Ariston Eurocombi help please - Fixed

[SEDIY]

Our combi has been working ok over the summer providing hot water on demand.
Worked ok for the cold snap a few weeks ago.

But now it will only run for 25-30 mins before the flame goes out (water up to temp?). This seems to be as it was before. However the boiler never re-ignites if we leave it running.

If I switch the boiler off then back on the boiler re-ignites and heats for 25-30 mins again.

If I switch the CH off then the DHW always works and re-ignites the boiler.

Any help appreciated.

Steve.

BTW I replaced the expansion vessel as after searching on the forum and running a few checks found that water was leaking out the air side of the EV.

 

[SEDIY]

I removed the primary and secondary heat exchangers and cleaned them in case they were blocked. Black sludge was evident and cleaned out.

Do I just fill the system with X400 and hope for the best?

Still only getting 25-30 mins of boiler runtime before the flame is shut off. Boiler still doesn't reignite by itself after this.

What else can I check?

 

[Agile]

There is obviously a fault!

But I am surprised that you are using hot water for so long and so regularly.

There are several possibilities as to why the boiler is stopping. Knowing what makes it come back on would be a clue!

Obviously correctly diagnosing the cause is the first step in repairing!

Tony

 

[SEDIY]

Thanks for the reply Tony.

Not really sure by what you meant with "Knowing what makes it come back on would be a clue!"
as the only way to get the boiler to come back on is to let the timer turn it on (if boiler is off) or to manually switch it off then back on.

Some more background;

We were changing 2 radiators and after draining and refilling a couple of times it is evident that we have a blockage somewhere in the pipes/radiators as the draining took ages as we waited for much gurgling and burbling.

We have now taken off an additional two radiators so far and flushed them through. Also flushed through from the upstairs bathroom towel warmer and vented out the ground floor hallway pipework until no more black magnetite sludge came out. Re-filled system and bled all radiators and now the boiler is vibrating and whistling. (Turning the CH temp down a little stops the noises). I have a bottle of X400 on standby to help flush the system once I know I won't have to drain it for a while.

Boiler is still only running for 25-30 mins before the flame is extinguished (pump and fan still run after flame is out)
Using the time clock to stop and start the boiler fires the flame up again for another 25-30 mins.

I just can't seem to figure this one out?

Thanks in advance for any pointers as to where I should look next.

Steve

 

[SEDIY]

Should the fan on the flue continue to run after the flames have been shut off?

 

[Agile]

Without being there its difficult for me to even guess at whats wrong.

If you could advise the flow and return temperatures when it goes off would be a start.

Tony

 

[SEDIY]

Thanks for pointing me in the right direction.

With the boiler temperature setting at maximum:
Looks like the flow temperature reaches between 80C-82C when the shut off occurs.
The return temperature is a maximum of 70C.

Is my boiler overheating, as when it reaches this stage the fan is still running?

I used the timer to switch off the CH and then ran the hot tap to cool the flow temp to 55C (flames still off)
I then shut the hot tap off and then a few seconds later turned it back on and the boiler fired into life again to provide hot water. Does this confirm that the boiler is overheating when running the CH?

 

[Agile]

On test can you replace the CH temperature sensor with a 6.8k resistor or a bit less and see what it does?

I dont thinks thats the fault but helps to eliminate it.

Does your model have a grey ignition control box in the middle? If so can you direct cool air from a hair dryer towards it and see if that changes the timing of the problem?

Or a black and red ignition box? Do the same!

Or a rusty orange ignition box? Do the same!

I think the clue is that the fan continues to operate. Unfortunately the next step I would want to take would require a registered engineer.

Tony

 

[DP]

OP I suspect the PCB is faulty. Your boiler reaches operational temperature, burner goes out and will not resquence ingnition. Switching the mains off, resets the controls until boiler goes to sleep again.

When boiler has shut down like this, does running a hot tap, get the burner to fire?

 

[Agile]

Usually I would agree.

However, as ( if ? ) the fan continues then that implies to me that the fault lies within the ignition sequence.

Chris R once said he had counted about 11 different versions of eurocombi so I dont know which ignition unit his has.

Its also possible ( probable ) the fault lies with a cheaper component which we dont advise DIYers about.

Substituting a resistor for the temp sensor test would eliminate the fault being the getting to temperature and failing to come back on again.

I always expect that to be a fault in the anticycle circuit. Seems very common on older worcesters.

Tony

 

[SEDIY]

Couldn't find a 6.8k or similar in my spares bin so had a go at measuring the resistance of the existing thermistor while operating.

CH thermistor starts at 13.4KOhms at 15C
down to 2.0KOhms at 70C
then 1.4KOhms at 82C when the burners shut down

The boiler is modulating from 68/70C up when I pull the wire off the thermistor to measure the resistance the burners flare up to high output.
Replacing the connection to the thermistor reduces the burner output once again.

Is this the ignition control box?

@DP
When the boiler shuts down the only way to allow it to fire up is to use the timer control to stop it and then run the tap or use the timer to turn it back on. It then fires up okay.

Thanks again for having a look.

Steve

 

[DP]

When the boiler shuts down the only way to allow it to fire up is to use the timer control to stop it and then run the tap or use the timer to turn it back on. It then fires up okay.

What I am getting at is, boiler shuts down during central heating demand, You do not do anything other than go to the sink and run a hot tap. Does the boiler recognise the HW demand and ignites the burner?

 

[mysteryman]

I would look at system cleanliness. Black sludge is never good news. Get it clean first; don't spend money on parts before that!

 

[mysteryman]

I would look at system cleanliness. Black sludge is never good news. Get it clean first; don't spend money on parts before that!

 

[SEDIY]

When the boiler shuts down the only way to allow it to fire up is to use the timer control to stop it and then run the tap or use the timer to turn it back on. It then fires up okay.

What I am getting at is, boiler shuts down during central heating demand, You do not do anything other than go to the sink and run a hot tap. Does the boiler recognise the HW demand and ignites the burner?

Once the boiler has shut down, running the hot tap doesn't ignite the boiler