HVAC · Lesson 11 of 13Beginner5 min read

Troubleshooting by Symptom

Plenty of no-heat and no-cooling calls end with a simple fix: a thermostat setting, a tripped breaker or a clogged filter. Check those safely, learn which signs mean stop, and you’ll also be able to tell a technician exactly what you saw.

Think it through in order

Safety first. A sounding CO alarm or a smell of gas is an emergency, not a repair (see below). So is smoke or a burning electrical smell.
Settings. Is the thermostat in the right mode (Heat or Cool), set past the room temperature, and powered? Is the fan on Auto or On?
Power. Is a breaker tripped? Many furnaces and air handlers also have a service switch nearby, and outdoor units have a disconnect. Reset a tripped breaker once; if it trips again, leave it off and call (Reset a Tripped Circuit Breaker).
Airflow. Is the filter clean? Are registers and return grilles open and clear? Is the outdoor unit free of leaves, snow and ice?
Water and ice. Water around the indoor unit or ice on the coil or lines points to drainage or airflow problems, or to low refrigerant.
Equipment. If all of that checks out, the problem is inside the equipment. Note what you saw and heard, any blinking fault light you can see, and when it started. That’s what a technician needs.

Work from the outside in: the safe, simple checks first, and the equipment last.

The finder covers common problems in typical home systems. Your equipment’s manual explains its own settings and any fault codes.

StopEmergencies

If the CO alarm sounds: immediately move everyone outside to fresh air, call 911 or the fire department, and don’t go back in until emergency responders say it’s safe. Don’t try to find the source. Don’t use a malfunctioning appliance again until it has been serviced (U.S. Consumer Product Safety Commission).

If you smell gas (gas companies add an odorant that smells like rotten eggs so leaks can be noticed): treat it as an emergency and follow your gas utility’s emergency instructions right away. The utility and the fire department handle suspected gas leaks.

If you see smoke or flames, get out and call 911.

Notes for the technician

Good notes save time and money. Write down the date and time, what the thermostat said (mode, setting, room temperature), what ran and what didn’t (indoor blower, outdoor fan, burners), any noises or smells, whether a breaker had tripped, and any blinking light you could see without opening a panel. Many control boards blink a status light in a pattern; the furnace’s manual explains it. If you have your equipment’s model and serial numbers from the nameplate, have them handy.

Check yourself

Tap an answer to check it, or open “Show the answer.”

The air conditioner isn’t cooling. Which should you check first?
Show the answer
The thermostat mode and setting, the breaker, and the filter. Settings, power and airflow are safe owner checks; refrigerant and electrical tests are for technicians.
A breaker for the outdoor unit trips again right after you reset it. What now?
Show the answer
Leave it off and call a qualified technician or electrician. A breaker that trips again is a warning. Leave it off and call.
Your CO alarm sounds. What comes first?
Show the answer
Move everyone outside to fresh air, then call 911 or the fire department. Outside first, then call. Don’t go back in until responders say it’s safe.

Go deeper

ENERGY STAR: Maintenance Checklist ENERGY STAR’s maintenance checklist, to prevent many of these problems.
U.S. Consumer Product Safety Commission: Carbon Monoxide Questions and Answers CPSC’s carbon monoxide questions and answers.
Being an Adult 101: Reset a Tripped Circuit Breaker Reset a tripped breaker safely.
Being an Adult 101: Change a Furnace Air Filter Change a furnace air filter.

Sources

The finder also draws on these sources:

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.