AC Not Cooling? Troubleshooting Guide for Ottawa Homes

Quick answer: If your AC is not cooling, check that the thermostat is on Cool with the fan on Auto, reset any tripped breaker, replace a dirty filter, clear debris and cottonwood fluff from the outdoor unit, and look for ice on the lines or a full condensate pump. Hissing, humming or ice that returns needs a technician.

Quick answer: check these first

Thermostat

Set to Cool, fan on Auto, setpoint at least 2 °C below room temperature, fresh batteries.

Power

Two breakers (indoor furnace and outdoor AC) plus the outdoor disconnect box beside the unit.

Furnace filter

A clogged filter is the top cause of frozen coils.

Outdoor unit

Fan spinning, fins clear of grass, leaves and cottonwood fluff, winter cover removed.

Frozen coil

Ice on the copper line or indoor coil means turn cooling off and let it thaw.

Condensate drain and pump

A full pump or clogged line trips a float switch that shuts the AC off.

Furnace door and vents

Door seated firmly, supply registers open, returns unblocked.

If these are fine and the house is still warm, the problem is likely electrical or refrigerant-related, and it needs a licensed refrigeration technician.

Step 1: thermostat and power

Start with the thermostat. Confirm it is on Cool, not Heat or Fan only, and that a schedule or eco mode is not holding a high setpoint. Set the fan to Auto. With the fan on On, the blower runs non-stop and re-evaporates moisture from the coil back into the house, which makes rooms feel muggy.

Central air uses two power circuits: one for the indoor furnace blower and one 240-volt circuit for the outdoor condenser. Check both breakers, and check the outdoor disconnect box mounted on the wall near the unit. Reset a tripped breaker once. If it trips again, leave it off and call, because a repeat trip usually points to a failing capacitor, contactor, fan motor or compressor.

After a power interruption, give the system 5 minutes before expecting the outdoor unit to start. Many systems have a built-in delay that protects the compressor.

Step 2: filter, vents and the furnace door

Your central AC uses the furnace blower and the furnace filter. A dirty filter cuts airflow over the indoor coil, the coil gets too cold and ices up, and cooling drops off. Replace it and check monthly during July and August. Our furnace filter guide explains MERV ratings and change intervals.

Make sure supply registers are open and big return grilles are not blocked by furniture. The furnace blower door must also be seated tight, because its safety switch stops the blower, and the AC with it, if the panel is loose.

Step 3: inspect the outdoor unit

With the AC calling for cooling, go outside and look and listen. The top fan should be spinning and you should hear the compressor humming steadily. Warm air should blow out the top.

Clear the fins.

In June, Ottawa’s cottonwood trees shed fluff that mats onto condenser coils like felt. Turn off the disconnect, then rinse the fins gently from the inside out with a garden hose. Never use a pressure washer.

Keep clearance.

Trim shrubs and fences back at least 45–60 cm on all sides and keep 1.5 metres clear above the unit.

Remove winter covers.

A cover left on traps heat and can damage the compressor.

Fan not spinning but humming?

Turn off the power. A failed run capacitor is a common, quick repair for a technician.

Step 4: frozen coil or ice on the lines

Ice on the large insulated copper line at the outdoor unit, or on the indoor coil above the furnace, means the coil has dropped below freezing. Running the AC frozen can push liquid refrigerant to the compressor and damage it.

Set the thermostat to Off and the fan to On to blow warm air over the coil.
Wait 2–4 hours until all the ice is gone. Put towels down for meltwater.
Replace the filter and open all vents.
Restart cooling. If ice returns within a day, the cause is usually low refrigerant from a leak or a dirty coil, which needs a technician.

Step 5: condensate drain and pump

On a humid Ottawa day with the humidex near 40, your AC can remove many litres of water from the air. That water drains through a trap and line to a floor drain or a small condensate pump. If algae clogs the line or the pump fails, a float switch shuts the AC off to prevent flooding, or water spills around the furnace.

Check the pump reservoir and listen for it cycling. Clear a clogged line with a wet/dry vacuum at the drain end. A splash of white vinegar in the trap each spring helps slow algae growth.

