Why is my HVAC system running all the time?

A heating or cooling system that never seems to stop is usually telling you one of three things: the weather is extreme, a setting or a blockage is making it work harder, or a part has failed. Most of the first two you can check yourself. The third needs a technician.

First, is it actually abnormal?

Long run times on the hottest and coldest days are expected. Trane describes a normal cooling cycle as lasting about 20 to 30 minutes, two to three cycles an hour,^[1] and Lennox puts it at about 15 to 20 minutes.^[2] It is normal for an air conditioner to run longer than usual in the hottest months.^[1][2]

Heat pumps work differently. Carrier says it can be completely normal for a heat pump to run all the time in extreme temperatures as it works to pull enough heat from the outdoor air.^[3] Carrier also notes that heat pumps have historically lost efficiency or performance in extremely cold temperatures, which can make them lean more heavily on a backup heat source, and that newer cold climate heat pumps have helped with that.^[3] How much your own system relies on backup heat depends on its model, so check the owner's manual or ask at the next service visit.

So the real warning sign is a system that runs nonstop in mild weather, or one that never reaches the temperature you set.

The thermostat

Start with the thermostat, and check the fan switch first: When the fan is set to On, the blower runs but the air conditioner itself does not.^[1] You hear air and feel it from the vents, but it is not cooling. Set it to Auto.

Next, the set point. Lennox notes that a thermostat set far too low keeps the system working toward a temperature it cannot reach.^[2] It also costs you: Virginia Cooperative Extension says every degree the thermostat is lowered increases cooling bills 3% to 5%.^[5]

Then look at what is near it. Virginia's State Corporation Commission warns that heat from lamps, televisions and other sources near the air conditioner thermostat may make the unit run longer than it should.^[4] Direct sun on the thermostat can have a similar effect. Older thermostats wear out too: dirt on the sensors, damaged wires and loose connections can interfere with the signal between the thermostat and the equipment.^[1][2]

Airflow: filter, vents and coils

A system that cannot move air has to run longer to move heat. Trane notes that a dirty or clogged air filter makes cooling cycles longer.^[1] Closed vents and furniture over grilles have the same effect, and a blocked outdoor unit cannot shed heat.

Coils are the next layer down. ENERGY STAR says dirty coils reduce the system's ability to cool your home and make it run longer.^[6] This one is not a homeowner job: Trane says it is best to have a professional clean the indoor evaporator coil, because there is a risk of electrical shock, the fins are easy to damage, and many household cleaners damage the copper or aluminum or start corrosion.^[1]

Ice is a related sign. A frozen evaporator coil can also keep an AC running constantly, and reduced airflow is one possible cause.^[2] On a heat pump in winter, Carrier explains that a light layer of frost, generally less than 1/4 inch, is normal on the outdoor coil in winter and should clear during the automatic defrost cycle.^[3] Thicker ice, or ice that stays, is a service call.

Size, ducts and worn parts

If the simple checks do not help, the cause may be built into the system. A system that is too small will run constantly.^[2][1] The opposite problem looks different: Virginia Cooperative Extension explains that an oversized air conditioner will short cycle, cooling the house too quickly to remove moisture well.^[5] ENERGY STAR adds that rooms that are too hot or too cold can point to improper equipment operation, duct problems or inadequate insulation.^[7]

Failed parts are the last group. Trane gives a common example: "If the compressor contactor becomes welded closed, the AC unit will continue running with constant power."^[1] For a heat pump that runs constantly in mild weather after recent service, Carrier lists a frozen coil, a faulty reversing valve, low refrigerant or a compressor problem as possible causes.^[3]

Stop and call a pro if

• You smell gas near a gas furnace that will not stop running. Columbia Gas of Virginia tells customers who smell gas or have carbon monoxide symptoms to "go outside and call 911 and then call us at 1-800-544-5606 (24/7)."^[11] If another company supplies your gas, use its emergency number.
• A carbon monoxide alarm sounds, or people in the house have headaches, dizziness, confusion or nausea while a gas or oil furnace is running. EPA lists severe headaches, dizziness, mental confusion, nausea and fainting among the symptoms of CO poisoning,^[12] and says that if anyone has symptoms when the alarm goes off, get them out of the house immediately and seek medical attention.^[12] If no one has symptoms, it says to ventilate the home with fresh air, turn off the furnace and other fuel-burning appliances, and have a qualified technician inspect them.^[12] Columbia Gas of Virginia gives the same instruction for carbon monoxide symptoms as for a gas smell: go outside and call 911, then the gas company.^[11]
• You smell burning or hot electrical insulation, or a breaker keeps tripping. Turn the system off at the thermostat. ENERGY STAR notes that faulty electrical connections can cause unsafe operation of your system,^[6] and Carrier advises that if a breaker trips repeatedly, not to keep resetting it but to contact a qualified HVAC professional.^[3]
• The outdoor unit keeps running with the thermostat off. That points to an electrical fault such as a stuck contactor, which is a repair for a technician.
• There is thick ice on either coil, or ice that does not clear during defrost. Turn the system off at the thermostat and call.
• It still runs nonstop in mild weather after you have checked the thermostat, filter, vents and outdoor unit.
• Someone suggests adding refrigerant. EPA rules say technicians who maintain, service or repair equipment that could release ozone-depleting refrigerants must be certified,^[8] and a low charge needs a reason found, not just a top-up.
• You are thinking of replacing the system because it seems too small. Ask for a load calculation first.

Virginia's Code defines a tradesman as anyone who does electrical, plumbing or heating, ventilation and air conditioning work for the general public for compensation,^[10] and no one may do that work without a tradesman licence unless they work under the supervision of a tradesman licensed in that specialty.^[9] A yearly check-up catches much of this early; see what an HVAC tune-up costs in Virginia and how often to change a furnace filter.

Getting it looked at

Loclis lists DPOR-licensed HVAC businesses with their licence number and class. Look up HVAC businesses in Blacksburg or another town, browse HVAC repair, or read more heating and cooling answers.

When you are ready, describe what the system is doing once and licensed local HVAC businesses quote it.