If you're running iOS 8, you control your HomeKit accessories with Siri; outside your home, Siri talks to your Apple TV.

Your third-generation Apple TV may be getting a bit long in the tooth hardware-wise, but it stil has some skin in the new software features game: Its 7.0 update included support for extending communication with your HomeKit accessories outside your local Wi-Fi network. This is primarily a feature aimed at users still on iOS 8; iOS 9 has this support built-in, no Apple TV required. Here's everything you need to know.

So what does the Apple TV do for HomeKit, anyway?

By default, HomeKit in iOS 8 is limited to your personal Wi-Fi network: All handshaking and Siri commanding happens within those boundaries. But your Apple TV — in combination with your Apple ID — gives your iPhone or iPad a way to securely talk to your HomeKit devices even when you're outside the house.

iOS 8? What about iOS 9 users?

Starting with Apple's iOS 9 update, your iPhone can securely chat with your HomeKit bridges and accessories even while out of the house. It can do so because of the new HomeKit Accessory Protocol (HAP), which uses secure networking over iCloud to talk to your connected HomeKit accessories. HAP doesn't work with prior versions of Apple's mobile operating system; as such, if you're still using iOS 8, you'll still need to use and set up an Apple TV for controlling your house outside your local Wi-Fi network.

How does the Apple TV connect to HomeKit in iOS 8?

It's an under-the-hood mixture of iCloud Keychain and HomeKit frameworks. Say you're on LTE: You press the Home button to pull up Siri and give the command "Turn on the lights." Without an Apple TV or HAP support, that command goes nowhere — Siri replies with "I can't do that."

With an Apple TV on your home Wi-Fi network, however, that command travels over your cellular network back to your set top box, where it handshakes with your Apple ID. "This is the owner of this HomeKit network," Apple TV then says to your installed HomeKit bridge. "Here's a command from them." The HomeKit bridge thusly executes the command, and the lights in your home pop on.

Now, if you're doing this on LTE sitting in front of your lights, you're going to see a slight delay between giving Siri the command and the command actually happening: This is because of that extra handshake that happens between the Apple TV and the HomeKit bridge; normally, when you're on Wi-Fi, your commands go directly to the bridge, then to your compatible accessories.

But I'm going to hazard a guess that 90 percent of commands given outside your Wi-Fi network won't be to turn on a light right in front of you; they're going to be to unlock your door, turn up the thermostat, or turn on the lights from your car. By the time you get to your door, the command should have been executed with time to spare.

There is one other limitation to the Apple TV's relay: It currently only supports commands from the owner of the HomeKit home. This is because the Apple TV can only be logged in to one iCloud/Apple ID at a time.

For instance, say you created a HomeKit network under john@icloud.com and also added sarah@icloud.com as an additional user. You're also logged in as john@icloud.com on your Apple TV. Sarah can use Siri to activate HomeKit when she's on your Wi-Fi network, but once she leaves the house, she'll be unable to issue commands. Because your Apple TV is only logged in to one account, it can only authenticate commands from that one account.

So it's just an extender? Why can't my Apple TV be a HomeKit bridge and talk to my non-HomeKit accessories like my Nest?

A HomeKit extender does not a HomeKit bridge make: Just because your Apple TV can talk to HomeKit doesn't mean it can link your Hue bulbs and Nest thermostat into that framework.

First of all, HomeKit hubs have to be built with certain bluetooth and Wi-Fi specifications, and the 2012 Apple TV doesn't have the hardware for such a venture. On top of that, for the box to be a HomeKit bridge, it would need to develop secure handshakes and APIs for every third-party accessory out there — no easy feat.

I'm not saying that there's never going to be a future Apple TV that also acts as a HomeKit bridge, but such a thing is nigh-impossible for your current Apple TV — and the new fourth-generation Apple TV has made no mention of having HomeKit bridge support. I'm afraid we're stuck with too many bridges for the time being.

Here's how to set up your Apple TV with HomeKit

If you're still on iOS 8 and have HomeKit-enabled accessories in your house, here's how you go about enabling your Apple TV for HomeKit.

  1. Make sure you've set up rooms and zones for your accessories with Siri.
  2. Turn on your Apple TV.
  3. Open the Settings app.

  4. Click on iCloud.
  5. If you're currently logged into iCloud, log out and log back in; otherwise, log in with your iCloud ID. If you're already logged in with iTunes, it may prompt you to log in with the same ID.

Give it a few minutes; after that, you should be able to use HomeKit commands with Siri even when you're not on your local Wi-Fi network.

If you can't get it to work after ten minutes or so, try restarting your Apple TV and repeating the above steps. It took me a couple of logouts/logins before my Apple TV successfully started routing Siri HomeKit commands.

Any other questions?

Anything else about Apple TV and HomeKit you're confused about? Holler in the comments.