worcester 28i cdi hot water problem

[plumberstool]

The hot water at the bar shower goes hot then cold. The shower manufactures sent me a new cartridge to try and this has not cured the problem.
If you run the tap at the basin this runs ok, I ran the shower and watched the flame picture boiler fires up, and does not seem to modulate to me, as it runs for a couple of minutes then cuts off then fires up again. I have been told by the customers they turned the water off when new w/machine fitted, and drained the water from the boiler, but kept the heating on. Could this have damaged a sensor/thermistor ? don't think it is the divertor valve as it fires on demand when the taps are turned on. Any help/ideas appreciated as there are no manufacturers instructions with the boiler thanks p.s hopefully you can now understand my post

 

[chrishutt]

Just take some of these ...........

They're full stops and are used at the end of sentences. You could even try Capital Letters at the beginning too. Then we might be able to understand what you're writing.

 

[scatmanjohn]

If it works using the basin ( i assume all the other taps in the house are ok).
I could well be a flow rate problem.

The shower may be reducing the hot water flow down too far for the boiler to fire.

The dhw diaphragm could well be worn.