combi boiler dhw

[godatnkg]

I have a Vokera excell 80SP combi boiler and I am having problems with the hot water.
The central heating appears to work ok.
The hot water is very temprental, some times is ok-hot for approx 20 seconds and then shuts down, sometimes just warm or doesn't work at all.
The heat exchanger is new as is the PCB.
The water filter, pressure and flow are ok.
The dhw flow valve and micro switch are working perfectly.
Any help on this would be appreciated as my ears are starting to ache.
Thanks, John

[gasman]

The Vokera excel 80 SP is well known to us engineer's as been under-powerd and rubish with showers and baths it only has a maximun output of 23.7kw with a very poor 11 litre/min delivery acheiving only a 30*C rise in temperature. Not very hot at all when the incoming mains water is only around 5*C at this time of year giving you a unsatisfactory luke warm 35*C if you are lucky. You can how-ever lower the water flow rate to around say 6 l/min and this will give you a higher temperature rise of around 45*C but it will take forever to fill a bath and forget trying to run a shower off it. They also degrade quite quickly and need regular diaphragm changes to keep the hot water at the best it can do (which is not very much ). And the Thermistors need changing quite frequently as-well. I think you are going to need an engineer round to fit a new Thermistor and a new Diaphragm from the sounds of it. And ask them to lower the flow rate while they are there if they can-not get a temperature of 45*C or more. In reality you should be able to achieve 50*C - 60*C if set up correctly but a very poor rate of delivery.

[godatnkg]

Thanks for the quick reply, it confirms my thoughts on the boiler, luckily we do not run the showers off it (both electrlc) and haven't had a bath since we moved in. I have a thermister on its way.
Thanks, John