Genus 27plus, plumber stumped

[jules.b]

Hello, I wonder if anyone out there has experienced this problem. An ariston Genus 27 heats water perfectly, but cycles indefinitely when set to DHW only or DHW + CH. Initially it cycles every 12-15 seconds. The frequency of cycling decreses with time. After 1/2 hour with no HW drawn the frequency drops to one brief cycle every 1.5 minutes. The pump operates continuously. The cycling on DHW prevents the boiler from providing CH automatically. If one switches mode to CH only, the boiler functions correctly, provides central heating, and does not cycle unduly, so the three way valve is working correctly. The DHW probe has been replaced by our plumber as suggested by the technicians at Ariston. This has made no difference. The engineer wonders if the air pressure switch might be causing the problem...is this the likely cause?

Thanks in advance to anyone who has read this far. When I mentioned to my friendly CORGI engineer that the problem was with an Ariston COMBI boiler he had palpitations! Ariston recommended an engineer but he seems to be scratching his head too. The only problems their technicians suggested were with the three way valve (tested and working fine) or the DHW probe (replaced by the engineer).

[heat tech]

hi,has both the thermistors been replaced and was wondering have you any dripping hot taps anywhere,on the microswitch DHW head is the microswitchs clearing and not jammed,i have come across this fault before there is also a downloadable sheet on the there web site,hope it may steer you in the right direction and will haelp all i can

[jules.b]

Thanks very much. No dripping taps in the place, and I don't know about the microswitch. I will get the engineer to check it.

[jules.b]

Thanks very much. No dripping taps in the place, and I don't know about the microswitch. I will get the engineer to check it.

[jules.b]

Where is the micro switch in question? I will point it out the the negineer returns. Also what is the second thermistor that you are referring to?

Thanks

[heat tech]

on some of the earlier models they had 2 thermistors normally they are green but some were white,they would make the boiler ramp up on the gas supply then go down,if you go to the ariston web site there is a full parts list and fault finding,the microswitch is on the end of the divertor assembly,by the way does the pressure via the gauge on the front of the boiler show a minimum of around 1mb,hope it helps,i was wondering what area are you in i am in kent
kind regards

heat tech

[jules.b]

I am in oxfordshire unfortuantely a long way from Kent. The boiler is a storage combi. It does not appear to have a microswitch in the DHW circuit. It has three thermistors - one on the storage tank, one in the pipe that runs through the heat exchanger, and an overheat sensor clipped on the the same pipe.

[Agile]

Its getting too late to correct it but this posting should have been in the technical section.

Now that we know that its a storage combi I would expect that its a problem with the setting temperature for the store compared with boiler flow.

To check on the problem I would set a slightly different storage temperature using a sensor substitute to see what effect this has on the problem and hopefully diagnose the actual problem.

Tony Glazier

[jules.b]

Thanks, Agile!

[DP]

Have worked on these boilers in the past. There is a design problem with this boiler. If you contact me on rannochservices999@ntlworld.com (remove 999 for the email to function).

I wrote MTS a letter suggesting why the HW EV fails on this boiler. They denied there was a problem with the design. On a subsequent call they suggested a modification as detiled in my letter to them.