A: I think you may have a faulty Thermal Cut Out
A faulty TCO will have no continuity across the two terminals (The black thing on top of the heating tank)
Therefore a good TCO will have power at both connections and a faulty TCO will have power at only one.
(The function of the TCO is, if the water over heats it cuts the power off to the elements until the water cools, this can take up to 15mins)
After fitting a new TCO
When fitting a new TCO listen to the shower, after it has completed its first on/off cycle, if when it is switched off you hear it continuing to heat (Noise like a kettle starting to boil) this will mean that there is a switch or a relay that is stuck in the on position, this is keeping an element energised and this is probably what caused the original TCO to fail.
If this is the case replace the faulty switch or relay and make sure that you switch the shower off at the isolating switch when the shower is not in use because this may cause the new TCO to fail.
The other possibility is it may be a faulty element in the heating tank (there are two, three in most 10kw showers) you can check them with a multi meter you should have a reading of around 12>18 ohms.
If you have power to the elements and no heat it's probably the elements, no power to the elements then it's probably a micro switch.
If you don’t have a multimeter we sell them at £10 each with a guide on how to use them for checking and testing the various parts of your shower.
Apart from helping to fix your shower, they are really useful for checking you have a power supply to appliances, for checking fuses, letting you know much power you have left in batteries
(so you know if they are dud or not) and lots of other jobs, in short they are well worth the £10.
http://www.showerdoc.com/installation/electrical-products/SKYTRO-600-005-Digital-Multi-Tester Answered 701 days ago | The Doc