Your thermostat says 22 °C. Your basement office feels like a walk-in cooler, the main floor is fine, and the upstairs bedroom your kid sleeps in is pushing 27 °C at bedtime. Nothing is broken. This is what a house designed around a furnace does when you ask it to run an air conditioner — and in Spruce Grove, Stony Plain and across the Greater Edmonton Area, almost every house was designed around a furnace.
Start With The Boring Explanations
Before anyone talks to you about zoning systems or new equipment, three unglamorous things cause most uneven-cooling calls, and all three are free to check.
- A loaded filter. A filter thick with dust chokes airflow to the rooms furthest from the furnace first. The close rooms still get air. The far rooms starve. If you cannot remember the last change, that is your answer.
- Blocked or closed registers. A couch over a floor vent, a rug across a return, a register someone shut last February and forgot. Walk the house and open everything.
- A filthy outdoor condenser. Cottonwood fluff, grass clippings and dust mat the outdoor coil every Alberta summer. A coil that cannot shed heat lowers the capacity of the whole system, and the rooms at the end of the duct run notice first.
Fix those, run the system for a full day, and re-measure with a cheap thermometer in each problem room. A surprising number of “my house cools unevenly” problems end right here.
Why Upstairs Is Always The Loser
Warm air is less dense than cool air, so it rises. That is not a metaphor — it is a pressure difference, and it operates in your house every hour of every day. Natural Resources Canada calls it the stack effect: warm air collects near the top of the house and pushes out through ceiling penetrations and upper-storey window cracks, while lower pressure near the bottom of the house pulls outside air in through basement windows and the rim joist.
In winter this is why your basement feels drafty. In summer it works against you differently: the heat that leaks into your attic all afternoon migrates down into the upper storey, while the cool air your air conditioner produces sinks toward the basement. Your thermostat, sitting on the main-floor hallway wall, is measuring the one part of the house that is behaving.
The stack effect gets stronger with a leakier envelope, a taller house and a bigger indoor-outdoor temperature difference. A two-storey home in Sherwood Park with an unsealed attic hatch is a chimney.
Your Ducts Were Sized For Heating, Not Cooling
This is the structural problem, and it is nearly universal in this region. Alberta homes are heating-dominated: for eight months of the year the job is pushing warm air, which wants to rise on its own. Ductwork sized and balanced around that task delivers warm air to upstairs bedrooms beautifully.
Ask that same duct system to push cold air — which wants to fall — up two storeys against its own buoyancy, and the upstairs branches simply do not carry enough volume. The system is not malfunctioning. It is doing exactly what it was designed to do, in the opposite season.
Then There Is What Never Reaches The Room At All
According to ENERGY STAR, in a typical house about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes and poorly connected joints. That conditioned air is not landing in your bedroom — it is landing in the joist space, the attic, or the crawl space.
Leakage is rarely distributed evenly. A single disconnected boot above a basement ceiling can quietly bleed off most of the air destined for the room above it. The longer and more convoluted the duct run, the more joints it crosses, and the more chances there are to lose air before the register.
Closing vents in unused rooms makes uneven cooling worse, not better. It is the single most common self-inflicted cause we see. Forced-air systems are designed to push against a maximum static pressure — typically 0.5 inches of water column. The National Comfort Institute, which has measured static pressure across a great many systems in the field, finds the typical system already running near 0.8 iwc, roughly 60% over spec. Close a few registers and you push it further. Duct leakage rises with pressure, so more of your air escapes into the joist space. A standard PSC blower moves less air; a variable-speed ECM blower ramps up and burns more electricity to compensate. And starve airflow across the indoor coil badly enough and it will drop below freezing, ice over, and stop cooling anything at all. You have not redirected the air. You have made the system fight itself.
The Room That Faces West
Solar gain is not evenly distributed either. A bedroom with large west-facing windows absorbs heat through the entire late afternoon — precisely when Edmonton-area temperatures peak and precisely when that room is about to be slept in. No duct system compensates for that automatically, because the thermostat downstairs has no idea it is happening.
Exterior shading, blinds closed before the sun swings west, and thermal curtains do more for that one room than any equipment change. This is the cheapest fix on the list and the one people skip.
Nobody Ever Checks The Returns
Air that goes into a room has to get out of it. Many Alberta homes were built with a single central return on each floor, sometimes only one for the whole house. Close a bedroom door and that room becomes a pressurized box: supply air pushes in, cannot easily get back to the furnace, and flow through that branch collapses.
- Test it. Leave the problem bedroom’s door open overnight. If it cools noticeably better, you have a return-air problem, not a supply problem.
- Cheap fixes exist. Undercutting the door, a transfer grille, or a jumper duct restores the return path without touching the equipment.
- Keep returns clear. Furniture and floor-length curtains across a return grille do the same damage as closing a supply vent.
When It Genuinely Is The Equipment
Sometimes the house is not the problem. Natural Resources Canada is blunt about oversizing: an air conditioner with more capacity than the cooling load “will result in short operating cycles, which will not adequately remove humidity, resulting in an unpleasantly cold and damp home.” An oversized unit satisfies the thermostat fast, shuts off, and never runs long enough to move air into the far corners of the house. The main floor gets cold. Everywhere else stays warm and clammy.
