Russ presented the following explanation :
Having just spent the weekend replacing the hot water cylinder,
immersion heater and thermostat - it was all working lovely last night
and this morning, but I've just come in from work to discover the
thermostat has tripped and I've had to reset it.

Why does this trip exactly? The thermostat is set to it's maximum
setting on the dial, so surely it's impossible to set the thermostat
too high?

Oh yes it is!

If the set point of the regular stat is higher than the safety stat,
then the safety will trip. That is its designed purpose. Set the
regular one a little lower until it doesn't trip.

Health and safety concerns over the hot water being too hot mean that
HW stat temperatures have been lowered anyway, so don't expect your
water to be as hot as it used to be.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk