Aranet4 Home CO2 Monitor review
Aranet4 Home Wireless Indoor CO2 & Air Quality Monitor
By Tom Calder
The best CO2 instrument in this cluster by a clear margin, with a manufacturer-stated NDIR accuracy of ±(30 ppm + 3% of reading) and a display you can genuinely read across a room. It does one job properly and makes no pretence otherwise: no particulates, no VOCs, no radon and — this is the point readers most often get wrong — no carbon monoxide whatsoever. Bluetooth only, so there is no checking in on it while you are out.
- Best for
- Knowing, from the other side of the room and without touching your phone, when to open a window.
- Skip it if
- You want particulates or radon as well
as of 4 Nov 2026, 20:07 GMT · Details
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.co.uk at the time of purchase will apply to the purchase of this product.We earn a commission if you buy through the retailer links on this page. It doesn't change our verdict. How we make money →
Researched, not used. We haven't used this product. This review is built from the maker's specifications, owner reports and our own sizing.
What we like
- Manufacturer-stated CO2 accuracy of ±(30 ppm + 3% of reading) from an NDIR sensor
- Large-digit e-ink display with traffic-light status, readable from across a room
- 2x AA lasts years — up to around four on alkalines, up to seven on lithium cells
Worth knowing
- CO2, temperature, humidity and pressure only — no PM2.5, VOC, radon or carbon monoxide
- Bluetooth only with no Wi-Fi, so no remote checking or alerts when you are out of range
- That accuracy figure is a manufacturer specification, not an independently certified one
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Our review
A palm-sized e-ink CO2 meter from SAF Tehnika that runs for years on two AA cells and shows one number large enough to read from the sofa. What sets it apart from cheaper meters is the sensing: a true NDIR cell with a stated accuracy of ±(30 ppm + 3% of reading). It suits anyone who wants to know when a room has gone stale and when a window has fixed it.
What the numbers mean
NDIR means the sensor shines infrared light through a chamber and measures how much the CO2 absorbs, the method HSE recommends for portable monitors. The datasheet gives a 0 to 9,999 ppm range, accuracy specified up to 5,000 ppm at 15 to 35°C and 0 to 80% humidity, and a 100-second time constant, so it reacts within a couple of minutes. The default colours are green below 1,000 ppm, yellow to 1,400 and red above, with the screen inverting at red and an optional buzzer. HSE's benchmark is that a room sitting above 1,500 ppm while occupied is poorly ventilated, and that 1,000 ppm corresponds to about 10 litres of fresh air per second per person. Temperature is to ±0.3°C and humidity to ±3%; pressure is logged to the app only.
In a British home
HSE's placement advice is head height, away from windows, doors and vents, and more than 50 cm from anyone, because a breath on it reads high. The rooms worth watching are a closed bedroom overnight, a home office through an afternoon and a kitchen with a gas hob. SAF Tehnika treats fresh outdoor air as around 420 ppm; a room that stays well above that is not being aired, and the same stale air holds the moisture behind winter condensation and mould. It does not measure carbon monoxide or particulates, so a CO alarm remains separate equipment.
How it compares
The Airthings View Plus adds radon, PM2.5 and VOCs with Wi-Fi and a screen, but its CO2 cell is rated at ±50 ppm ±5% on the spec sheet and it needs six AA cells for a claimed two years. The Netatmo monitor measures CO2 over Wi-Fi with phone alerts, but has no screen and must stay plugged in. The Airthings Wave Plus pairs CO2 with radon and no display. For a glanceable number the Aranet4 is the one; for remote alerts it is not. The rest are in the comparison.
