The house is warm, the thermostat is set to cool, and the system seems to be running — but the air coming out of the vents is not cold. This is a frustrating situation, especially during Calgary summers when you only have a few months to enjoy AC. The good news is that many of the common causes are either things you can check yourself or straightforward repairs.
Check the air filter
A clogged air filter is one of the most common and most overlooked causes of poor AC performance. When the filter is dirty, airflow across the evaporator coil is restricted. The coil gets too cold, ice forms on it, and the system loses its ability to cool the air. Check the filter and replace it if it looks dirty. If the coil has already frozen, turn the system to fan-only mode for a couple of hours to let the ice melt before trying cooling again with a clean filter.
Frozen evaporator coil
If you open the furnace cabinet and see ice on the coil or refrigerant lines, the system is not going to cool properly until that ice is gone. Dirty filters are the most common cause, but low refrigerant, a failing blower motor, or a blocked return vent can also cause freezing. Turn the system off or switch to fan-only, let it thaw completely, then address the underlying cause before restarting.
Thermostat issues
Confirm the thermostat is set to cool mode, not just fan. Check that the set temperature is below the current room temperature by at least a couple of degrees. If you have a programmable thermostat, check the schedule — it may be following a program that is not calling for cooling when you expect it to.
Outdoor unit not running
The condenser unit outside is where the heat gets rejected. If the outdoor unit is not running, the indoor coil will blow air but it will not be cold. Check the breaker panel for a tripped breaker on the AC circuit. Also check the disconnect box near the outdoor unit — it may have been switched off or the fuse inside may have blown.
If the outdoor unit is running but the fan is not spinning, the fan motor or its capacitor may have failed. If the compressor is humming but not starting, the compressor capacitor is likely the issue. These are common repairs that a technician can usually handle in a single visit.
Low refrigerant
Home AC systems do not use up refrigerant — if the charge is low, there is a leak. Low refrigerant reduces cooling capacity and eventually causes the evaporator coil to freeze. Signs include warm air from the vents, ice on the refrigerant lines, and the system running continuously without reaching the set temperature. Refrigerant work requires a licensed technician.
Dirty condenser coil
The outdoor condenser coil collects dirt, grass clippings, cottonwood fluff, and debris. If it is dirty, the system cannot reject heat efficiently and cooling performance drops. You can carefully rinse the coil with a garden hose from the inside out, but avoid using high pressure. For heavy buildup, a professional coil cleaning is more effective.
When to call a technician
If you have checked the filter, thermostat, and breakers and the system still is not cooling, call for service. Running an AC system with a problem — especially low refrigerant or a frozen coil — can cause compressor damage, which is a much more expensive repair.
YYC Mechanical provides residential AC repair and maintenance across Calgary. Whether it is a filter change or a refrigerant leak, we will get your home cooling again.