Low refrigerant is the other one. NRCan notes that even a small loss of refrigerant causes a significant drop in efficiency — and a system limping along at reduced capacity delivers to the nearest rooms and gives up on the rest. That is a service call, not a homeowner fix, and topping up refrigerant without finding the leak simply repeats the problem next summer.
The Order To Actually Do This In
Work cheapest to most expensive. Most homeowners get their comfort back somewhere in the first three steps.
- Change the filter and open every register and return in the house.
- Clear and rinse the outdoor coil, and cut back anything growing within a metre of it.
- Run the furnace fan continuously instead of on AUTO. Constant circulation is the single most effective free fix for a hot upstairs, because it keeps mixing the stratified air the stack effect keeps separating.
- Fix the return path for closed-door rooms before you spend a dollar on equipment.
- Have the ducts inspected and cleaned, and have obvious leaks and disconnected boots sealed while the system is open.
- Have static pressure measured before anyone sells you a bigger air conditioner. A system already at 0.8 iwc does not need more capacity. It needs less restriction.
Uneven cooling is almost never one dramatic failure. It is four or five small restrictions stacked on top of a duct system that was drawn for January, quietly compounding through a July afternoon.
Quick Answers
Why is my upstairs so much hotter than my main floor?
Three things stack up. Warm air naturally rises and collects at the top of the house — Natural Resources Canada calls this the stack effect — while the cool air your air conditioner produces sinks toward the basement. At the same time, heat soaks into your attic all afternoon and migrates down into the upper storey. And your ducts were sized to push warm air up in winter, not to push cold air up in summer. Running the furnace fan continuously instead of on AUTO is the most effective free fix, because it keeps mixing the air the stack effect keeps separating.
Should I close vents in rooms I do not use?
No. Forced-air systems are designed to push against a maximum static pressure, typically 0.5 inches of water column, and field measurement by the National Comfort Institute finds the typical system already running near 0.8 iwc. Closing registers raises pressure further, which increases duct leakage, reduces airflow with a standard PSC blower, increases electricity use with a variable-speed ECM blower, and can starve the indoor coil until it freezes over. It does not save money and it makes uneven cooling worse.
How much conditioned air is lost inside the ducts?
ENERGY STAR reports that in a typical house, about 20 to 30 percent of the air that moves through the duct system is lost to leaks, holes and poorly connected ducts. That loss is rarely spread evenly — one disconnected boot can bleed off most of the air destined for a single room, which is why one bedroom can be dramatically worse than the rest.
Will a bigger air conditioner fix uneven cooling?
Usually the opposite. Natural Resources Canada warns that oversizing an air conditioner results in short operating cycles that do not adequately remove humidity, producing an unpleasantly cold and damp home. A unit that satisfies the thermostat quickly and shuts off never runs long enough to move air into the far corners of the house. Have static pressure measured before anyone sells you more capacity.
Does closing a bedroom door make that room hotter?
It can, if the room has no return-air path. Supply air pushes into the room and cannot easily get back to the furnace, so flow through that branch collapses. Test it by leaving the door open overnight. If the room cools noticeably better, the fix is a return path — undercutting the door, a transfer grille, or a jumper duct — not more supply air.
Can duct cleaning help with uneven cooling?
It helps when restriction is part of the problem, and having the system open is the right moment to find disconnected boots and obvious leaks that are bleeding air into joist spaces. It is not a substitute for fixing a closed return, an oversized unit, or a duct system that was never sized for cooling. A reputable company should tell you which of those you actually have.
Trusted Sources
Every figure above is drawn from federal agencies, national statistics or published building-science measurement.
ENERGY STAR Duct Sealing
The 20–30% duct leakage figure, and what proper sealing does about it.
Read the guideNatural Resources Canada Stack Effect
Keeping the Heat In, Section 2 — how warm air, pressure and leakage move through a Canadian house.
See the scienceNRCan — Air Conditioning Your Home
Federal guidance on sizing, oversizing, short cycling, refrigerant loss and thermostat range.
Read the guidanceEnergy Vanguard Static Pressure
Building-science analysis of what closing vents does to static pressure, blower behaviour and coil temperature.
See the analysisLinks open official government and industry sites in a new tab. Home Pros Group isn’t affiliated with these organizations.
Cooling One Room At A Time Is No Way To Live
Home Pros Group has cleaned and inspected duct systems across Spruce Grove, Stony Plain and the Greater Edmonton Area since 2003. If your upstairs never keeps up, we’ll tell you honestly whether it’s restriction, leakage, or something a duct cleaning won’t solve.
This article is general home-maintenance information, not engineering advice for your specific system. Duct leakage figures are from ENERGY STAR; stack-effect and equipment-sizing guidance is summarized from Natural Resources Canada; typical measured static pressure is reported by the National Comfort Institute. Every house is different — airflow, sizing and refrigerant work should be assessed on site by a qualified HVAC technician.