You can hear the outdoor unit running. The thermostat says cooling. And there is nothing at all coming out of the registers.
Before you start worrying about refrigerant and compressors, here's the thing that reframes most of these calls: in a central system, your air conditioner doesn't move any air. The outdoor unit handles refrigerant. The thing that actually pushes air through your ducts is the blower inside your furnace — the same blower that heats the house in January.
So "the AC is on but no air is coming out" is usually a furnace-side or airflow problem wearing an air conditioning costume. That's good news, because most of the causes are cheaper than the one you're imagining.
We take more of these calls in Airdrie than anywhere else we work, and the reason is timing rather than equipment: a lot of the housing there is newer and came with AC from new, and it all gets switched on within about the same forty-eight hours each summer. The first genuinely hot weekend is when every marginal system in town announces itself at once.
Here's how to narrow it down.
Start by asking which vents
The pattern tells you where to look, so check more than one room before anything else.
- Nothing from any vent — the blower isn't running at all, or something is blocking airflow at the furnace itself. Almost always one of the causes in the next two sections.
- Nothing from some vents, normal from others — the blower is fine. You have a duct, damper or balancing problem.
- Weak air everywhere, but there is some — restriction. Filter, blower wheel, or a coil starting to ice up.
That one question saves an enormous amount of guessing.
Nothing from any vent: the blower isn't running
If the furnace blower isn't turning, no air moves regardless of what the outdoor unit is doing. Work through these in order:
The furnace switch. There's a switch that looks exactly like a light switch, usually on or beside the furnace. If someone was down there changing a filter or moving boxes, it gets knocked off more often than you'd think. No power, no blower.
The breaker. Check the furnace circuit at the panel. If it has tripped, reset it once. If it trips again, stop and call — a breaker that won't hold is telling you about a fault, not asking to be reset harder.
A blown low-voltage fuse on the control board. Furnaces carry a small automotive-style fuse, usually 3 or 5 amp, on the circuit board. When it goes, the board loses control power and the blower won't respond to a call. It's a cheap part and often a quick fix, but the fuse blew for a reason — we've written up why a furnace fuse keeps blowing if yours has gone more than once.
A failed blower capacitor or motor. The capacitor gives the motor the kick it needs to start. When it weakens, you sometimes get a hum from the furnace with no fan movement. A dead motor gives you nothing at all. Both are technician work.
A thermostat wiring fault. The blower is triggered by a specific wire from the thermostat. If that connection has failed, the outdoor unit can still receive its call to run while the indoor blower never gets told to start — which is exactly the symptom you're describing.
Nothing from any vent: a frozen evaporator coil
This is the one people don't expect, and it's common enough that it's worth checking early.
Inside your furnace, above the blower, sits the evaporator coil — the cold half of the system. If airflow across that coil drops too low, or refrigerant pressure is wrong, condensation on it freezes instead of draining away. The ice builds until it forms a solid block, and at that point almost nothing gets through.
How to tell:
- Ice or heavy frost visible on the copper refrigerant line where it enters the furnace
- Water pooling around the furnace once things start thawing
- The system was cooling normally a day or two ago and got gradually weaker before stopping
What causes it:
- A clogged filter, by a distance the most common reason
- Low refrigerant from a leak
- Blocked return air — furniture against a return grille, or too many closed registers
- Running the AC on an unusually cool night
What to do: turn the cooling off at the thermostat and set the fan to ON. That runs the blower without the compressor and melts the ice off with room-temperature air. It takes a few hours, sometimes most of a day for a solid block — be patient with it. While it thaws, change the filter and open every register and return in the house. Put a towel down if water is finding the floor.
What not to do: don't chip at the ice. The coil fins bend if you look at them wrong and the refrigerant tubing sits directly behind them, so a screwdriver turns a filter change into a refrigerant leak. And don't just switch the cooling back on and hope — it will refreeze within hours, and each cycle is hard on the compressor.
If it freezes again after a filter change, that points at refrigerant rather than airflow, and that needs a technician.
Nothing from some vents only
If other rooms are getting air normally, your blower is fine and the problem is between the furnace and those particular registers.
A disconnected duct. Flexible ducting is held on with clamps and mastic, and in an attic, crawlspace or unfinished basement it can pull loose — sometimes from nothing more than a run being stepped near. The blower is working perfectly; the air is being delivered into a ceiling cavity instead of your bedroom. The tell is one or two rooms with nothing at all while the rest of the house is normal.
A closed damper. Some homes have manual dampers in the trunk lines for seasonal balancing, and zoned systems have motorised ones. A damper left in its winter position, or a zone motor that has failed closed, cuts a whole branch.
A crushed or kinked run. Flex duct that's been compressed by storage, or pinched around a tight corner during a renovation, chokes everything downstream of it.
Registers closed or covered. Worth ruling out before assuming something dramatic — rugs, furniture and closed dampers in the register itself account for a fair share of these.
Weak air from everywhere
Some air, but nowhere near what you'd expect.
The filter. Change it rather than inspect it hopefully. And here's the Calgary-specific part: the filter serving your AC is the same one that served your furnace through a seven-month heating season. Plenty of people change it in autumn, forget it entirely by June, and then wonder why the cooling feels weak on the first hot day. If yours is grey and you can't see light through it, you've found your problem.
A dirty blower wheel. The blower is a squirrel-cage wheel with a lot of small blades. Over years, dust cakes onto those blades and the wheel simply stops moving the air it was designed to. It's a gradual decline, so people rarely notice until it's severe. Cleaning it is a proper service task, not a DIY one.
Undersized or leaking ductwork. Chronic rather than sudden. If the airflow has been disappointing since the system went in, that's a design issue rather than a fault.
What's safe to check yourself
- Which vents have air and which don't
- Thermostat set to Cool, setpoint below room temperature, fan on Auto
- The furnace switch, and the breaker at the panel
- The filter — replace it if it's dirty
- Every supply register open, every return grille clear of furniture
- Ice or frost on the refrigerant line at the furnace
- Whether the outdoor fan is actually spinning
That's the list. Past that you're into capacitors, refrigerant and the control board, and none of those reward improvisation.
When to call
Call if the blower won't run after you've checked the switch, the breaker and the filter. Call if the coil ices up again after a filter change — that's refrigerant, and refrigerant doesn't get used up, so it means a leak. Call if a breaker trips more than once, or if whole rooms get nothing while the rest of the house is fine.
One last thing worth saying, because it costs people money every summer: don't keep running a system that isn't moving air. A blower fighting a frozen coil, or a compressor running against a system that can't shed heat, is doing itself damage the entire time. Shutting it off for an afternoon while you sort the filter is the cheaper decision.
If you've worked through the list and you're still getting nothing at the registers, call 587-834-3668 and we'll find it properly. See our air conditioning repair page for what a diagnostic visit covers.
