IKEA’s ALPSTUGA has quickly become the go-to cheap CO2 monitor for smart home tinkerers, and plenty of owners are running into the same head-scratcher. I myself have been using a Airthings CO2 monitor at home for years, in my office, to make sure I have good air to stay sharp. But if you buy two or three, line them up on the same shelf, and each one reports a different CO2 level, sometimes hundreds of ppm apart. The natural assumption is that something’s broken.
For the most part, it isn’t. The ALPSTUGA is a €29 air quality sensor that shows CO2, PM2.5 particles, temperature and humidity on a small display, and doubles as a clock. It runs on Matter over Thread, so it slots into Apple Home, Google Home, Alexa, SmartThings or Home Assistant through a Thread border router, or through IKEA’s own DIRIGERA hub. The mismatched CO2 numbers come down to how the sensor inside works and how it calibrates itself, and the fix is mostly patience and fresh air. I’ll get to exactly what to do shortly, but the why comes first because it changes how you should be using the thing.
Why IKEA ALPSTUGA CO2 sensors show different readings
New ALPSTUGA units haven’t finished calibrating when you first plug them in, and even once they have, the CO2 sensor is a budget design that trades precision for price.
Inside is a SEN63C sensor module from Sensirion, a Swiss company with a solid reputation in this space. The catch is how it measures CO2. Proper CO2 monitors use NDIR (non-dispersive infrared) sensors, which shine infrared light through a sample of air and measure how much of it gets absorbed by CO2. That’s the gold standard, and it’s what you’ll find in the likes of the Aranet4 or AirGradient ONE.
The ALPSTUGA uses thermal conductivity instead. A tiny heater warms up and the sensor measures how quickly that heat escapes into the surrounding air. More CO2 changes that rate slightly, and the sensor works backwards from there. It’s cheap and tiny, which is how IKEA gets a CO2 monitor down to €29, but it can’t tell which gas caused the change, it leans heavily on temperature and humidity readings to correct itself, and it’s noisier as a result. Sensirion itself says the SEN63C’s CO2 measurement doesn’t meet the requirements of indoor air quality building standards like RESET and WELL. That tells you where it sits.
How ALPSTUGA calibration works
Sensors like this rely on automatic self-calibration. The sensor assumes that at some point it’ll be exposed to fresh outdoor air, which sits at roughly 400 to 420ppm, and uses its lowest readings as an anchor for everything else. Until it has that anchor, the numbers drift.
IKEA’s ALPSTUGA instructions say it can take up to 12 hours after plugging in before the CO2 reading is accurate. The same instructions warn that readings may be off at altitudes of 1,500 metres and above. Carrauntoohil tops out at just over 1,000 metres, so that one isn’t a worry here.
The bigger problem is a room that never gets properly aired out. A bedroom with the windows shut all winter never shows the sensor a true fresh-air level, so it anchors itself to the wrong number and every reading after that is skewed. That’s why units in different rooms, or units unboxed on different days, can disagree so wildly. Treat the 12 hours as a minimum; some units take a day or more to settle down.
How to calibrate your IKEA ALPSTUGA CO2 sensor
There’s no calibration button or setting, so the job is giving the sensor what it needs to calibrate itself.
Start by plugging it in and leaving it alone for at least 12 hours, ideally a full day. Don’t move it between sockets or unplug it during that time, because every restart kicks off the settling process again, and a freshly powered unit can briefly show nonsense readings (including 0ppm) while it gets going.
Next, give it fresh air. You’ll hear the advice to bring the sensor outside, and the principle is sound, but IKEA says the ALPSTUGA is for indoor use only, shouldn’t sit in direct sunlight and is rated for 0 to 40 degrees. An Irish back garden in October ticks none of those boxes. The better approach is opening the windows wide in the room it’s in for a good 20 to 30 minutes, ideally with a through draught, or placing it on an inside windowsill beside an open window, out of the sun. Do that once or twice over the first few days, then keep airing the room at least weekly so the sensor keeps finding its baseline.
If you want to compare units, only do it after they’ve all settled, and put them side by side for a few hours rather than glancing at them once.
If one sensor is still way off after a week of that, a factory reset forces a fresh start. Press and hold the system button for 10 seconds, or until the red light stops blinking. Bear in mind you’ll need to pair it with your smart home setup again afterwards.
One practical note on power. There’s no USB-C cable or plug in the box, so budget for IKEA’s €5 SMÅHAGEL power supply kit or use a decent USB-C charger you already own, and give each sensor its own spot where it won’t get unplugged every time someone needs to charge a phone.
How accurate is the IKEA ALPSTUGA CO2 sensor?
Accurate enough to tell you when a room is getting stuffy, not accurate enough to take any single number as gospel. It’s not a medical grade product.
Sensirion rates its standalone thermal conductivity CO2 sensor at plus or minus 100ppm plus 10% of the reading. Assuming the ALPSTUGA performs in the same ballpark, a reading of 800ppm could really be anywhere from 620 to 980ppm. Two perfectly healthy sensors could sit 360ppm apart and both be within spec. On top of that, the readings bounce around by dozens of ppm from minute to minute, even in an empty room with nothing changing.
So stop staring at the number on the display and look at the trend. This is where the ALPSTUGA is class. Pipe it into Home Assistant or your smart home app of choice and you’ll see the curve climb through the night in a bedroom with the door shut, then drop off a cliff when a window opens. The warning light kicks in at 1,000ppm, which is the level IKEA says indoor CO2 shouldn’t exceed, and that’s a perfectly useful nudge to crack a window.
Should you still buy one?
Yes, if you understand what you’re buying. For spotting stuffy rooms, building automations that switch on a fan or remind you to open a window, and getting a general feel for air quality around the house, nothing else comes close at this price. Having particle, temperature and humidity readings in the same box makes it even harder to argue with.
That said, if you need numbers you can rely on, for a home office you’re trying to keep within a specific range or for anyone in the house with respiratory concerns, save up for an NDIR-based monitor like the Aranet4 or AirGradient ONE. They cost several times as much, and that’s the price of precision.
The IKEA ALPSTUGA smart air quality sensor costs €29 from IKEA Ireland, with the USB-C cable and plug sold separately.
