could be sludge/scale in plate heat ex, or it could be ur thermistor..... u can do a resistance check on ur thermistor, or you could take the plate out and clean it......
no probs, just be wary about how long the boiler has been in, if its happened quickly (over 6 month or so) you could do with the system flushing through.
I would get in touch with your installer....it is only a month since you had your ariston replaced, there is no way an even half decent installation would already have a sludged up w2w heat ex.
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