Living with it
You choose a measurement interval of 1, 2, 5 or 10 minutes; the default is 5. On alkaline cells the datasheet gives 4.0 years at 5 minutes with Bluetooth off and 2.8 with it on, 6.5 years at 10 minutes and about a year at 1 minute; lithium AAs stretch the 10-minute figure to 9.4 years with Bluetooth off. The device keeps 15 days of history at the 5-minute interval and 30 at 10; the Aranet Home app stores 90 days and exports CSV. It is Bluetooth only: there are no alerts to your phone when you are away from the house. It is a 71 mm square, 24 mm deep and 104 g, with no mains option. SAF Tehnika suggests calibrating yearly; auto mode wants about eight hours of fresh air a month, and manual calibration is done outdoors from the button or the app. Removing the batteries wipes the stored history, so sync first. The warranty is two years, extendable to three by registering.
Our verdict
Buy it if ventilation is the question, for a stuffy bedroom, a home office or a classroom-sized room, and you want a trustworthy number without a subscription or a mains socket. Look elsewhere if you want alerts while you are out, which needs Wi-Fi, or particles and radon in the same box, which the View Plus provides.
A researched review, built from the maker's specs, manual and UK listings; facts checked 21 Oct 2026. Sources: Aranet — Aranet4 Home product page (sensor, accuracy, app, warranty); SAF Tehnika — Aranet4 HOME datasheet v3.6 (ranges, accuracy, battery life, memory, calibration); SAF Tehnika — Aranet4 HOME/PRO user guide (thresholds, buzzer, intervals, calibration); HSE — using CO2 monitors to check ventilation (1,500 ppm benchmark, NDIR, placement).
Who shouldn't buy this
Don't buy it if you want particulates or radon as well — the Airthings View Plus covers all three at a broadly similar price.
Key specs
| Measures | CO2 (NDIR), temp, humidity, pressure. No PM2.5, VOC, radon or CO. |
| Display | Large-digit e-ink, readable across a room, traffic-light status |
| Connectivity | Bluetooth only — no Wi-Fi; Aranet Home app for history/export |
| Power | 2x AA — up to ~4 years alkaline, up to 7 years on lithium |
This is a Researched review: we haven't used this product. We check every specification against at least two independent sources, re-check the UK price on a schedule where we can track it automatically, and say plainly who it isn't for. Commissions never change a rating.
If this isn’t the one
Best for One device covering radon, particulates and CO2 in the same room, with a screen you can actually read.
as of 4 Nov 2026, 20:07 GMT · Details
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.co.uk at the time of purchase will apply to the purchase of this product.
Best for Radon and CO2 monitoring in a home that already has, or is willing to add, an Airthings hub.
as of 4 Nov 2026, 20:07 GMT · Details
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.co.uk at the time of purchase will apply to the purchase of this product.
Best for A cheap, permanently plugged-in CO2 reading in a room where noise levels are also worth knowing.
as of 4 Nov 2026, 20:07 GMT · Details
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.co.uk at the time of purchase will apply to the purchase of this product.Questions
How accurate is the Aranet4 Home?
SAF Tehnika's datasheet states ±(30 ppm + 3% of reading), specified between 0 and 5,000 ppm at 15 to 35°C and up to 80% humidity. That is the maker's own figure; above 5,000 ppm the stated error widens to 10% of reading.
How long do the Aranet4 Home batteries last?
On two alkaline AAs at the default 5-minute interval the datasheet gives 4.0 years with Bluetooth off and 2.8 with it on; the 10-minute interval reaches 6.5 years and lithium cells 9.4 with Bluetooth off. The 1-minute interval drops to about a year.
Does the Aranet4 Home need Wi-Fi?
No, and it has none. It talks to the Aranet Home app over Bluetooth only, so readings sync when your phone is nearby, and it cannot alert you when you are out.
Does the Aranet4 Home detect carbon monoxide?
No. CO2 and CO are different gases and this sensor only reads carbon dioxide. A boiler, gas hob, stove or open fire still needs a certified carbon monoxide alarm.
How do I calibrate the Aranet4 Home?
SAF Tehnika recommends once a year. Auto mode needs about eight hours a month in fresh air of around 420 ppm; manual calibration is done in fresh outdoor air, at least a metre from anyone, from the app, or on the device itself using the battery-compartment button or switches, depending on the hardware version.
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