Consort LST heaters failing - constant beeping

[MisterBoy]

I had 4 Consort LST electric heaters installed in a small office building almost exactly 6 years ago - the PLSTi1500E model.

A week or so ago one started beeping constantly when using the building after the Xmas break - it's a simple repeating beep about once per second which continues indefinitely. No error was showing on the control panel and I couldn't see any way to make it stop other than turning off at the wall outlet.
I restarted it and it was working again but at some point in the 24 hours the same happened as next time I came back, the same had happened.

A little googling seems to show only "generic fault" which isn't helpful.
Yesterday, another unit did the exact same thing and this was in a bathroom - it got down to 2C in that room and I am worried about pipes freezing!

Can anyone give me any suggestions what to check, how to diagnose what is wrong?
They're not the cheapest thing especailly with 2 on the fritz. That they turn back on for a while seems to indicate the main unit is working, are they repairable?

Thanks for any advice.

 

[jj4091]

Do the on/off times need resetting after powering off. User guide here if that helps https://device.report/manual/8336064

 

[MisterBoy]

Do the on/off times need resetting after powering off. User guide here if that helps https://device.report/manual/8336064

as far as I can tell the clock loses its setting every time you turn it off at the wall which is irritating but that isn't the problem, I'm not running it with a schedule but a constant low temperature (10C) as the building is not in much use and the weather is so cold.
It runs fine after plugging back in, for some unknown period - a few hours it feels like.