Systemate III SM144 hot water not constant and BG are clueless

My mother moved into an apartment about 2 years ago and during the summer of 2008 was unable to get the hot water taps to stay hot. I was visiting the UK at the time and experienced the same. When you turned on the shower, the water would be hot but within seconds and sometimes minutes it would go completely cold. The radiators all worked fine and were hot.

She had a contract with British Gas and they sent an engineer (a glossy title for what I saw). The guy had no idea what he was doing and after 40 minutes of climbing in the loft to figure out how it all worked, decided to call a colleague to help him. After three hours on the job, they concluded it was the heat exchanger and would return another day to replace it. They did so but said the 300 pound part was not covered by the service plan and that she would have to enter in to a new agreement plan at a higher monthly rate if she wanted the part replaced. She agreed, it was replaced and the HW was working again.

After just 5 months, the problem returned. Again, the radiators are perfect but the exact same problem with the HW. BG came back out and replaced a valve but that didn’t fix the problem. They came out again and replaced a thermostat but still the same. Now, the engineer has concluded that it’s sludge and the system needs to be power flushed at a cost of 700 pound. His recommendation, however, is to pull it out completely and have them install a Worcester for several thousand pounds.

When she told me this, I started to research this issue and it seems like many people are reporting the same problems and BG engineers not being able to service these systems properly. Gledhill tech support suggested it could be the sensor, pump or the logic board, none of which BG have replaced.

Replacing the heat exchanger 5 months ago fixed it last time. If it was sludge, could this have caused the heat exchanger to fail last time and now cuased the new one to fail too? If the radiator side of the system is working, would that rule out sludge? Could it be another bad heat exchanger. It concerns me when BG want to charge 700 pound to flush the system and Gledhill tech support disagree with the diagnosis.

Any help would be appreciated.

I suggest the DHW heat sensor needs to be replaced. It is the the most likely problem and a new one needs to be fitted if only to rule it out as the cause of the problem.

It's such a shame BG are letting you down on this as the appliance is quite easy to diagnose/fix if you know it well.

Cheers, Mike