Why is my new furnace short cycling?

A new furnace that turns on, runs a few minutes and shuts off again may be telling you something about the installation. Common causes include a furnace that is too big for the house, airflow that is choked by a dirty filter, closed vents or undersized ducts, and a thermostat in the wrong spot or set up wrong. A venting problem or a dirty or faulty flame sensor can also trip a safety control. Check the filter and vents yourself; for everything else, call the contractor who installed it.

Is it really short cycling?

Short cycling means the furnace stops before it finishes a normal heating cycle and starts again soon after. Manufacturers describe a normal cycle differently. Lennox says: "It typically takes five minutes for a furnace to complete a full cycle."^[1] Carrier says a system that repeatedly runs for less than 5 to 10 minutes before shutting off may be short cycling,^[2] and Bryant says normal heating cycles commonly run for about 10 to 15 minutes or longer.^[3] Bryant adds that cycle length is a diagnostic clue, not a universal cutoff.^[3]

The practical test: if the furnace keeps stopping after a few minutes while the house is still below the thermostat setting, and rooms feel unevenly warm, something is wrong.

The furnace is too big for the house

Bryant lists oversized equipment among the common causes of short cycling,^[3] which makes size a fair question to ask about any new installation. It explains that an oversized furnace will heat your home too quickly, reaching the target temperature and shutting off before completing a full, efficient cycle.^[3] ENERGY STAR warns that oversized equipment may cycle too frequently, resulting in less comfort and a shortened lifespan,^[4] and Lennox says a larger-than-necessary furnace runs shorter cycles, and the frequent turning on and off puts additional wear and tear on it.^[5]

It can happen when the new furnace was sized to match the old one, or by square footage alone. Carrier's view is that a qualified professional should use a heating-load calculation to determine the appropriate furnace capacity.^[2] Ask your installer for the calculation they used. If there was none, that is a fair question to raise.

Not enough air is moving

A furnace needs a steady flow of air across its heat exchanger. The general manager of a Pennsylvania HVAC company, quoted by Carrier, puts it this way: "If the furnace cannot move enough air across the heat exchanger, the system can overheat and trigger a safety limit switch that shuts the furnace down."^[2] The furnace cools, restarts, overheats again, and the cycle repeats.

The airflow problems to look for are:

• A clogged filter. Carrier is direct: "A clogged filter is a leading cause of short cycling."^[2] Check it even on a new furnace.
• Closed supply registers or blocked returns. Carrier notes that closed vents are a frequent but overlooked problem, especially when homeowners forget to reopen them after cooling season.^[2]
• Ducts that do not suit the new furnace. Lennox warns: "Poorly designed, sized or sealed ductwork can restrict airflow".^[5] ENERGY STAR adds that poor system airflow can result in reduced efficiency, higher utility bills, overly damp air, reduced comfort and shortened equipment lifespan.^[4] If the new furnace went onto the old ducts, ask the installer whether they measured the airflow afterwards.

The thermostat is misreading the house

A thermostat is sometimes replaced along with the furnace, and a new one in the old spot can cause trouble. Lennox lists faulty wiring, outdated technology or placement^[1] among the thermostat causes of short cycling. Carrier recommends installing a thermostat on an interior wall away from direct sunlight, exterior doors, windows, air vents, drafts, and heat-producing appliances.^[2] A thermostat above a supply register or beside a sunny window will tell the furnace the house is warm long before it is. Its settings also have to match the furnace, which the installer should have set up.

Safety controls are shutting it down

Some short cycling is the furnace protecting itself.

Flame sensing. Bryant explains that if the flame sensor is dirty or faulty, the furnace may shut down the burners shortly after ignition because the control system does not detect the expected flame signal.^[3] Bryant also notes that ignition, wiring, grounding, gas supply, and control problems can produce similar behavior.^[3] All of those are inside the furnace and are the installer's to diagnose.

Venting. "Modern furnaces have safety controls that can shut the system down if combustion gases cannot safely escape."^[3] Bryant is clear: "Do not attempt to repair or modify furnace venting yourself."^[3] Lennox adds: "Inaccurately installed venting can also affect performance and reduce furnace lifespan."^[5]

Also worth checking on a high-efficiency furnace

While the installer is back, have them look at the condensate drain on a condensing model as well. Bryant notes that high-efficiency condensing furnaces require model-specific venting and condensate configurations,^[6] and Carrier warns: "Improper drainage in high efficiency furnaces can lead to internal damage."^[7]

What you can safely check

Only check what is outside the furnace cabinet. Bryant's advice is: "Replace a dirty filter and make sure vents and returns are open and unobstructed."^[3] It also suggests you make sure the thermostat is set to Heat and has fresh batteries,^[3] and that nothing near it, such as a lamp or a sunny window, is warming it. For filter types and schedules, see how often to change a furnace filter.

If the furnace still short cycles, Carrier and Bryant both say to turn the system off and contact a qualified HVAC professional.^[2][3] Bryant says it is not recommended to leave a short cycling furnace running,^[3] because of the strain on its parts.

When to call a licensed pro

For a furnace installed in the last season or two, the first call is to the business that installed it. Ask them to check the items Bryant lists for a short cycling furnace: blower motors, airflow, temperature rise, gas pressure, safety controls, venting, thermostat operation, and equipment sizing.^[3] Ask for the heating-load calculation and the start-up readings they recorded, and check what the contract and labor warranty say about callbacks.

Virginia's Board for Contractors treats "Negligence or incompetence in the practice of contracting" as a prohibited act,^[8] and its regulation says complaints against contractors may be filed with the Department of Professional and Occupational Regulation.^[8] Give the installer a fair chance, in writing, first.

If you need a second opinion, use a licensed business. Under Virginia law, no individual may work as a tradesman without a licence, and the Code's definition of a tradesman includes anyone doing heating, ventilation and air conditioning work for the public for pay.^[9][10] Check any business on DPOR's License Lookup.^[11] If you smell gas or have carbon monoxide symptoms, Columbia Gas of Virginia tells its customers to "go outside and call 911" and then call the gas company.^[12]

For what a correct installation includes, see what replacing a furnace involves; if the furnace runs but the house stays cold, read why a new HVAC system is not keeping up.

Getting it looked at in Virginia

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

Describe what the furnace is doing and licensed local HVAC businesses quote the diagnosis and repair.