Performa 28i Hot water problems

[Slender Mouse]

Hi,
I hope somebody can give me some advice as I am running out of ideas.

I have a Potterton Performa 28i that only gives warm/cold water.
I have changed the DHW plate heat exchanger along with all the bits I could get to on the diverter valve.
(both diaphragms and the main needle valve). The CH and HW out nuts are seized so could not change the whole thing.

The boiler now looks like its over heating because the how water is not flowing around the boiler. Once refilled with water the temp lights rise very quickly and the boiler flame shuts off and the water is still barely warm out of the tap.

could it be an air lock? when I drain and strip it back down there still seems to be a little pressure stored in the DHW plate heat exchanger, when I remove it.

Central heating works fine, its all in the hot water system.

Thank you for your time

 

[Slender Mouse]

forgot to mention that I had the system power flushed about 2 months ago and have also added some Sentinel X800 to the system in an effort to clean the DV.
I am also looking to replace the whole boiler in about 8 months when I start a house extension so just need to cobble this old thing together for a lille bit longer.

 

[ianmcd]

does your CH work ?

 

[Slender Mouse]

yeah central heating works ok, no overheating from the boiler and all rads get hot. It seems like the hot water part of the boiler is not circulating but the pump must be ok if its pumping the central heating??