vokera compact hot water cutting out intermittently

[loafer]

boiler is firing for between 20 and 30 seconds then then cuts out. water runs cool before boiler fires up again. This process is repeated. The dhw water runs very, very hot, regardless of adjusting the hot water manual dial on the face of the boiler. central heating working fine.

Diagnostic in installation and servicing manual states that this problem may occur for the following reasons.
hot and cold water pipes crossed
cold water regulator valve needs to be opened more
regulator valve may be blocked and needs cleaning

I have double checked the pipes tracing both hot water from the boiler and cold water from meter before re-routing pipework and installing the bathroom suite and shower mixer valve. I checked again when the problem with the hot water supply became apparent. All changes to pipework that I have installed were beyond the "T" where the cw feed branches to bathroom and kitchen. All new pipework immediately beyond this "T" was fitted with a ballfix valve on the branch serving the bathroom (similarly with the dhw) which I fitted first so that the cw and dhw could be reinstated for the kitchen whilst I worked on the bathroom. So any debris could not have ingressed to the boiler. All feeds to the cw and dhw drain off points have been traced and there is no crossed cw/dhw pipes.

Upon inspection the cold water reg valve was barely open. So it was opened fully and flow/delivery of dhw checked again... problem persisted, dhw flow rate seems low.

Regulator valve was removed and checked for blockage or damage to the jumper on the valve. Valve working ok and no blockages were encountered on the inlet filter. Delivery of cw was tested whilst the valve was off. Pressure test guage read 4.2 bar.

The pressure of the dhw at the drain off points is still very low, however which suggests a problem with the boiler.

From my limited experience, and this is purely speculative, the problem may be:

a blockage inside the boler???
a faulty diverter valve???
an electrical fault dhw thermistor which is perhaps unlikely as the hot water, when it does heat, is very, very hot so the thermistor may be causing the boiler to turn off the hot water as a precautionary measure to prevent scalding for example.

I would be intrigued to know what the likely problem is.

 

[Gasguru]

The flow restrictor is probably blocked, its in the connection onto the heat exchanger. Right hand rear pipe connection. Outer case has to come off.

 

[loafer]

thank you very much, gasguru, for the reply
have informed my customer of the possible cause. he is covered by a service and repairs policy so at least the diagnostic will save time for the ch engineers.

one more q.
There is an inlet filter on the cw regulator valve so how could the restrictor valve have become blocked? especially since all work i did was beyond the t branch serving the boiler and the bathroom branches were immediately fitted with ballfix valves?

thanks again

 

[Agile]

I originally thought that you had fitted a new boiler.

It now seems likely that you have fitted a new bathroom and the boiler is existing.

You should have tested the boiler before you started the work as it was probably faulty but you are now being blamed for the problem.

I would still take the view that the secondary heat exchanger is blocked. You mention a "low" flow rate. Unfortunately "low" means nothing to me! Can you measure it in litres per minute? Other aspects may apply.

You seem to be muddling static pressure with flow rate. The flow rate is the only measurement thats relevant.

Tony

 

[loafer]

thanks tony fer the advice re: static pressure and flow rates

Yes, I did install the new bathroom and the boiler was pre-existing. Thankfully the customer(s) did admit that the dhw flow was previously low and even more refreshingly, the customer(s) are really nice genuine people (which increasingly in this day and age is surprising) So no blame was apportioned. They're gonna get their service contractors to sort it out. I've even got 2 more jobs as recommendations from them!!! :

Im increasingly becoming aware that some customers try to pull fast ones. I will now always check boilers for basic faults before starting oor estimating for jobs. This precaution will be added to the list of checking for pre-existing water staining and cracked/powdery plasterwork on ceilings, pre-existing staining to floor coverings underneath rad valves, open jammed stop cocks and gate valves etc etc.

oh my!!!! such cynicism in one soooo young!!!

cheers again

stuart