system as long it's a supervised test. Don't do the test by turning it on and walking away from it.
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I have the same boiler which I installed about 5 years ago. It has similar fault as yours. start from cold flame comes on, goes out comes on, goes out, then fires normally, for a cycle, then same again, until it is warmed up a bit then its okay? I have an external bolt on stat on the pipe. I'll try the by pass and see if it fixes mine too. I have tried moving the pots for soft start,cycle delay, etc, no change
Keith wrote:

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Hot water getting red hot at sink next to boiler but only just above cold up stairs any ideas
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On 14/01/2016 17:52, snipped-for-privacy@gmail.com wrote:

Have you just moved into this property ?
If so, are you sure the upstairs hot tap really is the hot tap, and not the cold tap with the wrong cap ?. If it and the downstairs hot tap really are connected to the hot water output of a combi then turn on the upstairs tap and shout down the stairs for some one to turn on the downstairs hot tap full blast. You should notice a reduction in flow from the upstairs hot tap.
If there is no obvious drop in flow, then do you still have a hot tank connected as a heating zone ?. Not everyone removes the hot tank especially if it is a larger property and teenagers are resident.
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On 14/01/2016 17:52, snipped-for-privacy@gmail.com wrote:

It could be a failed shower mixer allowing cold water to pass though to the hot pipework.
Is there an isolator on the boiler on either the cold water feed, or the hot out? If so, turn that off and see if you still get water out the hot taps.
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Toby...
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