Carbon monoxide alarm going off — Barcelona, Catalonia

A carbon monoxide (CO) alarm going off is a life-safety emergency that should never be ignored. Carbon monoxide is an odorless, colorless gas produced by incomplete combustion in furnaces, water heaters, gas stoves, fireplaces, and attached-garage vehicles. At low concentrations it causes headaches, dizziness, and nausea; at high levels it is fatal within minutes. If your CO detector sounds, evacuate everyone — including pets — immediately, call 911 from outside, and do not re-enter until the fire department clears the home. Common sources include a cracked heat exchanger in the furnace, a blocked or disconnected flue pipe, a malfunctioning gas water heater, or backdrafting caused by negative house pressure. After the immediate danger is resolved, an HVAC technician should perform a combustion safety inspection of every gas appliance. A heat-exchanger inspection runs $80–$150; if the exchanger is cracked, replacement costs $600–$1,500 or a full furnace replacement may be recommended ($3,000–$7,000). Flue pipe repair or reconnection typically costs $150–$400. Annual furnace maintenance ($80–$150 per visit) is the best prevention — the technician checks for CO at every burner and verifies proper venting. Install CO detectors on every level and near sleeping areas; replace units every 5–7 years.