The major new feature of the 3rd-gen Nest Thermostat was control of hot water systems. However, almost a year after its launch hot water control is still not exposed via the Cloud API. The hot water control functionality in the thermostat and iOS app is minimal, so there is a lot of scope for integrating with other devices and making the behaviour smarter.
Are there any plans to add hot water control to the API? If so, when?