The story is always the same. You buy your first Zigbee sensors, pair them and everything works. Then, a few weeks later, a door sensor stops updating, a button no longer responds, the bedroom temperature sensor has shown the same value for three days. You change the batteries, re-pair them and they work again. Until the next time.
Before blaming cheap sensors (sometimes they are to blame, more on that later), check something almost no installation guide mentions: Zigbee and your home Wi-Fi use the same frequency band, 2.4 GHz. If the channels overlap, Wi-Fi, which is much stronger, drowns out the weak signals of battery-powered sensors.
How the channels overlap
2.4 GHz Wi-Fi uses channels numbered 1 to 13, each about 20 MHz wide. Zigbee uses channels 11 to 26, each only 2 MHz wide and 5 MHz apart. Laid on top of each other:
| Wi-Fi channel | Frequencies used (approx.) | Zigbee channels drowned out |
|---|---|---|
| 1 | 2402-2422 MHz | 11, 12, 13, 14 |
| 6 | 2427-2447 MHz | 16, 17, 18, 19 |
| 11 | 2452-2472 MHz | 21, 22, 23, 24 |
| 13 | 2462-2482 MHz | 23, 24, 25, 26 |
In the gaps between Wi-Fi channels 1, 6 and 11 you have Zigbee channels 15 (2425 MHz), 20 (2450 MHz) and 25 (2475 MHz). Not by chance, these are the defaults on many commercial bridges.
Step 1: find out which channels your Wi-Fi and your neighbours’ use
Your router shows its channel in the admin panel, under 2.4 GHz Wi-Fi. For the neighbours you need an app: on Android, the free and open-source WiFiAnalyzer draws a chart of every network and its channel. On Windows, from the command prompt, netsh wlan show networks mode=bssid lists visible networks with their channel.
Two router settings to check straight away:
- a fixed channel instead of “auto”: with automatic selection the router may move to the very channel that covers your Zigbee network;
- 20 MHz channel width on 2.4 GHz: the 40 MHz option doubles the space used and leaves almost no room. The real speed comes from 5 GHz anyway.
Step 2: pick the right pair
- Wi-Fi on channel 1: Zigbee on 20 or 25;
- Wi-Fi on channel 6: Zigbee on 11 or 25;
- Wi-Fi on channel 11: Zigbee on 15.
Be careful with channel 26: it’s free almost everywhere, but some devices support it poorly or transmit at reduced power there. And watch out for Wi-Fi channel 13, allowed in Europe: it covers Zigbee channels 25 and 26, exactly the ones that look the quietest.
One more detail that is becoming important with Matter: Thread also uses channels 11 to 26. If you have a Thread border router at home (many recent hubs and smart speakers are), make sure Zigbee and Thread aren’t on the same channel. For example, Zigbee on 25 and Thread on 15.
Step 3: change the Zigbee channel without disaster
Changing the channel of an already populated network isn’t painless, so it’s best done early. In Zigbee2MQTT the channel is set in the configuration, but changing it later usually means re-pairing every device. Home Assistant’s ZHA integration has an option to migrate the channel from its network settings: mains-powered devices usually follow, some battery devices need re-pairing. The Philips Hue app can also change the Zigbee channel from the bridge settings.
After the change, give it a few hours before drawing conclusions: the network needs time to rebuild its routes.
The other common cause: the USB stick plugged into the PC
If you use a Zigbee USB stick, be aware that USB 3.0 ports and USB 3.0 external drives create interference precisely in the 2.4 GHz band. A stick plugged straight into the back of a mini PC, next to an external SSD, is working in the worst possible conditions. The fix costs a few euros: a 50 cm to 1 m USB extension cable, a USB 2.0 port if available, and the stick placed away from routers, PCs and metal surfaces.
Finally: the mesh needs “repeaters”
In Zigbee, mains-powered devices such as smart plugs, bulbs and in-wall switches act as routers and repeat the signal for others. Battery sensors don’t. If you only have the coordinator and ten battery sensors, the distant ones are at the edge of their range. Adding two or three Zigbee smart plugs in strategic spots, halfway down the hallway or upstairs, fixes more problems than any battery change.
A last word on very cheap sensors: some models are known not to get along with Zigbee routers from other brands and drop off the network. If a single device keeps disconnecting after you’ve sorted out channels and mesh, search for its model on the Home Assistant or Zigbee2MQTT forums: chances are someone has already found out why.