You should be the one to judge based on your situation.
But IF I were you, then I will be already grateful that they let me stay for free.
I actually would be ashamed as well, I will definitely offered them to pay for the rent of their place, if it's not will be considered as 'rude' to them.
Anything beyond that is my own responsibility.
If the boiler broke down, since I'm in 'free staying' state, then the choice is either I'm ok with the broke down boiler or I will fix the boiler and bear all the cost myself. I see it as the least I can do to return the favor,
There's no right or wrong, it again go back to the judgement of each individual, I just share you what I will do.