Installation of new 3-way switch located at the top of stairs on the quad switch bank that controls chandelier . We were called out today to look at smoke detectors that were chirping. Customer explained to us that he had just replaced all the units in his house and that one was chirping. We de-energized the smoke detector circuit, disconnected the units and opened their battery doors, and still heard the chirping. We determined that the chirping was caused by a plug-in carbon monoxide unit that was failing. While assessing the panel we also noted signs of overheating wires caused by multi wire branch circuits not on 2-pole breakers. We provided a quote to replace single pole breakers feeding multi wire branch circuits with 2-pole breakers. While on site we also replaced a three-way switch controlling the foyer light at the top of the stairs that was not working.