Glow-worm Xtramax HE - DHW problems - F14

[TParker]

Hi guys,
I'm after a bit of advice please.
I have a Glow-worm Xtramax HE boiler and a few months ago had an engineer replace the secondary heat exchanger for the hot water, quickly followed by the CH pump which sprung a leak.
We've also fitted a magnaflow unit to help clean out any rubbish floating round the system.
Everything has been working fine with the exception of one thing that has been happening for a long time.
When the DHW is switched on and in use the pump continues to cycle for about half an hour after the hot taps/shower have been switched off. This can cause problems with the heating as I assume the boiler thinks the hot water is still being demanded so the pump is diverted to DHW and the radiators are no longer fed.
Sometimes the burners will fire up and heat the DHW and the pump will cycle again even when not called for - in the night for example.
These problems stop if the DHW is switched to the 'off' position. We have therefore been operating our DHW by manually switching the water on when we want it.

Now an extra problem is happening.
When someone is using the shower and the DHW is being heated and used, several times an F14 error occurs (Flow temperature >95c) and the integrated hot water tank appears to dump its contents to the radiators. They begin to heat up even though the central heating isn't switched on.
The F14 fault can display 3 times for about 20sec and then continue normally during a 10 minute period of hot water use (shower).

We have changed 3 of the 4 possible thermistors just in case (not the one on the hot water tank)

Can anyone shed any light on these errors all during DHW operation:

- Pump continuing to cycle a long time after there is no call for it?
- Radiators warming up whilst hot water in use?
- F14 error message?

I'm having real problems getting hold of the engineer that fixed the problems before. He had suggested in the past to change the main control board and that 'may fix the pump over-run issues'. I wasn't keen to spend hundreds of £'s on trying something that MAY fix it!

If anyone has any views or thoughts, they would be much appreciated.

Many thanks.

Tim
Reading,
Berkshire

 

[naanbread]

F14 on these appliances is that the central heating flow thermistor is reading a temperature above 95 degrees. If its only happening on hot water then its not the thermistor as it would happen in heating too if that were the problem. The only difference between the hot water and heating circuit as far as the thermistor is concerned is the Plate to plate heatexchanger. You will probably find that its blocked or restriced causing a high temperature build up on the flow pipe causing the high reading.
The radiators are probably heating up as the boiler will try its best to clear any heat, pushing it into the heating circuit.

Hope this helps

NB

 

[TParker]

Thanks NB.
Really helpful.
The central heating is running fine with temperatures not getting as high as 95c.
Do you think the heat exchanger could be blocked again after it's been replaced 8 months ago? Is there potential for the internal small hot water tank itself to get blocked? We live in a very hard water area.
Would you have a view on the pump over-run for half an hour after calling for hot water?
Thanks again for your help.

Tim

 

[Agile]

Its most likely that the plate was previously blocked by lime rather than system dirt.

You need a mag water conditioner rather than a mag filter.

Its possible for the plate to become scaled up in weeks in your area!

I expect you have a dripping hot tap!

Tony

 

[naanbread]

Thanks NB.
Really helpful.
The central heating is running fine with temperatures not getting as high as 95c.
Do you think the heat exchanger could be blocked again after it's been replaced 8 months ago? Is there potential for the internal small hot water tank itself to get blocked? We live in a very hard water area.
Would you have a view on the pump over-run for half an hour after calling for hot water?
Thanks again for your help.

Tim

You're more than welcome. As Tony has stated there are ways around re-occuring problems with hard water.

It is definately possible for the heat exchanger to be blocked again within the time stated, particularly if the cause of the problem isnt rectified. Its not a heatexchanger issue but the heatexchanger is the effected part if you see what im saying.
I would probably not look towards the storage tank, based on the fact that this is simply for storage, it doesnt really have an effect on the flow temperature like the plate heatexchanger does. If the tank was blocked/restricted you would probably have issues with flow rate from your taps.

the pump over run is based on the temperature as well as time. some over run tries to drop the appliance below a certain temperature but would continue to run if thats not achieved. For example, the over run might aim to get below 20 degrees after 5 minutes. If after 5 minutes its still above 20, it would continue to run until it is below 20. Does this make sense?

its probably continuing to run trying to achieve a lower temperature which can be linked to the high temperature you are getting.

Hope this helps,
Sorry if none of what i have said makes sense (it does in my head)

NB