Homekit and Honeywell T5+ Thermostat

Subsequent session with Honeywell resulted in successful "add-accessory" function that's the equivalent of this set of steps.

Here's my understanding: Honeywell made an undocumented change in its product. The device code printed on the device and literature stopped working. The new device code is now displayed on the device screen.

I came this conclusion only after completely, and unnecessarily, wiping out my HomeKit and router settings, rebuilding them from scratch, and six hours on phone or chat with support agents.

After our phone call, Honeywell followed up with two suggested actions for its case 14128285:

1. Perform HomeKit reset to make Thermostat re-Enter into pairing mode. OR
2. Restart (power cycle) Thermostat to re-Enter into pairing mode.

I don't think HomeKit can be "reset". However, I did remove the Thermostat from its power source by removing and replacing from the wall mount. Upon replacing, the device appeared to retain its settings. (In a subsequent call, Apple suggested using the "reset" menu to restore the Thermostat to factory or out-of-the-box state, which I did do. )

This query converted to a phone conversation with Apple and Honeywell.

After research and discovery, Honeywell provides the following clue: "Devices unable to find correct IP address." This seems consistent with behavior during the Add Accessory workflow: HomeKit discovers the device, but does not make a connection.

With Honeywell, we conducted extensive troubleshooting, including DHCP settings on the router and WAC setting on the Thermostat.

Our conclusion after troubleshooting was that "Device unable to find correct IP address".

While reviewing the DHCP settings on the router using the Linksys dashboard into my Linksys Velop MX4200C, I noticed this message:

"Remove your router from the Apple Home app if you want separate Wi-Fi bands, or to change your security or network mode."

This clue reminded me of the one recent change made to the network's WLAN channels. Last week ago I installed a Linksys Range Extender using direct-connect wired connection from laptop. I encountered great difficulty in getting the Range Extender to connect as shown in the documentation. (I purchased the Range Extender off the shelf at a physical Staples store. After difficulty in getting it operational, I called Linksys, who informed that the Range Extender was out of warrantee and no longer supported. Staples is selling unsupported devices.)

The SSID logical network was carried on both the 2.4 GHz and 5.0 GHz bands, and the Linksys app suggested combining the two bands. I did combine the bands, which immediately resulted in the Range Extender operating as expected.

This is from the Honeywell Thermostat product manual:

"thermostat operates in the 2.4 GHz range. That may be why the thermostat does not see the network but the smartphone, which operates in both teh2.4 and 5.0 GHz range, sees the network. Most routers will broadcast two networks, one SSID on the 5.0 GHz radio and one SSID on teh 2.4 GHz radio."

The call with Honeywell did not resolve the problem. Honeywell demonstrated that the problem was off its device. I followed up with another call to Apple for the HomeKit senior staff, having been invited to escalate.

Apple's senior staffer was re-briefed on the problem, the device environment and the workflow. To prove Honeywell's claim that the Thermostat was operating properly, we reset the Thermostat to factory state. The problem was not resolved: HomeKit continued to display the device icon in its "Add Accessory" workflow, but continued to display the error message.

Conclusion from Apple was that both the device and HomeKit were operating normally, therefore the problem must be located in the network.

Apple suggested the following actions: disconnect all mesh routers and Range Extenders, connect the Thermostat to a single router.

Before I take that step, I will investigate whether dual-band SSID is the problem.

I noticed that when HomeKit is attempting to add the accessory, the Thermostat displays two eight-digit numeric codes. The codes appear sequentially, and quickly disappear.

Perhaps these are related to the two SSID radio bands.

The Linksys app shows the Thermostat connected on the 2.4GHz band.

HomeKit offers no mechanism to "remove" the router.

HomeKit Accessory Security is OFF. I will attempt to use this feature to restrict the Thermostat to HomeKit only.

Unplugged the Range Extender and Mesh Node. Cold rebooted the main Linksys Velop router.

Deleted the Router from HomeKit. Re-installed the Router to HomeKit, with accessory security.

Reviewed support page from Linksys about connecting IOT devices.

Problem not resolved.

Using the Linksys app, I renamed the SSID for the 5GHz band. Restarted Thermostat. Attempted to Add Accessory, but now the accessory icon does not appear in the Home app workflow.

Problem not resolved.

Reversed this action by bonding both bands to same SSID on Linksys and re-installing router on HomeKit.

Toggling Accessory Security: the Thermostat does not appear.

I have restored Accessory Security to OFF.

(Previously, I discovered a "Remove Router" action in the device window. This is how I disconnected Router from HomeKit.