Saswell SEA801 - Cheap Zigbee Thermostat that Works with Home Assistant

After trying many different smart home solutions, I finally found a tech-stack that I'm happy with:

  • Home Assistant
  • Zigbee2Mqtt
  • Mosquitto

One great thing about Zigbee is the large amount of affordable lamps, sockets and sensors. However, when I was planning to also control my radiators via Home Assistant I was struggling to find a good and affordable thermostat. The options that I found on Amazon where either overpriced or very badly rated.
After some research, I stumbled across the Saswell brand and found out that their thermostats are supported by zigbee2mqtt. Despite finding them really ugly, I decided to buy two for 25 Euros each on Aliexpress.

Installation

The thermostats are really easy to install. They replace the standard radiator thermostat.
They also came with lots of adapters so they should fit on almost every radiator. They require two AA batteries which are easy to insert. Unfortunately I can't remember whether or not batteries were included.

Pairing

There is not much to say about the pairing process. It was one of the simplest to pair Zigbee products that I ever bought.

Home Assistant Integration

They show as a standard HVAC device in Home Assistant and worked well out of the box.

Calibration

After some days of use, I realized that their calibration was way off. They were measuring the temperature too low which made them overheating the room. Fortunately, they expose the attribute local_temperature_calibration which allows to specify a temperature offset. After some experiments, I set this value to 2°C which seems to be a good value for me.

Operation Modes

The thermostat has three operation modes: heat, auto and off. And the modes DON'T work like you'd expect!

  • off: Is simple, the radiator stays cool, no problems here.
  • heat: I would expect that the radiator stays turned on when this mode is active, but that`s not the case! It basically does what I expected from the auto setting: It heats until the target temperature was reached and then turns off again.
  • auto: Now it gets weird. In auto setting, the thermostat follows a heating plan that can be programmed using the buttons. As this is a real pain in the A**, I recommend that you don't use this setting at all!

I only use the heat mode for normal operation and the off mode when I don't want to heat (the window is open or I'm on vacation).

Heating

The thermostat seems to regulate the temperature quite well. My only complaint is the relatively large hysteresis interval. One example: The thermostat is set to 21°. When the temperature falls below 20°, then the thermostat heats until the room reaches 23° and then turns off the heater. That's fine for larger rooms as the heat needs to spread but causes smaller rooms to be overheated. I therefore recommend to set the temperature lower for smaller rooms.

Batteries

Battery drain is ok. Dependent on the usage you will have to replace them every 2-4 months. I recommend to replace batteries early: When the battery runs out while the radiator is on, the radiator might stay on! I was able to extend the battery life drastically by upgrading to high quality Eneloop rechargeable batteries.

Overall I'm quite happy with the thermostats. They come with some design flaws but are a cheap alternative to more expensive thermostats.

TL;DR

Positive

  • Price
  • Easy paring
  • Good Zigbee2Mqtt and HomeAssistant Integration
  • Can be calibrated via Zigbee
  • Heating works fairly well

Negative

  • Heater might stay on when batteries run out
  • Confusing operation modes
  • Bad initial calibration
  • Overshoots while heating