IPhone – Does the Apple Watch (new serie 1) couple with iPhone 5 iOS 9

Apple Watch uses Bluetooth to communicate with iPhone. iPhone 5 is the first device that had Bluetooth LE support, which is used in cases where you want a low-energy-usage always-on connection, so it would follow that is the earliest model that can actually communicate with Apple Watch.

While Apple Watch can, in some circumstances, use Wi-Fi to communicate with iPhone, it has to be a network that iPhone was connected to while previously paired with Apple Watch over Bluetooth. Since devices prior to iPhone 5 likely cannot establish a Bluetooth connection to Apple Watch, no Wi-Fi network would ever be trusted by Apple Watch to use for communication with iPhone.

I have since solved the problem. Something was seemingly corrupted in the iPhone's Health database. Nothing could read or write data to the Health database, including step data (which should be automatically added from the Mwhatever chip) and manually entered data such as weight.

To solve the issue, I unpaired the phone and watch, then set up both the phone and watch as new. Setting up the phone from a backup did not solve the problem because the (hypothesized) corrupt database was (seemingly) included in the backup.

Since setting up the phone as new and re-pairing the watch, Health data works fine.