I had an older First Alert CO detector, and the alarm was beeping. I changed the battery and this did not solve the problem. I figured the older unit had aged out and needed to be replaced, but I turned off my heating system and opened windows and ventilated the house, I also heated with my wood fireplace in 21 degree weather for 2 days, and immediately bought this CO400 unit. When this alarm went off again after turning on the heating system, I called my heating contractor. The entire furnace had to be replaced as the heat exchanger was full of holes (it looked like a swiss cheese) and was allowing significant CO exhaust fumes into my hot air vents. I could have been killed. I am very happy I put in this new unit immediately.
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