Question on parameters options.

First off, I can confirm I've done the online training and to be honest a monkey could pass this. I found it still lacking in some key details.
Second my system is described here:-
http://www.automatedhome.co.uk/vbull...ference-so-far

My questions.

1. Fail Safe: is it good practice for it to be enabled, if so why is it disabled by default?
2. Hot water: what is the best setting for overrun? What is the best setting for Differential?

FYI, I am electrically qualified but not a plumber.

Thanks for any help offered.

Neil