Furnace Short Cycling

Restricted airflow causes the heat exchanger to overheat. The high-limit switch shuts off burners. Blower keeps running until temperature drops, then burners restart. Cycle repeats every few minutes.

Air filter heavily clogged or many registers closed. Burners run 2-5 minutes then shut off while blower continues. After 30-60 seconds, burners reignite. Pattern is consistent and repeated.

Same root cause as "blowing cold air" but the cycle restarts faster. Burners ignite, sensor doesn't detect flame, shutoff, restart attempt — repeating in 30-second cycles.

Burners ignite for 5-10 seconds then shut off. After short pause, ignitor glows and burners try again. After 3 attempts, control board locks out completely.

If the furnace is too large for the space, it heats too quickly, hits the thermostat setpoint, shuts off. Then the room cools rapidly (because oversized systems don't run long enough to evenly distribute heat). Cycle repeats endlessly.

Furnace runs for less than 5 minutes per cycle, hits setpoint, shuts off. Room cools quickly, restart in 10-15 minutes. No overheating signs. New construction or recent furnace upgrade.

Thermostat is in a location that sees temperature swings unrelated to the space being heated. Examples: near a supply register (heated quickly), in direct sunlight (rises fast), near a drafty door (cools fast).

Look at thermostat location. Is it on an exterior wall? Near a vent? In direct sun? In a hallway not actively heated? Compare its reading to a thermometer in the main living space.

The high-limit switch (a safety control) has failed and is opening at too low a temperature — shutting off burners prematurely. Different from genuine overheating because filter is clean and airflow is fine.

Filter is clean, all vents open, but furnace still cycles like it's overheating. Burners shut off without true overheat condition. Multimeter test confirms.

If the blower can't move enough air, the heat exchanger overheats fast and the limit switch trips. Sometimes the blower runs at reduced speed (failing capacitor) so airflow is just barely insufficient.

Hold your hand at a supply register — airflow weak compared to past performance. Listen at the indoor unit for blower running roughly. Check blower wheel for dust/dirt buildup.