Only shower hot water fluctuates; boiler turns on & off often

[DIY_somewhat]

Hi All,

New to the forum; but read through a lot of threads and solutions for possibilities around combi boiler, before posting.

Bought a flat that has a Ariston MicroGenus (not sure how old). The hot water in the shower (both through shower head and bathtub tap) fluctuates from hot to barely lukewarm every few seconds. When the temp on the boiler hits 80, the flame turns off till the temp goes down to 60, and then turns back again - all within matter of 5-10 secs. But this doesn't happen for the kitchen or bathroom sink, where temp stays at 70 steady.

Any thoughts? Should I be checking for limescale/sludge? Should I be look at the Main HE or Secondary HE?

Any advice on how to troubleshoot this would be super helpful.

 

[Rampage77]

Chances are it will be the secondary heat exchanger blocked ('normally' is this with your symptoms)

 

[DIY_somewhat]

Is it normal for it to be a Secondary HE scaling issue when it's only affecting the shower and not the rest of the taps?? On the shower, this occurs when only the hot water tap is open, so it's not a case of cold water mixing in. Thanks.