A07 error Vokera unica i28

[Tom__40]

Hi. As I understand it this is a primary flow thermistor error (temperature overheat). I've just had this happen again after having no problems for a while. It's only ever happened after turning on the hot water, doesn't seem to happen when it's doing the central heating. I wondered if in theory this could be caused by setting the pump speed too low (the lowest setting), therefore the flow's not fast enough within the boiler and it heats up too much? I haven't had the error in a few months and have recently turned the pump speed down from high (fine, no issue) to medium (also no issue) to low (fine for a couple of days, then just had this error again after not seeing it in a long time). My other guess would be the plate heat exchanger on the DHW side is somewhat blocked, but I cleaned it a while ago using an aquarium pump to run vinegar through it for a couple of hours, also fitted an electrolytic and magnetic scale inhibitor on it to prevent more limescale. I turned pump speed back to medium and reset, seems fine again so far, time will tell.

I did have one other question if anyone knows the answer. I know the internal expansion vessel in the boiler is shot, so fitted an external vessel on the central heating return some time ago. But how does this take up expansion when it's only doing the hot water?! Looking at the diagram in the manual it shows the internal vessel as attached to the pump. It obviously works somehow but I don't understand it, to me it looks like when the hot water is running the central heating circuit would not be involved.

cheers,

Tom

 

[Johntheo5]

Looking at the schematic it does look allright as if the CH return + the externalk expansion vessel will be isolated when DHW on, what is the boiler pressure during this period

 

[DP]

Suggest you leave the pump speed at max
And a proper service.

You are tinkering with the boiler for no reason. Just because there is a switch, does not mean it should be moved
At low speed the differential on primary side will be too great during HW mode.

How do you know the EV is shot?

 

[vulcancontinental]

Suggest you leave the pump speed at max
And a proper service.

You are tinkering with the boiler for no reason. Just because there is a switch, does not mean it should be moved
At low speed the differential on primary side will be too great during HW mode.

How do you know the EV is shot?

Couldn't have written it better myself, thanks DP.

 

[Tom__40]

Hi. The internal vessel is gone as the pressure was getting too high when in heating mode. It was a long time ago now, but I think it was in the region of 2.5-3 bar. I tried repressurising it with a pump, but couldn't get it to repressurise, so assumed the diagphragm had gone and decided to fit a 12 litre external one - pressure since then when heating is on is reliably around 1.25-1.5 bar hot. The original system only had 3 rads downstairs, so it seemed odd that the pump needed to be on high for such a small system.

Pressure when running the hot water is about 1.25 bar. I still don't get how the expansion is taken up when the external vessel is on the CH circuit...I don't think there's a dedicated DHW expansion vessel looking in the manual.

I agree it is probably time for a service! I just like to sort what I can out myself (which is why we're all here after all!).

 

[ianmcd]

The original system only had 3 rads downstairs, so it seemed odd that the pump needed to be on high for such a small system.

The pump needs to be on 3 for domestic HW, in an ideal situation, the pump speed would not be adjustable, but all manus use similar pumps that are available and used on different scenarios, your pump needs to be set a 3 and not adjusted down

 

[DP]

There are aspects of this boiler that will be beyond ability of almost all house holders. Furthermore, you are unlikely to have the equipment to carry out some of the checks required. You have already shown your lack of knowledge by adding additional EV when the original is unlikely to be defective. On subject of expansion vessel, how did you attempt to pressurise the EV