The checks
- The thermostat is on, set to the right mode and temperature, and has working batteries or power
- The air filter is clean
- The breakers for the indoor and outdoor units are on — reset a tripped one once, and if it trips again, leave it off
- The switch at the furnace or air handler, which often looks like a light switch, is on
- Supply vents and return grilles are open and unblocked
- Outdoor vent pipes on high-efficiency furnaces are clear of snow and debris
Stop and get help instead
- Any smell of gas: leave the building and call the gas utility or 911 from outside
- A carbon monoxide alarm: get everyone out and call 911
- A burning or electrical smell: switch the system off at the thermostat, and at its breaker if it is safe to reach
- Water pooling near the equipment or outlets: keep clear of it
What helps when you call
What the system is doing, when it started, any error lights or codes on the thermostat or equipment, what you already checked, and the make and age of the system if you know them. It gives the technician a head start.