Here's a complaint we get every July, mostly from homes between Morrisburg and Iroquois and along the St. Lawrence waterfront: "The AC is running constantly, the thermostat shows 22°C, but the house still feels muggy."
The answer is almost never that you need a colder AC. The answer is almost always that you have a dehumidification problem disguised as a cooling problem.
What your AC is actually supposed to do
A central air conditioner does two jobs simultaneously:
- Sensible cooling — lowering the air temperature
- Latent cooling — removing moisture from the air
When air passes over the cold evaporator coil, water vapour condenses out and drips into the condensate pan. That's the puddle you sometimes see under the outdoor unit when the drain is plugged.
A properly sized and properly operating AC removes both heat and humidity. When the system is sized wrong or runs wrong, it removes heat but leaves the humidity. The room temperature reads 22°C and you're still sweating because the relative humidity is at 65%.
For comfort, indoor relative humidity should be around 40-50% in summer. Above 55% you start to feel sticky regardless of temperature. Above 60% you start to grow mold somewhere in the house — typically in basements, behind furniture against outside walls, or in the HVAC equipment itself.
Why the St. Lawrence makes this worse
The river is a massive evaporative surface. From May through September, the air mass over the St. Lawrence Valley carries higher absolute humidity than air over inland farmland. Homes along the waterfront in Morrisburg, Iroquois, and Cardinal pull this humid air in every time a door opens, every time the bath fan runs, and continuously through normal envelope leakage.
The result: the latent load on your AC is significantly higher than what the equipment-sizing tables assume for "Eastern Ontario." If your AC was sized off a generic chart instead of a proper load calculation that accounts for your specific location, it's probably undersized on the dehumidification side, even if it's correctly sized on the cooling side.
Why oversizing makes it worse, not better
Counterintuitively, the worst thing you can do is install a bigger AC.
An oversized AC cools the room temperature down to the thermostat setpoint quickly, then shuts off. The problem is that humidity removal requires runtime — the longer the system runs, the more moisture comes out. A short cycle of 6 minutes cools the air but barely touches the humidity.
Bigger system → faster cycles → less dehumidification → muggier house.
This is one of the most common installation mistakes we see when we're called in to diagnose comfort complaints. A previous contractor saw a 2,000 sq ft house and quoted a 3-ton AC because that's the rule-of-thumb. A proper load calculation on the same house often comes back at 2 or 2.5 tons, and the smaller unit dehumidifies dramatically better because it runs longer cycles.
The two real fixes
If your house is already in this situation, you have two options:
Fix 1: Add a whole-home dehumidifier
A standalone whole-home dehumidifier (Aprilaire, Honeywell, Santa Fe) installs in the basement or mechanical room and works independently of your AC. It pulls humidity out of the air whether or not your AC is running.
Cost: typically $1,800 to $3,500 installed.
Best for: homes where the existing AC is the right capacity but humidity is still high (often St. Lawrence waterfront homes), or homes where the AC is oversized and replacement isn't on the table yet.
Fix 2: Replace the AC with a two-stage or variable-capacity unit
Two-stage AC units can run at low capacity (typically 60-65% of rated output) for most operating hours, kicking up to full capacity only when the house is hot. Variable-capacity (inverter-driven) units modulate continuously between roughly 30% and 100% of capacity.
The result for your house: the system runs longer at lower output, which is much more effective at dehumidification while using less total electricity than a single-stage unit running shorter, harder cycles.
Cost: $1,500-$3,000 premium over a single-stage AC of the same nominal capacity.
Best for: homes that are due for AC replacement anyway, or homes where the existing AC is oversized and undersized at the same time (yes, this is possible).
What we do differently in Morrisburg/Iroquois specifically
We've stopped quoting single-stage AC in Morrisburg, Iroquois, and the St. Lawrence-adjacent parts of our service area. The river-side latent load makes single-stage equipment a comfort failure here — we've had too many post-install complaints and we'd rather quote the right system from the start.
For inland SD&G — Winchester, Chesterville, the rural North Dundas concessions — single-stage cooling still makes sense in many homes because the humidity load is lower.
Geography matters more in HVAC than people realize.
What to do if your house is sticky right now
Three quick diagnostic steps you can run yourself:
- Buy a $15 hygrometer at Canadian Tire and put it in the room that feels worst. Read the relative humidity.
- Check your thermostat for cycle length. If the AC runs less than 10 minutes per cycle on a hot day, it's almost certainly oversized.
- Walk to the outdoor unit when the system is running. Touch the larger of the two copper lines (the insulated one). It should be cold and sweating slightly. If it's cold but bone-dry, your refrigerant charge may be low.
Then give us a call and tell us what you found. We can usually figure out the right fix in one conversation — and if a service visit is needed, we'll tell you what to expect.
See also: our AC repair and AC installation service pages for more on what we cover.