No problem! I do actually have an idea that just popped into my head.
If you know the game Random Access Mayhem, one of the playable characters uses a flamethrower, and if you use it too long you start to overheat and eventually go kaboom.
Perhaps something like that could work? Holding down the flamethrower for too long causing heat to build up in your suit, making you rapidly burn oxygen till you cool down again? (Probably no need to include the kaboom part tho :p)
Gives an extra avenue for upgrades, based on both managing and maybe even using the heat buildup. But that's just my own personal idea.