Ariston Genus 27 burner keeps cycling for DHW

On this boiler the temperature lights indicate the temp of the water in the heating loop and not the hot water cylinder temperature. Having the heating on would help rule out a few causes. As long as there's pressure in the system the rads removed shouldn't cause this.
If you turn down the water thermostat it will only reduce the temperature in the cylinder, the boiler has a hot water sensor in a phial underneath that goes into the cylinder itself.
Try drawing off water until it goes cold ( boiler off) start the boiler heating the hot water from scratch and note how quickly the temperature light go to 80deg. If the burner cycles straight away,ie when the cylinder is cold And the temperature is low at the lights it's not likely a blockage.
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Thanks 1983. I'm doing that now. I'll post back when I've emptied the cylinder of hot water.
HI 1983
It took 70 seconds to light the 80 led. The boiler stopped heating at 3 mins 30 seconds but didn't cycle. I checked the water and could keep my hand under. I would say 45 to 50 degrees. The water temp is set to minimum on the boiler.
That's not too bad, if it took 70 secs to get up to temp ( remember the loop going to the cylinder is small) it doesn't appear to be blocked. At worst it could be a thermistor.
Another thing to consider is that on these the hot water flow rate is stated at around 18l per minute... But... When the initial volume of hot water in the cylinder is exhausted the boiler can really only produce about 12l per minute.
So you need to restrict the incoming water isolator so you get 12l per minute at the tap, therefore you won't 'run out' of hot water.
Try this and set the hot water temp to just below half ( you can fine tune this to suit your own requirements) let us know how you get on.
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Hi 1983
So I semi closed the isolator and I can visibly tell that the flow rate has gone down. I set the HW temp to just below half and its now difficult to hold my hand under the tap. The boiler stays fired up as long as the tap is flowing. When i turn the tap off, the boiler continues to run for approx. 60 seconds then begins cycling. It cycles for approx. 3 or 4 mins before shutting down.

Does that help at all?

Appreciate your time on this.
To be honest I think your boiler s actually working pretty well, think it needs a bloody good set up, gas,water ect to get the best out of it.
That trick seems to have sorted out you running out of hot water, just keep reducing the hot water temperature until your comfortable with it. The cycling thing sounds ok to, it's just maintaining the temp in the cylinder. If it shuts down its doing its job. It should then only come back on if the water temp drops.
Thanks 1983,
I'll work a little more on the flowrate vs temp settings. I'll get the 7 bar valve and the leaky EXV replaced and see where we are.

I was going to replace it with an Intergas combi but money is being diverted elsewhere at the moment. I also like the idea of an unvented cylinder so this boiler makes a good halfway house.

Again, thanks for your help this evening.
Sparer, new expansion vessel will die prematurely too. You need a heating engineer to modify the boiler as suggested by Ariston
What is the modification DP? How can I tell if it has already been done? I want to be sure before getting the EXV replaced.

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