Upon recommendation for ability, and the fact our current CO detector is pushing seven during renovations. It attaches to the ceiling. Figured replacement early based on quality of function for new device and huge savings to buy two as singles if worth it. As vintage ceiling tile are being installed and this device is flexible as to where it sets up. Can be plugged in with the back up battery or wall mounted with battery alone.Although it’s a bit large has a keen sense of detection. There’s no price or device size that matters with safety. But if ya can score a discount kudos.No matter what detector you buy make sure it has a digital read on how much CO2 is invading when it does.To know what’s safe … 0 ? all seriousness, under 30. The booklet explains the levels. How the device will function in a warning situation. Symptoms o look out for. How long it’ll take to read CO2 depending on how severe it is.It’s recommended to take one on trips when staying at hotels and this detector is easy to set up anywhere. The two pronged action is helpful since this vintage house has a mix of two and three. But for a trip the 9 volt battery is good to go.For RVers this is a must have. Even in a van camping. Better to be safe than sorry.Tip: Write the date of install in permanent marker on the face. This way no need to remember.Tip 2: Same with smoke and fire detectors. For those , what I do is write the yr and month the battery is changed every time. Thin permanent marker should leave plenty of room.Unless hardwired and in ten you just emplacements it. Still, add the date of install. Same with fire extinguishers. Doesn’t hurt to have the fire department inspect it periodically.I know people who’ve lost their hem to fire. Not fun. I know people, like myself, a CO detector got us outta the house.What I like about this detector is the directions have great specifics for set up to avoid false positives and a freak out. Yeah, I did. It was scary. Thank goodness the two times it was for real we were safe.Though at that time we didn’t have a level readout. That’s what makes this gem such a peace.Not sure how long the sale is for. But the singles are a great price now. Hope you score a savings, too.Hope I never hear these defectors go off. But I feel save knowing new ones are in place every ten or less years, if your renovating, where to restock ?