Vokera Linea 28 combi boiler. DHW problem

[Enforcer777]

Hi

I wonder if anyone can advise me on what the problem might be with my Vokera Linea 28 combi boiler.

It is 9 years old and has been fairly trouble free over the years.
This morning, although the central heating was working normally, the shower was quite cold & I found that the domestic hot water would only heat to about 41 degrees before the flame which has modulated down, cuts out.

The flame then comes on again at about 30 degrees and keeps repeating and maintaining the temp between these figures (just too cold for a comfortable shower).

Fan seems to be working normally. I have replaced the DHW thermistor, but this has made no difference. I also checked the pressure
differential switch and tubes. They seem ok. We live in a soft water area & have no problems with scale build-up. There are no fault codes showing. Central heating is operating normally.

Any help or advice greatly appreciated.

Enforcer777.

 

[Agile]

You may have broken the hot water knob shaft.

I can fold down the PCB tray and open the back and see if the shaft is turning the pot on the PCB. If so then rather than bother with a new knob it usually suffices to just turn the pot higher.

Tony

 

[DP]

When display shows 40 degrees or so, ask someone to turn off the HW tap and note temperature displayed. High temperature (85 to 90 degrees) indicated blocked HW heat exchanger. I suspect system muck is cause of erratic HW delivery.

 

[Kristoflampie]

Yep have come across a couple recently with the choked plate heat exchanger and the test DP describes is spot on, even us Scots with our top quality water get crap through the heating side of the plate.

 

[DP]

I do not know where your tap water is sourced but our water quality is going downhill. Now have to resort to using bottled water to make tea else 'safe water' as the water authority likes to put it, turns good tea to bad.

All those years could drink water straight from the tap but sadly good old days no more. Now it is bottled water

 

[PUTWADE]

I seem to be having the same problem as you Enforcer777. My Vokera Linea 36HE combi (3 years old) has started running hot then cold when drawing through the DHW. (The central heating is fine).

I called a heating engineer over the weekend and got a new Thermistor fitted which, to be fair, when he tested the system the hot tap stayed running for 10mins without any problems and we therefore thought it was all fixed. This morning however, it went back to the same problem.

Also, we do not have limescale problems (Edinburgh area) but I will try DP and Kristoflampie's suggestions, i.e. test the system then remove and clean the domestic hot water heat exchanger....is it a difficult job to remove this secondary HE? Other posts say fairly easy but looks like it may be quite tricky to get too on my system? Has anyone come across any diagrams/ images (for the 36HE) to do this outside those in the manufacturer's installation booklet?

Thanks for any advice in advance.
GW

 

[gas4you]

Which thermistor did he replace? I would have replaced the primary (ch) one.

 

[Agile]

If the plate is blocked then if its not too bad it needs to be tested at the full flow rate the boiler can give otherwise the problem is not always apparent.

The problem is dirt on the sealed primary system.

The best diagnosis is to measure the flow and return temperatures on the primary. They should only be 15-20° differential.

Tony

 

[PUTWADE]

Gas4you; the original change was the thermistor on the DHW side, not the CH side. (I'll change this if the fault continues).

Agile: The engineer has actually just been back out after my first post and he looked inside the condense trap to find 'a lot of gunk' (his words!) which he reckons could have caused the problem. There was also a loose piece of metal in amongst the 'gunk' which I've no idea how that could get in there. He removed all this gunk and the metal and tested again for 10 mins and it all seems fine.

I intend wait to see if this solves the problem then try some central heating cleaner and ultimately put in some inhibitor after draining and re-filling etc.
Does this seem like a sensible approach?

Thanks again, truly appreciated comments from experts.

GW

 

[gas4you]

Usually I would expect a blocked condensate trap to make the boiler fail completely, not just cycle the HW hot and cold.

It is sounding more like the plate HE blocked on the system side from here.

 

[PUTWADE]

Ah...., but perhaps it has been failing on the CH side and I just hadn't noticed. Is it possible the burner would just start /fail more often like it does for the HW? Would the effect not therefore be warm radiators that I would have (incorrectly) assumed that this meant the CH was okay?
GW

 

[DP]

A combi boiler works hardest when in HW mode. In essence, boiler is getting too hot so it is switching off leading to HW cooling. Boiler cools and burner fires. So the cycles begins

Nothing to do with CH or thermistor in this case- neither condensate trap.

 

[PUTWADE]

Thanks DP. As you suspected, the boiler failed again this evening after running it for 10mins. I tried your initial post test of allowing the DHW to reach 40°C then turning off at the tap but the resultant temperature was only around 60°C when I did so....does this change the likelihood of the plate heat exchanger being the culprit?
GW

 

[Enforcer777]

Hi folks

Many thanks for the advice. I have checked the temperature knob shaft and it is not broken. It is turning the 'pot' ok. In fact prior to this fault I can't remember when the temp knob was last altered. I have always had it set at maximum.

Also tried turning off the hot water tap when water temp was at maximum (41 degrees) The temperature it then displayed was 50 degrees, so I take it that this does not point to a clogged heat exchanger.

Our water is from the pentland hills reservoirs and as well as being soft has never given any problems.

Enforcer777

 

[gas4you]

I would say the system side of the plate HE is blocked, not the domestic side.

Failing that replace the primary ntc, but take the plate off first to clean it.