If you're really so concerned about this problem,
@Mistwell, then there's a relatively simple fix:
Let a creature wearing heavy armor "vent" heat, at the cost of being Incapacitated until the start of their next turn (possibly having them pass a save to do so?) Should they fail the save, they can then choose whether to take the damage, or take one level of exhaustion; any exhaustion caused by this spell can be removed by recuperating during a short or long rest and using up one use of a Healer's Kit (two uses if no one has Healer's Kit proficiency.) TL;DR: You can spend your turn cooling off (and doing nothing else) if you pass a save; if you fail it, you can take the damage or take some exhaustion, which can be removed by rubbing on some aloe vera.
That way, the spell still has a sting, but people with heavy armor have a chance to do
something about it. It becomes very effective crowd control against armored targets,
or damage if . If you still feel that's "unfair" to creatures in metal armor, adding a range or sight requirement