Vokera Linea 24 cycles on and off only when hot.

[PenguinApproved]

Dear All,

I require some advice in diagnosing a fault with a Vokera Linea 24.

When a demand for hot water is made AND the central heating has been on for some time (e.g. 1.5 hours or more), then the boiler gets stuck into a constant ‘on/off cycle’ that can only be cancelled by turning the boiler to off and letting it cool down. By ‘on/off cycle’ I mean flame-ignited/flame-extinguished. Approximately 2-3 seconds for each.

If the boiler is not given a chance to cool down and is switched back on and there is a demand (either water or heating) it will go straight into the ‘on/off cycle’ again.

When the boiler is hot, it sometimes seems to get locked into the ‘on/off cycle’ when there has been no demand for hot water. However, it could be that the flow switch has been momentarily triggered by a fluctuation in the mains water pressure, perhaps by the washing machine or dishwasher or the cold tap.


When the boiler is in the ‘on/off cycle’, it has been observed that the ignition of the gas alternates between a soft start and a more aggressive start. This may be of no consequence; I just thought I would mention it.

The pump and fan still run when the boiler is in this ‘on/off cycle’.

The boiler has never reverted to fault mode and locked-out.

The boiler seems to operate normally at all other times. I think it is reasonable to suggest that a high ambient temperature of the boiler and then a demand for hot water may be triggers to the problem.

So far:
-The domestic heat exchanger has been replaced. This was to address a previous problem of the water going hot and cold. This problem was successfully resolved.
-The system has been cleaned and flushed; though not Powerflushed.
-Both thermistors have been replaced. I.e. in the primary heat exchanger and the manifold (for the domestic heat exchanger).
-The fan has been cleaned.


Any solutions, or other tests recommended for further diagnosis would be most welcome.

Best Regards,
PMHN.

 

[Agile]

The prime suspect is the ignition PCB.

However, a professional would first perform checks on the gas supply, gas valve, ignition electrodes and APS before fitting an expensive part like that. Those aspects should only be investigated by a competent CORGI !

Tony

 

[PenguinApproved]

Tony,

Rest assured, I am not Corgi registered and consequently will not attempt to implement your suggestions. I am however an engineer by profession and merely seek to understand how the boiler functions (or not!). It is also a useful means of vetting a suitable Corgi registered engineer who is either experienced with this particular boiler or is able to approach the problem in a logical fashion. Replacing expensive parts as a first option is exactly what I’d prefer to avoid!!

Your comments have reminded me of something I forgot to mention in my previous post. The boiler was in the house when I moved in approximately 18months ago. I had to turn the gas on at the mains before I could turn the boiler on. I suppose I thought the previous owner turned the gas off as a matter of course.
In hindsight, I should have got the gas system and boiler re-commissioned before attempting to switch the mains on, because I quickly discovered there was a gas leak somewhere in the house!

A Corgi registered engineer was called out. He quickly located the leak, but also noted that the pipe from the meter to the boiler was 15mm for about 50% of its length; about 7m in total. He said that would likely restrict the boiler from reaching its full output potential. Made sense to me so I gave him the go-ahead to fit 22mm just short of the boiler. He also ‘serviced’ the boiler.

More recently, I downloaded a copy of the MI’s (for installation and servicing) and noticed that when commissioning the boiler, the installer should calibrate the gas valve for the gas pressure. Presumably the guy who fitted the 22mm pipe should have re-calibrated the valve. I don’t know for certain if he did and I would have expected him to mention that he had done it.

Could replacing the supply pipe with a larger bore and not re-calibrating the gas valve be a likely cause?


Interestingly, the misses thinks that even though the shower doesn’t go hot and cold anymore (thanks to the new domestic heat exchanger), it still doesn’t go as hot as it used to. Maybe it is a PCB fault? Maybe it’s just that it’s winter and mains water is colder. Or is that just a fallacy?

 

[Agile]

You can check the hot water performance, 9 li/min at a temperature rise of 35° C

Certainly the CORGI should have checked the gas valve max/min settings after upgrading the gas supply as it may well have been fiddled to improve the performanance with an undersized supply.

There is no reason for him to have specifically told you that he was doing that though.

Its also one of the first checks to make now as part of the fault diagnosis procedure.

I always recommend anyone moving into a new property to have the gas supply and boiler checked for safety and serviced within the first few days.

Tony

 

[scatmanjohn]

Forget everything else, its the ignition pcb