I cannot tolerate summer heat in my city Chennai and I have to spend the daytimes in my airconditioned bedroom. I also get an immediate boost in energy when I take bath. I think it involves both cooling of body and removal of salt deposit from sweat on my skin. I get energy boost during winter when I take a hot bath as it is always above 90% humidity in my city throughout the year.
This also correlates to how I (and many people, even if they don't have ME/CFS) feel throughout the day. Each day around 10 am, barometric pressure is at its highest, and I feel more energetic. Conversely, it is at its lowest around 4pm, which is when I feel most sluggish.
Similarly, I feel better if I have a fan blowing on me. Not sure if this is because of all the oxygen the fan is sending my way, the air pressure that the fan creates, or both.
And I would argue the same phenomenon occurs when we take baths or showers - they cause a dramatic (although temporary) increase in blood flow (and therefore oxygen delivery), and we feel better.