Although the repair of your floors is the most important issue to be dealt with here, there is one thing that you’ve described that concerns me a little.
You’ve mentioned that
“In reality, the one that is worst was left at least 2 weeks before switching on, and has only been used approx 10 times?”
The reason that you have to wait before turning the heating on is to allow the adhesives to cure fully before heating, and 3 days is insufficient.
I appreciate you waited two weeks for the worst floor, but I instruct clients to wait 28 days, that’s the length of time it can take for cement based products to cure fully.
In fact adhesive doesn’t achieve its maximum bond strength until after 28 days.
Secondly, your method of operation, yes you were instructed to increase the temperature slowly at a rate of 1 degree a day, (which would be a good method for the first heating cycle,) but it should also be turned off in the same manor to complete the first cycle, before day to day use
However, was it explained to you that every time you turn the heating on and off, it should ALWAYS be done gradually?
There’s no need for it to be done in 1 degree increments after the first commissioning, but it should be done over a couple of days.
And the reasoning behind it is that you’re trying to prevent ‘thermal shock’ within the adhesive.
Thermal shock can happen at anytime!
It’s not unique to the first operation.
So if for example the way you use it is to leave it at operating temp on the thermostat and just switch it on via the fuse spur, it means it’s going from cold to operating temp straight away, or vice versa, this can possibly lead to thermal shock within the adhesive, which in turn can cause partial or catastrophic failure.
Just something to be aware of.
When the tiles are lifted and before they are
re-instated, look at the adhesive beneath them, typically thermal shock presents as ‘crazy paving’ or multiple micro cracks in the adhesive.