Vailant aquaplus combi - hot water runs out!

[theoldsilverfox]

Hi looking for ideas on this one.
before i start - No fault code shows on the boiler.
customer could run hot water say to fill a bath, which it would do no problem i am told for a deep bath.
Now it gets to half full and the hot water starts to run cold, or at least lukewarm.
if you turn off the tap and wait 5 mins.
Run again it will run piping hot.
Seems the quick start tank is working and heating water, but the combi side is not heating the water while running.
i could be wrong though.
ideas please as i dont know where to go with this one.
cheers

 

[picasso]

Press the "I" button when the tap is running and see is it reads s14.

 

[theoldsilverfox]

Press the "I" button when the tap is running and see is it reads s14.

Tried that - with water running it shows S.30

 

[Piers1989]

Press the "I" button when the tap is running and see is it reads s14.

Tried that - with water running it shows S.30

S.30 is no demand for heating or hot water so it doesnt know you have the tap on.

As stated above it should show s.14 in hot water mode.

they have a hall effects flow sensor which looks for the movement of water. it will most likely be this thats the problem but further testing is required.

looking at the manual online it would appear they have a HW storage so the hot water you are getting will be from this store, so once its gone its not being replaced.

I hope this helps

 

[theoldsilverfox]

that does help - thanks

so is the sensor easy to replace, just a sensor or is it part of flow device or pump.
had a look in the manual not sure which part I need.

cheers

 

[ollski]

71-1933 is for just the sensor.
19-4819 for the whole flow switch including the sensor

 

[theoldsilverfox]

Thanks everyone