This rule's real problem is that it favors constitution as an ability score way too much.
Con is already a really nice stat. There is no reason to make it even more important.
I'd rather make a rule that you only recover 3 surges per rest or something. That is it. Maybe add a feat that lets you recover more, that has a con prerequisite.
Absolutely.Of course, we still need rules on how to tailor the system to more or less combats between long rests.
Reduncing the rate of surge recovery would, to me, serve a different purpose than increasing "grittiness"- it would help solve the "1st to 30th level in a month" problem. Which, imho, is a huge potential issue. I really believe that a 30th level character ought to have been adventuring for (game) years, not months.
YMMV, of course.
What about getting:
2 surges if you get get 4hp pr level
3 surges if you get get 5hp pr level
4 surges if you get get 6hp pr level
5 surges if you get get 7hp pr level
+2 surges for resting indoors
+1 surge if you have the durable feat
I am NOT a fan of this getting tied to the con score.
As it was previously stated, limiting surges recovered, limits the amount of combats per day.
How that improves the grittines is not clear to me.
For starters, it the amount of surges you have is a problem for what you want to do, then you rest more until you recover all your surges.
So instead of taking one rest, the party will rest two days.
Ok, I see.
But how can't you accomplish this with the normal rules but with these houserules you can?
I've made several adventures where there where more encounters than the party would be able to handle. So they had to choose which encounters to have, the consequences of doing each encounter, etc.
I can see how recovering less surges you can span a conflict between the party and enemies across long rests/days instead of solving them all in one day.
But that doesn't seem to be the core of gritty.