How much heat can an iPhone 7 handle?

Saunas are probably a bad idea.

Environmental Requirements

• Operating ambient temperature: 32° to 95° F (0° to 35° C)
• Nonoperating temperature: ‑4° to 113° F (‑20° to 45° C)
• Relative humidity: 5% to 95% noncondensing
• Operating altitude: tested up to 10,000 feet (3000 m)

https://support.apple.com/kb/SP743?locale=en_US

As others have correctly commented... If you have to ask this and cannot simply buy another iPhone to replace this sauna’d one—heat and humidity aren’t known for great compatibility with electronics, as you’ve already discovered—or to replace the damage or the dunking from your other adventures whether summiting the Matterhorn or parasailing in the warm tropical waters off St Barths, you don’t want to do this.

IdrisSeabright wrote:

• Saunas are probably a bad idea. .....

I totaly agree, but I would also avoid the adverb "probably", and use "for sure" :)

Giulio

ProustGiulio wrote:

I totaly agree, but I would also avoid the adverb "probably", and use "for sure" :)

I was engaging in intentional understatement. Trying to be polite here. ;-)

I see ;-)