Should every home have a carbon monoxide detector? Where should it be installed?

Yes, we have two carbon monoxide detectors, combined with smoke detectors. They work for smoke detection, or carbon monoxide detection. We have one on the wall coming out of the cellar, where our furnace is located, and the gas hot water heater, another on the wall going upstairs to the bedrooms. When I test them a lady's voice will say warning! warning! smoke is detected, then she will say warning! warning! carbon monoxide detected, and a very loud siren will wake us up. Luckily, it never went off for real, just testing them!  more