Ariston Genus 27 burner keeps cycling for DHW

7 Dec 2012
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United Kingdom
I'm trying to keep my Ariston Genus 27 going but noticed recently that whilst the DHW is heating, the burner keeps going out then relighting. The pump continues to run and this happens until the water reaches the required temperature.

The DHW expansion vessel requires replacement since water comes from the schrader valve when tested. The 7 bar temp and pressure relief valve also leaks a little but constantly; more so when the pump/burner is running. I can see this from the tundish and have ruled out the other valves.

My question: is the knackered expansion vessel responsible for the burner cycling and also does it contribute to the leaking 7 bar valve? Is it likely I will need to replace the 7 bar valve or should I do the expansion vessel first and suck it and see?

Any advice appreciated.

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Does it keep cycling once it has reach temperature.
What is the water delivery like? Can it fill a bath without a problem or is it cycling on and off all the time?

Sounds like blocked plate heat exchanger.

It needs to be taken off and flushed.

This boiler is a bitch to work on.
Hi dc
Yes it stops cycling when it reaches temperature at the same time as the pump stops running. It does run out of hot water if you shower for too long. Like it cant keep up with dhw delivery. I've also noticed the burner dims before it goes out.
A combi boiler has a 3 bar PRV and does not usually have a tun dish!

Are you sure that you don't have an unvented cylinder?

Have you tried letting all the water out of the EXV ? It may only have a little and take a repressure?

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Hi Tony,
The boiler has a 60 litre unvented cylinder inbuilt. I already replaced the 3 bar PRV in error and later established that it's the 7 bar temp and pressure valve on the unvented cylinder that is leaking.

I'll try to let the water out of the EXV and pump it up. Does that explain the burner dying out though?

Right you have a storage combi then.

It will have a plate heat exchanger which might be partly blocked but there are different ways for them to be interconnected and unless I saw it there could be a different cause.

Do you ever go to the Castle Balti?

I'm a good 15 miles from Warwick but I'll take that as a recommendation. I like a good Balti. Both yourself and dc have suggested the heat exchanger. Sounds like a likely cause. :idea:
OP you do not have a plate heat exchanger. There is a design issue on This boiler that kills the the white vessel. There would appear to other issues that Could easily be resolved. Not something that end user can undertake though.

The anodes in the tank will need replacing
The anodes are just to prevent corrosion.

The Castle Balti is strongly recommended and you can take your own wine! Good size portions and they don't mind you taking some home!

I go there so often they know what starter and sweet I usually choose!

That's a long way to go for a meal. I take it London is not your only place of residence?

So i'll replace the anode as a precaution but what might be causing the cycling.

look at the lights that indicate temperature. Can you watch and post the result

if the heat generated by the burner is not used up by the load, in your case the storage tank, burner either downsizes or goes out. Bore reduction in the primary loop to cylinder, air lock etc could cause this.

Havw worked on a few but recall is hazy so will need to open the manual to have a gander

Is your water hard?

Is the boiler during central heating mode cycling in a similar manner?

While it is difficult to pinpoint the cause at this moment in time sitting looking at PC screen, corrosion could be the cause of your problems.
The expansion vessel problem won't be causing the cycling but it does need replaced anyway. From what I can see from your video the boiler is firing ok and going from its ignition rate straight down to its lowest gas setting.
As DP said it would help if you could tell us if this is happening on heating as well.
The boiler is sitting up at 80 deg so either it is at this heat or the heating thermistor is faulty and providing the pcb with the wrong info.
There are other faulty components that can cause this fault but they are safety critical so we can't advise on here.
Does the hot water get scalding hot?
I've turned down the temperature control to minimum to see whether the boiler still heats to 80 degrees. That should indicate a faulty thermistor (possibly)...right?

I don't really want to turn the heating on cos I haven't got any rads connected. I'm doing the house up and going to replace them all and reroute some pipework. Just got a 22mm loop at the moment to pressurise the system. Could that be causing the fault?


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