Smart thermostats never actually look at your windows

Published Sep 24, 2026, 5:00 PM EDT Shivansh began his tech journey as an avid reader of tech publications, which inspired him to launch his career in video media at Sillycorns. He has led content across multiple tech YouTube channels and Instagram accounts, crafted go-to-market (GTM) strategies for Vivo, and consulted on digital growth. Coming full circle to his roots in written media, he now delivers deep-dive tech journalism. Outside of his professional work, Shivansh loves watching movies, riding bikes, and playing cricket. You crack the bedroom window on a cold morning, then get pulled into your day. An hour later, it's still open. If your smart thermostat has open window detection, you'd assume it has this covered. It only covers part of that hour. The feature never looks at the window. It reads its temperature sensor, sees the room cool too fast, and guesses. Most guesses come with a timer, so the heat switches back on while the window is still open. One of us at XDA has observed that energy spikes every time a window stays open. A cheap sensor on the frame knows the truth, but most thermostats never ask for it. Open window detection is a guess It doesn't watch the window You'd expect a feature called open window detection to detect a window. But actually, it detects a draft. Tado says its version looks for sudden changes in temperature or humidity. Netatmo's smart radiator valves watch for a drop in temperature. Nothing in either system ever touches the window. Netatmo is proud of those sensors. Its own guide says the valves measure temperature to within about a tenth of a degree Celsius. That's roughly two-tenths of a degree Fahrenheit, a change your skin would never notice. It's impressive, and it's still only a reading of the air. It works like a motion-sensor light. The light never sees you. It notices something warm moving and clicks on. The thermostat notices a sudden chill and switches the heat off. Spotting the chill is the easy half. Knowing when it ends is where the guess falls apart. Stand still, and the porch light clicks off Your thermostat has the same blind spot For Tado, that 15-minute timer in the screenshot above isn't laziness. It's the only way to guess. The thermostat's only clue is the change itself, and once the room is cold, nothing is changing anymore. A cold, steady room looks exactly like a room with the window shut. It's the porch light again. Stand still under one long enough, and it clicks off with you right there. A cold, steady room looks exactly like a room with the window shut. Netatmo's help page is the most candid one I found. Its valves need at least three minutes to spot an open window. The heat then stays off for about half an hour and comes back on, even if the window is still open. After that, detection switches off for another 30 minutes. Now picture a bedroom window left open for an hour. By Netatmo's rules, the heat is off for roughly the first half. For most of the second half, the radiator heats a room with an open window, and nothing is stopping it. Tado lets you extend its timer, but that just moves the problem. A sharper guess won't fix any of this. Something has to touch the window. Else, say hello to bigger electricity bills! Which smart thermostats use window sensors Ecobee does, and Google leaves it to you A contact sensor is the thing that touches it. It comes in two small pieces: a magnet on the window and a switch on the frame. When they pull apart, the sensor knows the window is open. When they meet again, it knows the window is shut. There's no guessing in either direction. Ecobee is the clearest example of a thermostat that uses one. Its Pause When Open feature waits until a window with an Ecobee sensor has been open for five minutes, then pauses the heat. It resumes once the window has been shut for 30 seconds. For that hour-long bedroom window, the heat stays off from minute five until you close it. Google's Nest thermostats don't come with anything like this. The newest Nest can even remind you to open your windows, though Google's help page doesn't say how it would know you closed them. Google Home's script editor can fill the gap by changing a thermostat's mode when a window sensor opens. Home Assistant goes the furthest. A community add-on called Better Thermostat lists "let your windows disable your heating" among its features. But think about it: getting the answer right every time has to cost more than a guess. And that "more" is about $8 here. The porch light still covers the whole yard A sensor only knows about one door The guess has one real advantage, and the porch light has it too. One motion sensor covers the whole front yard, and nobody wires a switch into every gate. A guessing thermostat watches every window in the room, with no extra hardware to mount. A contact sensor only knows about the window it's stuck to. Each sensor in the gallery above watches exactly one window. Three windows means three sensors and three batteries to replace. Then there's the monthly bill. Ecobee's plan table, in the screenshot above, shows an odd split. The free plan already alerts you when a window opens. Pausing the heat needs the Plus plan, at $5 a month. Your thermostat knows the window is open for free. Doing something about it costs extra. The free market world, as we know. Bosch's smart radiator thermostats are really smart here. They watch for a falling temperature on their own, and then shut themselves down as soon as a window opens. Setup How it knows Heat goes off Heat comes back What it costs Tado Guesses from a sudden temperature or humidity change Once it spots the change After the shut-off timer, 15 minutes by default AI Assist plan ($3.99 a month) to switch off on its own Netatmo Guesses from a sudden temperature change After at least 3 minutes About 30 minutes in, even if the window is still open Nothing extra Ecobee Checks with its door and window sensor After 5 minutes open 30 seconds after you close the window $60 sensor two-pack, plus $5 a month Home Assistant Checks with any contact sensor you add When your automation says so When you close the window About $8 to $12 per sensor, plus your Home Assistant setup Put a sensor on the window you actually open Tado's promise describes exactly what people want. Open a window, and the heat stays off until you close it. A guess can't keep that promise, because it never knows when you close the window. A magnet on the frame can. To see which kind you have, open your thermostat's help page and search for sudden or temperature drop. If either one shows up, your thermostat is guessing. You don't need a sensor on every window. Start with the one you open most, like the bedroom window you crack every morning. Let the guess watch the rest, the way a porch light watches the yard.