Quick Answer
An air conditioner that cools but doesn't dehumidify is usually oversized — it satisfies the thermostat too quickly to wring moisture from the air. Other causes include a fan set to ON instead of AUTO (which re-evaporates moisture off the coil), low refrigerant, leaky ductwork pulling in humid air, or simply extreme GTA summer humidity overwhelming a marginal system.
Comfort in a GTA summer is as much about humidity as temperature. Air at 24°C and 65% relative humidity feels muggier than 26°C at 45%. Your AC is supposed to handle both jobs — every minute it runs, water condenses on the indoor coil and drains away. When a house stays clammy with the AC running, one of these mechanisms is broken.
The usual suspects
- Oversized AC — the most common cause in the GTA. A unit with too much capacity blasts the temperature down in 8–10 minutes and shuts off before meaningful dehumidification happens. If your AC runs in short bursts on humid days, this is likely it.
- Thermostat fan set to ON — between cooling cycles, the fan keeps blowing across the wet coil and evaporates the moisture right back into your ducts. Set it to AUTO.
- Low refrigerant or a dirty coil — reduces the coil's ability to condense moisture; often accompanied by weak cooling. Related: why your AC isn't cooling.
- Leaky return ducts — returns running through a humid basement, garage or attic pull moist air into the system. Common in older Toronto homes. Duct sealing fixes this.
- Clogged condensate drain — condensed water that can't drain re-evaporates into the airstream.
- Lifestyle load — aquariums, line-drying laundry indoors, long showers without exhaust fans, and dozens of houseplants all add litres of moisture per day.
What healthy indoor humidity looks like
Aim for 40–55% relative humidity in summer. Above 60%, dust mites and mould thrive, and the house feels sticky regardless of temperature. A $20 hygrometer tells you where you stand; many smart thermostats display it too.
Fixes, from free to permanent
- Free: set fan to AUTO, run bathroom/kitchen exhaust fans, close windows on muggy days
- Cheap: replace a clogged filter, have the condensate drain flushed
- Moderate: AC tune-up with refrigerant check and coil cleaning ($120–$250)
- Structural: seal return-duct leaks, add a whole-home dehumidifier ($1,500–$3,000 installed) for chronically damp houses
- Long-term: when replacing the AC, insist on a proper load calculation — or choose a two-stage or variable-speed unit, which runs longer and gentler cycles that excel at moisture removal
Persistent humidity is also an indoor air quality issue — it feeds mould and dust mites that aggravate allergies. If your home never feels dry in summer, ZK Mechanical can measure the system's actual performance and fix the cause rather than the symptom. Book a visit anywhere in the GTA.
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