Stooping to name calling and making claims of ignorance is not doing your argument any favour. It would behoove you not to make assumptions about what individual DIY'ers do or do not know about anything, and what they are capable of.
Of course it matters. You're implying that "we" are routinely touching things that we shouldn't be, so I've asked you to provide a specific example of what your concerns are rather than a hand wavy "oh its too scary and dangerous, don't touch it, only us experts know what we're doing" assertions.
You're trying to imply even removing the covers on boilers needs gas safe training - nonsense. My boiler has a user removable cover (without screws) that exposes all the gubbins with exposed gas pipes, gas control unit, mains wiring, the whole lot all just sitting there in plain view - you have to take the front cover off and expose all this just to adjust the thermostat or light it!
I'm sure there are boiler designs where you can't get at some of the non gas stuff (plumbing, temperature sensors, PCB etc) without disturbing the gas pipes, gas control unit etc - in that case fair do's. But that is on a boiler by boiler basis, and depends on what exactly you're trying to get at in the unit.
So turning off the power before working on a system is bad advice ? Ok.... I'll file that piece of advice away in my waste paper bin...
I misread the original posters post - I thought they were breaking into the plumbing side of the system without ensuring the system was properly off - and as pointed out, Evohome has a much broader and greyer definition of "Off" than most standard control systems, so my advice was to make sure the system was really turned off at the mains before cracking open any plumbing, least the Evohome surprise you by turning the boiler back on and pumping water everywhere... I think this is a perfectly legitimate piece of advice when most people don't realise what "Off" really means on an Evohome.
And it should also be turned off before working on any of the electrical wiring, including changing a thermistor - anyone changing something like that without turning the power off isn't as qualified as they think they are.
Of course "checks" will require the power to be on again after the work is done. Duh.... so I'm not even sure why you mention that.
Carry on...
