Against my better judgment, I purchased two Honeywell RTH8580WF WiFi thermostats several years ago. I did this only after verifying that the thermostats work as expected, even when not connected to the network. One can program schedules into them and they control the heating system, even when they can’t connect to the network. This gave me the confidence to try them. In the worst case, they become annoying-to-program-but-at-least-programmable thermostats.

Do not read any further. Read the next article for the real reason why these thermostats stopped working, and then come back here for a laugh. No, really.

The devices worked just fine for several years. I could control them over the internet. I could program them relatively easily. I could even turn the heat on just before embarking on the 1-hour-long trip from the airport to home after a winter trip. I could turn the heat on from bed if I happened to wake up a little early (or a little late). I decided that I like the devices.

And then they fell off the network and wouldn’t climb back on. For days. Ugh.

I didn’t think I’d changed anything—at least not recently. Yes, we changed routers, but several weeks prior. Suddenly, and with no recourse to useful diagnostic messages, neither thermostat could get an IP address from the router’s DHCP service. I contacted Honeywell support and, as I expected, they didn’t tell me anything beyond what I’d already read in the user manual. Turn WiFi off, then on. Enter WiFi setup mode. Nothing helped.

And then they started working again… for a few days. And then they stopped again. Oy. This illustrates exactly why we shouldn’t put everything on the network. This illustrates in particular why we should never put anything on the network that doesn’t also work offline!

And then it hit me: if these devices can’t get IP address reliably, then let’s cut some steps out. Specifically, let’s just ask the router to give them the same IP address every time. Maybe that would help.

I do not know enough about networks to call myself anything more than an Advanced Beginner, so if you know more and can suggest something, then please contact me and tell me what I should have done. Thanks.

Fortunately, the thermostats display their MAC addresses on a little sticker, so I logged on to the router and asked it to reserve fixed IP addresses for the two WiFi thermostats using their MAC addresses. I had to change the first three bytes of the MAC addresses to 02:0f:b5, because of virtual MAC addresses, so I even learned something in the process. Something I didn’t care to learn, but that’s sometimes how it goes.

Now—for the moment, at least—the thermostats have IP addresses and I feel happy about that. I hope it stays that way.

If you have these thermostats, then I recommend that you consider configuring your router to reserve fixed IP addresses for them, instead of negotiating new ones every time. This might save you some headache, especially in the winter, when you actually need these things to work.