They can be a pain when fitted in thermal stores which need higher temps to work properly (the DHW is either being heated via the primary water with the coil & blending valve or plate heat exchanger).
So you turn em up to 75, and get a call the next day as the high limit stat has tripped and the cylinders cold.
I still come across a few stats with no high limit stat, and used to squirel the working ones away for thermal stores, but now I either fit manufacturers factory set ones, or don't set em so high.
The legislation is good though. Who wants that on their hands. Only last year I got called to a cylinder full of boiling water, the open vent was pouring over with steaming water and guess where the tank was....right over the customers bed.
The tank was allready spongy and starting to warp. It was about 5pm, it could have been bad.