AC symptoms and what they usually mean

Symptom	Common causes	What to do
AC will not turn on	Thermostat, breaker, disconnect, float switch, door switch	DIY checks; call if breaker trips again
Blowing warm air	Outdoor unit off, failed capacitor, low refrigerant	Check outdoor fan; call
Weak airflow	Dirty filter, frozen coil, blower issue, closed dampers	Change filter, thaw coil
Ice on lines or coil	Low airflow, refrigerant leak, dirty coil	Thaw; call if it returns
Hissing or bubbling	Refrigerant leak	Call
Vibration or rattling outside	Loose panels, debris in fan, unbalanced fan blade, unlevel pad, worn motor or compressor mounts	Turn off; clear debris; call
Water around the furnace	Clogged drain, failed pump, frozen coil melting	Clear drain; call if it continues
Runs non-stop on hot days	Undersized system, dirty coil, low charge, heat wave beyond design	Check filter and coil; call

Why is my AC vibrating?

A gentle hum is normal. Heavy shaking, rattling or a thumping noise is not. Shut the AC off at the thermostat to avoid further damage, then check the outdoor unit with the disconnect pulled.

Sticks, leaves or toys inside the fan guard after a summer storm
Loose access panels or screws
A concrete pad that has settled out of level, common after Ottawa frost heave
A bent or out-of-balance fan blade, or a failing fan motor
Worn compressor mounts, or a compressor struggling to start
Indoors, an unbalanced blower wheel can shake the ductwork

Clearing debris and tightening panels is safe DIY. Motor, blade and compressor issues need a technician.

When to call, and what repairs cost

Call a technician if a breaker trips twice, ice returns after thawing, you hear hissing, the outdoor fan hums without spinning, or the house is still warm after the checks above. Refrigerant work in Ontario must be done by a licensed technician; every Oria tech holds a refrigeration licence.

Most AC repairs in Ottawa cost $150 – $1,200. A compressor replacement typically runs $1,800 – $3,500, and a full central air conditioner replacement $4,000 – $8,000. Final prices depend on the home, the equipment and the line set and electrical. Older systems that use R-22 refrigerant, which is phased out, are often better replaced than recharged. Our AC repair cost guide has more detail, and our repair or replace guide helps you decide.

If the system works but some rooms stay hot, read why your house is hot with the AC on. For fast help, book air conditioning repair in Ottawa or call (613) 866-1869. Same-day repair is often available, and Oria answers 24/7.

Frequently Asked Questions

Why is my AC running but not cooling the house?

The most common causes are a dirty furnace filter, a frozen indoor coil, an outdoor unit clogged with debris or cottonwood fluff, or an outdoor fan or compressor that is not running. Low refrigerant from a leak and a failed capacitor are also common. Check the filter, thermostat and outdoor unit first, then call a licensed technician.

How do I unfreeze my air conditioner?

Turn the thermostat to Off and set the fan to On so the blower moves warm air over the coil. Let it run 2 to 4 hours until all ice is gone, with towels down for the meltwater. Replace the filter and open the vents before restarting. If the coil freezes again, it likely has a refrigerant leak or dirty coil.

Should I reset my AC breaker if it trips?

You can reset a tripped AC breaker once. Wait a few minutes before switching cooling back on. If the breaker trips again, leave it off and call for service. Repeated tripping usually means a failing capacitor, contactor, fan motor or compressor, or a wiring fault, and resetting it over and over can damage the system.

How cold should air from my AC vents be?

Air from the supply vents should be about 8–11 °C cooler than the air going into the return grille. Measure both with a simple thermometer after the AC has run for 15 minutes. A smaller difference points to low refrigerant, a dirty coil or an airflow problem, especially if the house is not reaching the setpoint.

Why is my AC unit vibrating or rattling?

Common causes are debris inside the outdoor fan guard, loose panels or screws, a pad that has settled out of level after frost heave, a bent fan blade, a failing fan motor or worn compressor mounts. Turn the AC off and remove any debris with the power disconnected. If shaking continues, call a technician before it damages the compressor.

Is it normal for my AC to run constantly during a heat wave?

On 32 °C days with a high humidex, a properly sized AC may run almost continuously in the afternoon and still keep the house a few degrees warmer than the setpoint. That is normal. If it runs non-stop on milder days, or never gets the house below about 25 °C, have the filter, coil and refrigerant charge checked.