Signs your furnace, AC or heat pump needs a professional

Updated November 10, 2026 · KeystoneProGuide editorial team

Which signs mean you should act right away?

Two signs are emergencies, not service calls: a carbon monoxide alarm, or symptoms that could be carbon monoxide poisoning, and the smell of gas. Leave the house first in both cases and call from outside.

A carbon monoxide alarm. The U.S. Consumer Product Safety Commission (CPSC) says never to ignore one and not to try to find the source. Move outside to fresh air, call 911 or the fire department, count heads, and stay out until responders say it is safe. If the cause turns out to be a malfunctioning appliance, the CPSC says do not run it again until trained personnel have serviced it.

Symptoms with no alarm. The CPSC lists headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion, and says to get outside and then call 911 if you suspect carbon monoxide. Both the CPSC and the Centers for Disease Control and Prevention (CDC) describe the symptoms as flu-like. The CPSC adds that slowly developing carbon monoxide problems at home are sometimes mistaken for flu by families and doctors, and the U.S. Environmental Protection Agency notes symptoms that clear up after you leave home. The CDC warns that people who are asleep can die before they have symptoms, which is why it recommends alarms near every sleeping area.

A gas smell. In Philadelphia, Philadelphia Gas Works says to leave immediately, call its emergency line from a safe place, and do not use phones or other electrical devices, open flames or appliance controls on the way out. Do not try to find the leak or shut off valves; wait for the utility to say it is safe. Outside Philadelphia, use your own gas utility's emergency number.

What if there is no heat at all?

Check the thermostat setting and the filter first; checking thermostat settings is the first item on ENERGY STAR's maintenance list, and a clogged filter restricts airflow. If both are fine, it is a service call. In a Pennsylvania winter it may also qualify for help: the state's LIHEAP emergency services treat broken heating equipment or leaking lines that must be fixed or replaced as a heating emergency for eligible households, and can pay for repair or for replacing a system that cannot be repaired. Apply through your county assistance office.

Do not heat the house with the oven or range. Both the CPSC and the CDC say never to use gas cooking appliances to heat a home.

Which signs call for a service visit?

ENERGY STAR's list of reasons to call a professional contractor is the most useful guide here, because it ties each symptom to a likely cause:

Some rooms too hot or too cold. Possible causes it names: improper equipment operation, duct problems or inadequate insulation.
Humidity problems. Air that is too dry in winter or too humid in summer can come from poor equipment operation, equipment that is not up to the job, or leaky ducts.
Excessive dust. Leaky ducts can pull dust and air from attics, crawl spaces and basements and spread it through the house. ENERGY STAR suggests duct sealing may be the answer.
A noisy system. It may point to an undersized duct system or a problem with the indoor coil of the cooling equipment.
Rising bills and frequent repairs. Together, ENERGY STAR says, these suggest the equipment has become less efficient.

Some signs come from the maintenance checklist instead:

Water near the furnace, air handler or indoor coil. ENERGY STAR says a plugged condensate drain can cause water damage and affect indoor humidity.
A system that will not start, or will not shut off. Checking the starting cycle, so the system starts, runs and shuts off properly, is part of a standard tune-up.
Weak airflow. ENERGY STAR says airflow problems can reduce a system's efficiency by up to 15 percent, and that a dirty filter slows airflow and makes the system work harder. Change the filter before you book the visit.
A new system that never seems right. ENERGY STAR says oversized equipment may cycle too often, and that poor airflow after installation can leave the air overly damp. Both are installation problems, so call the installer first.

Which signs point toward replacement?

Age, mainly, combined with the signs above. ENERGY STAR suggests considering a new unit when a heat pump or air conditioner is more than 10 years old, or a furnace or boiler more than 15. For a home that is empty for long periods of the day and has no programmable thermostat, it suggests installing one, a much smaller job than new equipment.

None of these signs by itself proves a system needs replacing. A symptom tells you to have the system looked at; the diagnosis should come with a written explanation of what failed and what it would cost to fix. How to weigh that against the cost of new equipment is in the cost guide.

What signs should never wait for a convenient day?

Anything involving the burner, the flue or the gas line. The CPSC says a yearly inspection should check chimneys and flues for blockages, corrosion and disconnections. The EPA lists a flue that is blocked, disconnected or leaking among the sources of carbon monoxide in homes, so a vent pipe that has come loose is a reason to call the same day, not at the next tune-up. Questions worth asking when the technician arrives are in questions to ask an HVAC contractor, and contractors are listed on the HVAC contractors directory page.

Sources

Carbon Monoxide Fact Sheet, U.S. Consumer Product Safety Commission
Carbon Monoxide, U.S. Consumer Product Safety Commission
Carbon Monoxide (CO) Poisoning Basics, Centers for Disease Control and Prevention
Natural Gas Safety, Philadelphia Gas Works (PGW)
When is it time to replace?, ENERGY STAR
Maintenance Checklist, ENERGY STAR
Heat & Cool Efficiently, ENERGY STAR
HVAC Quality Installation, ENERGY STAR
Carbon Monoxide's Impact on Indoor Air Quality, U.S. Environmental Protection Agency
Pennsylvania LIHEAP Emergency Services Program, Pennsylvania Department of Community and Economic Development