Shower alternating hot and cold - Potterton boiler

[judep2000]

I have an Aqualisa shower that just gets to the right temp and then cuts out to cold over and over again.

Checking some of the other posts on this site I have:
1. Looked at my Potterton Puma 100e (about 9 yrs old) to see what it was doing when the shower is on. The flame light will come on for 28 seconds and then go off for 52 seconds exactly over and over again.
2. Put on a hot water tap to see what the boiler does and the flame light does exactly the same but the water from the tap remains at constant temp and doesn't go cold.
3. Checked the thermostat coil thing inside the shower and it is spotless.
4. Noticed that the shower head is not the original Aqualisa and is more of a cheap replacement.
Anyone got any idea why I can't get steady hot water, is it the shower or the boiler? and is there a fix?
Thanks

 

[Slugbabydotcom]

It appears that you need a better flow rate for this to work properly.
Start with cleaning the shower head
If that doesn't work replace it with an original part
After that try running a hot tap slowly when you get a shower until you get to the bottom of the problem
The bottom of the problem could be with your combi and the possibilty that the flame is not modulating down [reducing according to demand] Amongst other things this may be down to a faulty sensor or gas valve. Whatever you will need an RGI to figure that bit out.

 

[bengasman]

what is the flowrate?

 

[judep2000]

Have worked out the flow rate from the shower now

Normally it is 2700ml per minute
When it is having it's short burst of normal temperature it is 3300ml per minute
If I run the bathroom hot tap (gently) at the same time as the shower it is 1200ml per minute.

Have worked out that the shower is an old model (stopped manufactuing in 1990) so it is older than the boiler, but there was a time when it worked much better than it is at the moment.

Just trying to work out how big a job this might be (replace shower or just get someone in to look at boiler?0

Thanks