No DHW on Ariston Eurocombi A/27 MFFI

[gasgasgas]

Called to an Ariston Eurocombi A/27 MFFI as customer complained that DHW only working when central heating on full. Figured on arrival that only residual DHW seemed present when running tap. Diverter valve pin not activating microswitch so thought DV was problem. Activated microswitch with screwdriver and DHW aplenty. Installed new DV and auto air vent, full movement restored and loads of DHW last night and this morning. Customer called this evening to say DHW no longer working (again), went to take a look and boiler firing on demand but hot water only tepid......seems as though plate heat exchanger at fault but so suddenly? What else could have have caused such an instant failure of DHW?

 

[Gasguru]

........Installed new DV

Probably only needed a diaphragm. Is the diverter genuine? Does the heating flow pipe get hot in DHW mode? Blocked plate exchanger normally results in hot/cold/hot/cold with burner modulation dropping quickly from max to min (or off) and a significant delay before re-lighting..

 

[gasgasgas]

Yes genuine replacement DV. Boiler remains alight water runs continuously tepid without much sign of shift in modulation - do you reckon thermistors/flow switches/stats etc all ok? I figure boiler would not keep going on demand with these being out of action??

 

[mrgas]

Is it a boiler problem at all?
You may have hot to cold to hot letting bye somewhere?

 

[gasgasgas]

Thanks Gasguru & MrGas - bizarrely received a call from customer this am early to say 'it's working fine now, plenty of hot water this morning and late last evening'

Suspecting the worst will happen and there will be a recurrence of the problem, and so another call-back some time in nr future, I called v helpful tech at Ariston who suggested several possibilities to explore:
1) Check gas rate on DHW demand to ensure valve is opening to max - should be kettling in heat exchanger - pointing back to pcb/thermistor
2) If not up to temp check resistance on Primary Flow thermistor.
3) If thermistor ok check pcb for voltage
4) Also plate heat exchanger may be blocked - he said he had 'never successfully cleaned one' so poss replacement.
5) On the valve assembly at back right hand side housing PRV, there is large hex on left - remove and clean to keep bellows on DV optimised.

Bottom line is that if LHS pcb has failed, they recommend replacing both for compatibility so 2 x £100 = £200, plus if plate heat exchanger failed - another £100.........plus labour plus what customer has already spent starts to make a replacement boiler look like best call. Will gird loins and speak to customer.......

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[Agile]

I called v helpful tech at Ariston who suggested several possibilities to explore:

1) Check gas rate on DHW demand to ensure valve is opening to max - should be kettling in heat exchanger - pointing back to pcb/thermistor
2) If not up to temp check resistance on Primary Flow thermistor.
3) If thermistor ok check pcb for voltage
4) Also plate heat exchanger may be blocked - he said he had 'never successfully cleaned one' so poss replacement.
5) On the valve assembly at back right hand side housing PRV, there is large hex on left - remove and clean to keep bellows on DV optimised.

But all those are only what any competent boiler engineer would be checking.

I manage to clean 99% of plate heat exchangers fairly easily. its just the odd one which has become blocked with calcium compounds that I have to use HCl to clear.

Only one, the water way in a 24i which was totally blocked, defeated me!

Tony

 

[ChrisR]

Try asking in the Combustion Chamber