Quick answer
No air coming out of your vents means one of two things: the blower is not turning, or it is turning and the air is blocked. Stand at the furnace and listen. A silent blower points to a door switch, a blown 3 or 5 amp board fuse, a capacitor or the motor. A running blower points to an iced coil, a collapsed filter or a closed damper.
When no air is coming out of your vents, the fault is in one of two places, and you can tell which in about a minute without tools. Set the thermostat fan to ON, walk to the furnace and listen at the lower cabinet. If the blower is silent, the problem is electrical: power, a safety switch, a fuse, a capacitor or the motor itself. If the blower is humming along and the registers are still dead, the air is being stopped after the fan, by ice, a filter or the ductwork.
Blower silent: the five faults that stop it from turning
A blower that will not run with the fan set to ON has one of five faults, and the first two cost nothing to fix.
- Furnace switch off. The switch on or beside the cabinet looks like a light switch and gets flipped by accident during a basement clean-up. Check it, then check the breaker marked furnace.
- Door switch open. The blower door presses a small plunger switch that cuts all power when the panel is off or seated crooked. If the filter was changed recently, reseat the lower door until it sits flush.
- Blown board fuse. The control board carries a small automotive-style blade fuse, usually 3 or 5 amp, that blows when a low-voltage wire shorts: a thermostat cable nicked by a staple, or a humidifier wire rubbing on sheet metal. A fuse that blows again right away has a short that needs finding, not a bigger fuse.
- Failed run capacitor. On a PSC motor the capacitor gives the motor its starting kick. When it fails the motor hums, gets hot and never spins.
- Failed motor or ECM module. Variable-speed (ECM) blowers have an electronic module bolted to the back of the motor, and the module fails more often than the windings.
The page on why a furnace blower is not turning on goes deeper on the electrical tests. Leave anything past the switch, the door and a look at the fuse to a technician: the capacitor holds a charge with the power off.
Blower running but nothing at the registers: ice, a filter or a damper
If you can hear the blower and feel nothing upstairs, the air is hitting a wall. In cooling season that wall is usually ice.
- Evaporator coil frozen solid. The coil sits in the plenum above the furnace. Low airflow or low refrigerant lets it drop below freezing, the condensate turns to frost, and the frost grows into a block that no air passes. Signs: frost on the larger copper line at the furnace, water on the floor, an outdoor unit that never stops.
- Filter sucked into the blower. A flimsy 1-inch filter that is overdue can bow, pull out of its rack and fold against the blower inlet. Look into the filter slot: if the filter is missing or bent into the cabinet, you have found it.
- Zone damper closed or failed. Zoned systems use motorized dampers in the trunk lines. A damper motor that fails closed, or a zone panel that has lost its transformer, shuts a whole floor off while the blower runs normally.
- Disconnected or crushed trunk duct. A supply trunk that has pulled off the plenum, or a run flattened above a finished basement ceiling, dumps the air into the basement or joist space instead of the rooms.
Disconnected ducts show up after other work: a basement finish, a new furnace set under a plenum that was never screwed and taped. If the furnace room is suddenly the warmest or coldest room in the house and you can feel air blowing along the ceiling, follow the trunk with a flashlight. In condos with fan coils the same symptom is usually the fan coil motor or its relay, and who is responsible for a condo fan coil unit matters before you call anyone.
Turn the cooling off if the coil is iced
If you see frost on the refrigerant line or the coil, set the thermostat to Off for cooling and the fan to ON. Running the compressor against a block of ice sends liquid refrigerant back to it, and the compressor is the most expensive part of the air conditioner. Thawing takes several hours, with towels or a pan under the furnace for the meltwater. Replace the filter while you wait, and if the coil ices again with a clean filter and open registers, the cause is refrigerant charge or a dirty coil, covered in why an AC freezes up.
In heating season the risk runs the other way. Burners firing with no air moving across the heat exchanger trip the high-limit switch, the burners shut off and the cycle repeats. Do not let it keep doing that: set the thermostat to Off and leave it there until the airflow is fixed.
When to call now, and what a no-airflow repair costs
Call the same day if any of these apply:
- A burning or hot-electrical smell from the furnace, or a motor too hot to touch.
- The furnace lights, runs briefly and shuts down over and over with no air at the vents.
- Ice returns after a full thaw and a new filter.
- There is no heat and the outdoor temperature is below freezing, with pipes on outside walls at risk.
Typical prices for the parts on this page:
- Diagnostic visit: $90–$150, folded into the repair if you go ahead.
- Capacitor: $250–$500.
- Blower motor: $500–$1,500, with ECM motors and modules at the upper end.
- Control board: $500–$1,200 when a short has taken out more than the fuse.
Duct reconnection and damper motors are priced on the visit because access decides the cost; you get the figure in writing before work starts. ZK Mechanical handles no-airflow calls as furnace repair across Burlington, Oakville and Hamilton, and the 24/7 line carries no after-hours surcharge. Call (647) 801-1252 or book a visit. More airflow questions are answered in the air quality and ducts hub.