Multiple iPhone Duo reviews covered the device's thermal properties, and reviewer opinions varied. Some found that it was just warm, while others thought it was running too hot at times.
Joanna Stern said the back of the device got warm when using FaceTime and taking photos. She had to end a FaceTime call during testing because it got too hot. "Yeah, I'm gonna burn my hand off," she said. "This thing is getting really warm."
Apple told her the warmth was a sign of the iPhone Duo's thermal design moving heat away from the chip to keep performance up. "But to me, it just feels like the phone is really warm," she said.
The iPhone Duo has the same custom vapor chamber setup and A20 Pro chip as the iPhone 18 Pro, but the vapor chamber is thinner. Apple added a flexible graphite pipe in the hinge to transfer heat between the two halves, giving more surface area for heat dissipation.
The Verge said the iPhone Duo heats up when using the camera for a long time, especially if both displays are on for features like Duo Preview. "You can just feel the phone getting significantly hotter when this happens," reads the review. "The phone gets hot enough to start noticeably lagging after a while." The site also said it gets hot when running third-party apps, particularly video apps and apps from Google.
PetaPixel's review unit got warm when gaming, photo editing, or doing anything that stressed the processor, with the heat felt on the right side of the unfolded device.
Mrwhosetheboss did a 20-minute stress test with the iPhone Duo and iPhone 18 Pro Max. He found the iPhone Duo trapped heat inside when it was closed, and iPhone Duo performance was below the iPhone 18 Pro Max. When open, the iPhone Duo has a large high-resolution display that also affects performance. Mrwhosetheboss focused more on performance than heat, and said the difference between the iPhone Duo and iPhone 18 Pro Max isn't "massively" noticeable in day-to-day use.
HTX Studio did a 30-minute gaming test with the iPhone Duo and iPhone 18 Pro models. The iPhone Duo was 46.5°C, while the iPhone 18 Pro and Pro Max were at 43.8°C and 43.2°C, respectively. The iPhone Duo had a 20 percent lower frame rate than the iPhone 18 Pro models and it couldn't maintain 300 nits brightness, dropping to 185 nits.
9to5Mac said the iPhone Duo "runs a bit hotter than the iPhone 18 Pro, but not in a way that impacts performance." Tom's Guide did a benchmark heat test and found that the iPhone Duo was able to spread heat evenly across the body of the device, but it was a few degrees hotter than the Galaxy Z Fold8 the site compared it to. The iPhone Duo scored 75.9 percent stability score on the 3DMark stress test, compared to 79 percent for the iPhone 18 Pro Max and 72 percent for the Z Fold8.
Gizmodo didn't find heat to be a problem, saying the vapor chamber "does a solid job cooling it down to prevent lag or slowdown" when apps use "all the CPU and GPU cores."
The iPhone Duo launches this Friday, so we don't have long to wait to see if heat is going to be a problem for the everyday user.
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