Sandstorm says the following about fire resistance, in the context of environmental danger:
A character with a spell or effect granting fire resistance applies this resistance to lethal and non-lethal damage from hot temperatures.
Since someone with a minor fire resistance ring has a fire resistance of 10, and no temperature band is capable of doing more than 10 non-lethal damage, and non-lethal damage due to exposure to heat is the cause of heat stroke, this person is immune to heat stroke and corresponding fatigue as long as they keep their ring on. (Maybe the exact moment of things is not specified if they remove the ring, but I don't care at all)
It's fine, but the very next section of Sandstorm covers dehydration.
… the creature must pass a Constitutional test every hour (10 DC, +1 for each previous test) or take 1d6 points of non-fatal damage. In particularly hot environments (those above 90 ° F), the time a creature can spend without water before doing Constitution checks is reduced
Nothing in this document mentions the fire resistance having an impact on the time before the start of the dehydration checks. Nor does it say that the damage is "caused by high temperatures", which, according to the first quote, reduces fire resistance.
Does this mean that a half-red half-demon dragon, both immune to fire and resistant to fire 10, is dehydrated by hot temperatures in exactly the same way as Joe Average the human ?
Same problem with sunburn:
If a character is caught in the sun and completely unprotected … the character is slightly burned by the sun and takes 1 point of non-fatal damage. After an additional 3 hours of exposure, the character develops severe sunburn and immediately suffers 2d6 points of non-lethal damage and a penalty of -2 on saving saves damage or fatigue due to danger from heat until non-lethal damage is healed.
The damage here is also not "due to hot temperatures". And even if that were the case, the penalty on Fortitude backups is not even "damage". Does this mean that our scaly demon once again suffers from sunburn as quickly and as severely as Mr. Human, or at least from a reduced form of severe sunburn, despite its ability to resist temperatures of the literal surface of the sun?