Reprinted from TidBITS by permission; reuse governed by Creative Commons license BY-NC-ND 3.0. TidBITS has offered years of thoughtful commentary on Apple and Internet topics. For free email subscriptions and access to the entire TidBITS archive, visit http://www.tidbits.com/ Reconnect a HomeKit Accessory over a 2.4 GHz Wi-Fi Network Glenn Fleishman Nearly two years ago, my family swapped out a natural gas furnace for a two-stage heat pump that provides winter heating and summer cooling. As part of this changeover, we needed a new thermostat. I wanted something with HomeKit support, so I chose a [1]Sensi Touch from Copeland that came with a significant rebate from our local utility. I remember having a few false starts configuring it for HomeKit, but I got it working fairly quickly. I added a schedule for day and night and automations for when we all leave the house or return, and it has worked well for the last 20 months. Recently, however, when I woke up, the house was cold. The thermostat and thermometer both read 62°F, our default overnight setting'well below our daytime setting. Using the manual control, I bumped it to our regular daytime level, 67°F, and the heat pump kicked in. Checking my iPhone, I found the Sensi Touch wasn't communicating with HomeKit; using the device's panel, I discovered it couldn't connect to Wi-Fi. (Perhaps some gremlins caused a router weirdness overnight? I'll never know.) A quick fix, I thought! I turned its Wi-Fi off and on'no luck. I rebooted some of my routers. No change. I tried to re-pair the thermostat: sorry, it's already added to HomeKit. The Home app claimed the deleted device was still present even after I deleted the thermostat entry and tried to add it back. Maybe it was Apple TV that served as our HomeKit hub? I restarted the Apple TV and, for good measure, powered down the two iPads on our network even though they aren't HomeKit hubs under the new Home architecture. Nothing. With advice from friend-of-TidBITS Geoff Duncan, I logged out and back in from my account on our Apple TV, thinking that might clear some HomeKit corruption. Nope. Finally, after taking pictures of all the deep settings screens for our HVAC configuration on the Sensi Touch, I did a factory restart. (It retained all the heat pump settings, which was great!) Now, I was able to try to add the Sensi Touch to the Home app, but the process never completed'the Sensi Touch kept saying it couldn't connect to the network. This time, I noticed a detail I hadn't paid attention to in the Sensi app when performing setup, as you can use the Sensi Touch with Copeland's free 'cloud' (its term) for scheduling, usage, and other details, or opt it into HomeKit. The app noted that the Sensi Touch requires a 2.4 GHz Wi-Fi network, and one's iPhone should be connected to 2.4 GHz. I hadn't given this any thought before, as all the routers on my network are dual-band (2.4 GHz and 5 GHz), and all have the same network name and password to allow roaming around the house and ensure that all the devices can 'see' one another, regardless of band. Therein lay the problem: apparently, my iPhone consistently used a faster 5 GHz network. Because my networks were named the same, I couldn't force my iPhone to use 2.4 GHz. Here's what I did: 1. On my primary router, a NetGear model (which is also the closest one to the thermostat), I set up a guest network using the 2.4 GHz band. Many routers let you set up 2.4 GHz and 5 GHz guest networks with separate names, security, and other details. 2. I gave the 2.4 GHz guest network a unique name and password. 3. I enabled a setting on the router called 'Allow guests to see each other and access my local network.' Most routers have a similar setting. 4. On my iPhone, I connected to the 2.4 GHz guest network and then began the HomeKit pairing process. Success! Note that Step 3 is critical. When I omitted it on my first pass, the Sensi Touch connected to its proprietary cloud but not HomeKit. When I mentioned this on Mastodon, someone reminded me that guest networks typically separate the Bonjour (mDNS) and other network broadcast traffic from your main network for security. Enabling the feature in Step 3 allowed mDNS broadcasts, which in turn provided HomeKit discovery. Since this happened, I did more research and discovered that most HomeKit devices only include a 2.4 GHz radio. This makes sense to keep costs and power usage low while simplifying configuration. Smart devices with HomeKit support usually need to send and receive only small amounts of data'the Sensi Touch is probably sending only a few megabytes per day, if that. In most cases, 2.4 GHz networks provide better range than 5 GHz networks. (That's because obstructions more readily block 5 GHz; devices with higher bandwidth requirements generally prefer 5 GHz because it offers substantially more throughput than 2.4 GHz over short distances.) There are 5 GHz outliers, like HomeKit-capable security cameras, which can benefit from higher throughput connection. But smart thermostats, outlets, and light bulbs typically use only 2.4 GHz. The ease of Wi-Fi and HomeKit made me overlook the obvious solution! I consider myself something of a Wi-Fi expert, yet I turned a 5-minute problem into one that took over an hour because I'd started at the wrong end of the problem. With luck, this article will help others who find themselves in a similar situation. References Visible links 1. https://sensi.copeland.com/en-us/products/touch-thermostat Hidden links: 2. https://tidbits.com/wp/../uploads/2024/02/Sensi-Touch.png 3. https://tidbits.com/wp/../uploads/2024/02/NetGear-2.4-guest-network.png .