Heatrae Sadia Megaflo water heater

[Toblerone]

I have recently moved into an apartment which is 10 years old and fitted with a Heatrae Sadia Megaflo water heater which has two switches next to it, one marked "off peak" and one marked "boost". The only way I can get the water hot is to use the "boost" which heats the water VERY hot. The off peak switch lights up and it sounds as if it is heating the water when it kicks in at about 1am til 8am but the water is never hot.
An engineer came to look and said that the thermostat didnt need replacing, he took it out, looked at it and put it back in saying that it would be ok but it still didn't heat the water. Unfortunately there's no way of knowing until 1am whether it is working on the off peak and I have to take time off work to be here for the engineer to come out so I dont want to waste any more time if he doesn't really know what needs replacing. Any ideas???

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[Steve]

Any chance of some pictures? Of the connections to the heater and the wall switches?

 

[RF Lighting]

Have you checked that the bottom stat is set to a decent temperature?

It's possible that the element has failed. I have seen this where the elemnent still makes some noise as if it is heating the water, but it isn't.

This can be easily checked with a continuity test and an insulation resistance test.

You could try swapping the top and bottom stat over and see if the off peak element heats the water and the peak doesn't any more.