Vokera Compact 24, DHW good CH bad.

[Bladerunner67]

Good afternoon,

I am wondering if anyone can shed any light on what might be wrong with my boiler. Firstly it was in the house when I bought it a little of 12 months ago, I have not installed it / had installed myself.

The symptoms are:
DHW runs warm, briefly cold then stays hot, I think this is a secondary NTC problem but I can live with it.
CH runs from cold up to 51-53 degrees and then the flame cuts out, the temperature drops to 40 degrees and it fires again until 51-53 degrees. It has done this all afternoon. I am measuring the temp with a Bluetooth temperature probe. The probe is attached to the side of the heat exchanger by the CH flow pipe. The rads are not warming as the water doesn't seem to be getting hot enough?

So far I have carried out the following:

Bled the boiler by closing the return valve and using the filling loop and the PRV until I can't hear any more gurgling. Open the return valve

Remove the screw from the centre of the pump to check for freedom of movement of the spindle and to remove any air. The spindle rotates easily. The three-speed switch on the pump alters the light hum of the pump motor.

replaced the primary NTC with a new OEM NTC. The new one is 10.6 Kohms at room temp the old one is 9.6 Kohms at room temp.

Checked the High Temp cutout and it shows 1 ohm at room temp, can't test at 80 degrees because the boiler doesn't get that hot before shutting down. I have bridged the thermal cut out as well in case it was operating at a lower temp. The boiler stills shuts down at 51-53 degrees.

I can't find any air in any of the rads all the valves are open.

The pump remains on when the flame extinguishes and the light flashes green indicating demand?

I have cleaned the system with X800 Sentinel and flushed it through. I have also removed and hosed out two rads downstairs which only ever got lukewarm

This model of boiler doesn't seem to have a mechanical diverter valve as such.

It doesn't seem to be air lock, iso valves, thermistors or pump.

My next thoughts are PCB? Any other suggestions?

Thanks in advance and sorry for the lengthy post...

 

[Gasguru]

What happens if you crank the DHW temp control to max and run the hot tap? Does the burner remain lit with constant hot water over does it cycle?

 

[Bladerunner67]

Hi Gasguru, I've run the hot tap in the bathroom for 10 minutes and the boiler stays lit. The CH side only reaches 48 degrees while the tap is running. The water out of the tap is 60 degrees. As soon as the tap is turned off the CH side continues to rise to 51 degrees and then goes out...

 

[Gasguru]

You won't have any heating whilst there is a hot water demand....the pump doesn't run.
The hot water test was to rule out air flow issues.
Check the heating control knob is turning the potentiometer on the pcb...the spindle may have broken.

 

[Bladerunner67]

The control knobs haven't been moved, I've always had them set on max. I can check but that seems unlikely?

 

[ianmcd]

there are lots of Vokera compacts and the temp control knob may very well be snapped , its very common, if your model is the bi thermal heat exchanger check the two braided hoses on the left hand side of the heat exchanger, if they are solid and crunchy that will be your fault, dont squeeze too hard or the crud will be pushed into the pump assembly and a *******s to get out

 

[Bladerunner67]

Hi Ian, I've depressurised the boiler with the PRV and the two braided pipes ate rock solid but not crunchy. If I take one off to look inside it, will it be obvious if it's shot and also will I be able to put it back without it leaking if it looks OK? Also would that account for it always shutting down at exactly the same temp of 51-53 degrees?

 

[Gasguru]

When I've had the hoses solid with debris the boiler just cooks, temp rockets.

 

[ianmcd]

Hi Ian, I've depressurised the boiler with the PRV and the two braided pipes ate rock solid but not crunchy. If I take one off to look inside it, will it be obvious if it's shot and also will I be able to put it back without it leaking if it looks OK? Also would that account for it always shutting down at exactly the same temp of 51-53 degrees?

yes you can remove them and clean them out, will be obvious if they are full of crud, as @Gasguru says the boiler usually overheats but ave seen them shutting down too early, you wont do any harm by removing and cleaning them out, as already said dont crush them in situ or you will release all the crap into the pump housing, you will be surprised how muck keek accumulates in them

 

[Bladerunner67]

OK represurised to 1.5 and now its cycling between 53, 54,55 low and 66 high. So the point it was shutting down is now the point its firing up... It has done this for 4 cycles now, I'm guessing the pump must be good because the water in the heat exchanger drops by 10 degrees in 2 minutes when the flame is out.

 

[Bladerunner67]

Overnight the boiler ran for 4 minutes at 70-71, this morning it is back to a high of 66 and a low of 55-56. I will check the flexi pipes later on and report back.

 

[Bladerunner67]

Thanks, Ianmcd and Gasguru for your help. The flow pipe was almost completely full of black charcoal-like stuff and the return was also very crunchy. I emptied about a mug full of grit out of the two pipes. The rads downstairs are hotter now than they ever have been. I'll put another descaler through now it has a chance to circulate and then an inhibitor. Is this a normal build up or a lack of maintenance?

 

[ianmcd]

its very common on that model, putting in a clenser then an inhibitor will help

 

[Gasguru]

Rubber hoses in boilers are perfect filters..they block in Vaillants and Viessmanns too.

 

[Bladerunner67]

Well it's certainly made a difference. I was going to get a new PCB and then look at a pump and heat exchanger or throw in the towel and get a new boiler. I would never have thought of those pipes. Top job thanks.