Air is moving, the blower's clearly alive, and what comes out of the vents is disappointment. "Running but cold" splits into five stories — two of them fixable at no cost, one of them urgent — and the pattern of when it blows cold tells you which story is yours.
The no-cost ones first
Fan set to ON: at the thermostat, fan ON means the blower circulates room-temperature air between heat cycles — which registers as "cold air" from a vent. Set fan to AUTO; if cold-blowing between cycles stops, that was the whole case. Filter loaded to felt: a suffocating filter overheats the furnace, the high-limit switch cuts the burners for safety, and the blower keeps running to cool things — warm-then-cold-then-warm cycling is the signature. New filter, and watch whether the pattern breaks.
The classic dropout
Flame sensor: burners light, heat arrives for a few seconds or minutes, then flames slam off while the blower coasts cold. An oxide-coated sensor rod losing sight of the flame is one of the most common furnace faults in existence — and one of the most modest repairs. If your cold air arrives in a light-die-retry rhythm, this paragraph is probably your diagnosis.
The overheat loop
Limit switch trips beyond the filter: a dying blower motor or capacitor moving too little air, a matted indoor coil (on homes with AC), or closed/blocked vents starving airflow. The furnace makes heat, can't ship it, overheats, cuts burners — repeat. This one deserves a visit soon: chronic overheating is how heat exchangers age before their time.
The never-warm and the urgent footnote
Cold from the first second, always: ignition never happened — dead igniter, gas supply, pressure switch chain; see our no-ignition guide. And the override: if you ever smell gas, skip diagnosis entirely — leave the house, call PSE at 1-888-225-5773 or 911 from outside, then call us. Cold air is a comfort problem; gas smell is not.
One duct note for older homes
In some Everett and Shoreline houses the furnace makes perfect heat that arrives lukewarm — disconnected or leaking ducts in the crawlspace are heating the underside of the house instead of the rooms. If registers far from the furnace blow noticeably cooler, say so when you call; it changes where we look first. A written quote comes once the pattern and the measurements point to one of the five.